Generation of dynamo magnetic fields in protoplanetary and other astrophysical accretion disks

NASA Technical Reports Server (NTRS)

Stepinski, T. F.; Levy, E. H.

1988-01-01

A computational method for treating the generation of dynamo magnetic fields in astrophysical disks is presented. The numerical difficulty of handling the boundary condition at infinity in the cylindrical disk geometry is overcome by embedding the disk in a spherical computational space and matching the solutions to analytically tractable spherical functions in the surrounding space. The lowest lying dynamo normal modes for a 'thick' astrophysical disk are calculated. The generated modes found are all oscillatory and spatially localized. Tha potential implications of the results for the properties of dynamo magnetic fields in real astrophysical disks are discussed.

Generation of large-scale magnetic fields by small-scale dynamo in shear flows

NASA Astrophysics Data System (ADS)

Squire, Jonathan; Bhattacharjee, Amitava

2015-11-01

A new mechanism for turbulent mean-field dynamo is proposed, in which the magnetic fluctuations resulting from a small-scale dynamo drive the generation of large-scale magnetic fields. This is in stark contrast to the common idea that small-scale magnetic fields should be harmful to large-scale dynamo action. These dynamos occur in the presence of large-scale velocity shear and do not require net helicity, resulting from off-diagonal components of the turbulent resistivity tensor as the magnetic analogue of the ``shear-current'' effect. The dynamo is studied using a variety of computational and analytic techniques, both when the magnetic fluctuations arise self-consistently through the small-scale dynamo and in lower Reynolds number regimes. Given the inevitable existence of non-helical small-scale magnetic fields in turbulent plasmas, as well as the generic nature of velocity shear, the suggested mechanism may help to explain generation of large-scale magnetic fields across a wide range of astrophysical objects. This work was supported by a Procter Fellowship at Princeton University, and the US Department of Energy Grant DE-AC02-09-CH11466.

Effects due to induced azimuthal eddy currents in a self-exciting Faraday disk homopolar dynamo with a nonlinear series motor. I.. Two special cases

NASA Astrophysics Data System (ADS)

Hide, Raymond; Moroz, Irene M.

1999-10-01

The elucidation of the behaviour of physically realistic self-exciting Faraday-disk dynamos bears inter alia on attempts by theoretical geophysicists to interpret observations of geomagnetic polarity reversals. Hide [The nonlinear differential equations governing a hierarchy of self-exciting coupled Faraday-disk homopolar dynamos, Phys. Earth Planet. Interiors 103 (1997) 281-291; Nonlinear quenching of current fluctuations in a self-exciting homopolar dynamo, Nonlinear Processes in Geophysics 4 (1998) 201-205] has introduced a novel 4-mode set of nonlinear ordinary differential equations to describe such a dynamo in which a nonlinear electric motor is connected in series with the coil. The applied couple, α, driving the disk is steady and the Lorentz couple driving the motor is a quadratic function, x(1-ɛ)+ɛσx 2, of the dynamo-generated current x, with 0≤ɛ≤1. When there are no additional biasing effects due to background magnetic fields etc., the behaviour of the dynamo is determined by eight independent non-negative control parameters. These include ρ, proportional to the resistance of the disk to azimuthal eddy currents, and β, an inverse measure of the moment of inertia of the armature of the motor. When β=0 (the case when the motor is absent and ɛ and σ are redundant) and ρ -1≠0 , the 4-mode dynamo equations reduce to the 3-mode Lorenz equations, which can behave chaotically [E. Knobloch, Chaos in the segmented disc dynamo, Phys. Lett. A 82 (1981) 439-440]. When β≠0 but ρ -1=0 , the 4-mode set of equations reduces to a 3-mode dynamo [R. Hide (1997), see above], which can also behave chaotically when ɛ=0 [R. Hide, A.C. Skeldon, D.J. Acheson, A study of two novel self-exciting single-disk homopolar dynamos: theory, Proc. R. Soc. Lond. A 452 (1996) 1369-1395] but not when ɛ=1 [R. Hide (1998), see above]. In the latter case, however, all persistent fluctuations are completely quenched [R. Hide (1998), see above]. In this paper we investigate two limiting cases of ɛ=0 and ɛ=1 in the 4-mode dynamo when azimuthal eddy currents are allowed to flow i.e. cases when ρ -1=0 ; in a companion paper [I.M. Moroz, R. Hide, Effects due to induced azimuthal eddy currents in the Faraday disk self-exciting homopolar dynamo with a nonlinear series motor: II The general case, 1999, submitted] we extend the present analysis to the general case of 0≤ɛ≤1. When ɛ=0, chaotic behaviour occurs even more extensively in parameter space in the presence of azimuthal eddy currents than in their absence. When ɛ=1, the quenching of chaotic and all other non-steady dynamo action is no longer complete, for aperiodic solutions are found within limited regions of parameter space where β is very small and α is very large.

Shear-driven dynamo waves at high magnetic Reynolds number.

PubMed

Tobias, S M; Cattaneo, F

2013-05-23

Astrophysical magnetic fields often display remarkable organization, despite being generated by dynamo action driven by turbulent flows at high conductivity. An example is the eleven-year solar cycle, which shows spatial coherence over the entire solar surface. The difficulty in understanding the emergence of this large-scale organization is that whereas at low conductivity (measured by the magnetic Reynolds number, Rm) dynamo fields are well organized, at high Rm their structure is dominated by rapidly varying small-scale fluctuations. This arises because the smallest scales have the highest rate of strain, and can amplify magnetic field most efficiently. Therefore most of the effort to find flows whose large-scale dynamo properties persist at high Rm has been frustrated. Here we report high-resolution simulations of a dynamo that can generate organized fields at high Rm; indeed, the generation mechanism, which involves the interaction between helical flows and shear, only becomes effective at large Rm. The shear does not enhance generation at large scales, as is commonly thought; instead it reduces generation at small scales. The solution consists of propagating dynamo waves, whose existence was postulated more than 60 years ago and which have since been used to model the solar cycle.

Grand Minima and Equatorward Propagation in a Cycling Stellar Convective Dynamo

NASA Astrophysics Data System (ADS)

Augustson, Kyle C.; Brun, Allan Sacha; Miesch, Mark; Toomre, Juri

2015-08-01

The 3-D magnetohydrodynamic (MHD) Anelastic Spherical Harmonic (ASH) code, using slope-limited diffusion, is employed to capture convective and dynamo processes achieved in a global-scale stellar convection simulation for a model solar-mass star rotating at three times the solar rate. The dynamo generated magnetic fields possesses many time scales, with a prominent polarity cycle occurring roughly every 6.2 years. The magnetic field forms large-scale toroidal wreaths, whose formation is tied to the low Rossby number of the convection in this simulation. The polarity reversals are linked to the weakened differential rotation and a resistive collapse of the large-scale magnetic field. An equatorial migration of the magnetic field is seen, which is due to the strong modulation of the differential rotation rather than a dynamo wave. A poleward migration of magnetic flux from the equator eventually leads to the reversal of the polarity of the high-latitude magnetic field. This simulation also enters an interval with reduced magnetic energy at low latitudes lasting roughly 16 years (about 2.5 polarity cycles), during which the polarity cycles are disrupted and after which the dynamo recovers its regular polarity cycles. An analysis of this grand minimum reveals that it likely arises through the interplay of symmetric and antisymmetric dynamo families. This intermittent dynamo state potentially results from the simulations relatively low magnetic Prandtl number. A mean-field-based analysis of this dynamo simulation demonstrates that it is of the α-Ω type. The time scales that appear to be relevant to the magnetic polarity reversal are also identified.

Grand Minima and Equatorward Propagation in a Cycling Stellar Convective Dynamo

NASA Astrophysics Data System (ADS)

Augustson, Kyle; Brun, Allan Sacha; Miesch, Mark; Toomre, Juri

2015-08-01

The 3D MHD Anelastic Spherical Harmonic code, using slope-limited diffusion, is employed to capture convective and dynamo processes achieved in a global-scale stellar convection simulation for a model solar-mass star rotating at three times the solar rate. The dynamo-generated magnetic fields possesses many timescales, with a prominent polarity cycle occurring roughly every 6.2 years. The magnetic field forms large-scale toroidal wreaths, whose formation is tied to the low Rossby number of the convection in this simulation. The polarity reversals are linked to the weakened differential rotation and a resistive collapse of the large-scale magnetic field. An equatorial migration of the magnetic field is seen, which is due to the strong modulation of the differential rotation rather than a dynamo wave. A poleward migration of magnetic flux from the equator eventually leads to the reversal of the polarity of the high-latitude magnetic field. This simulation also enters an interval with reduced magnetic energy at low latitudes lasting roughly 16 years (about 2.5 polarity cycles), during which the polarity cycles are disrupted and after which the dynamo recovers its regular polarity cycles. An analysis of this grand minimum reveals that it likely arises through the interplay of symmetric and antisymmetric dynamo families. This intermittent dynamo state potentially results from the simulation’s relatively low magnetic Prandtl number. A mean-field-based analysis of this dynamo simulation demonstrates that it is of the α-Ω type. The timescales that appear to be relevant to the magnetic polarity reversal are also identified.

GRAND MINIMA AND EQUATORWARD PROPAGATION IN A CYCLING STELLAR CONVECTIVE DYNAMO

DOE Office of Scientific and Technical Information (OSTI.GOV)

Augustson, Kyle; Miesch, Mark; Brun, Allan Sacha

2015-08-20

The 3D MHD Anelastic Spherical Harmonic code, using slope-limited diffusion, is employed to capture convective and dynamo processes achieved in a global-scale stellar convection simulation for a model solar-mass star rotating at three times the solar rate. The dynamo-generated magnetic fields possesses many timescales, with a prominent polarity cycle occurring roughly every 6.2 years. The magnetic field forms large-scale toroidal wreaths, whose formation is tied to the low Rossby number of the convection in this simulation. The polarity reversals are linked to the weakened differential rotation and a resistive collapse of the large-scale magnetic field. An equatorial migration of themore » magnetic field is seen, which is due to the strong modulation of the differential rotation rather than a dynamo wave. A poleward migration of magnetic flux from the equator eventually leads to the reversal of the polarity of the high-latitude magnetic field. This simulation also enters an interval with reduced magnetic energy at low latitudes lasting roughly 16 years (about 2.5 polarity cycles), during which the polarity cycles are disrupted and after which the dynamo recovers its regular polarity cycles. An analysis of this grand minimum reveals that it likely arises through the interplay of symmetric and antisymmetric dynamo families. This intermittent dynamo state potentially results from the simulation’s relatively low magnetic Prandtl number. A mean-field-based analysis of this dynamo simulation demonstrates that it is of the α-Ω type. The timescales that appear to be relevant to the magnetic polarity reversal are also identified.« less

Exploring the Flux Tube Paradigm in Solar-like Convection Zones

NASA Astrophysics Data System (ADS)

Weber, Maria A.; Nelson, Nicholas; Browning, Matthew

2017-08-01

In the solar context, important insight into the flux emergence process has been obtained by assuming the magnetism giving rise to sunspots consists partly of idealized flux tubes. Global-scale dynamo models are only now beginning to capture some aspects of flux emergence. In certain regimes, these simulations self-consistently generate magnetic flux structures that rise buoyantly through the computational domain. How similar are these dynamo-generated, rising flux structures to traditional flux tube models? The work we present here is a step toward addressing this question. We utilize the thin flux tube (TFT) approximation to simply model the evolution of flux tubes in a global, three-dimensional geometry. The TFTs are embedded in convective flows taken from a global dynamo simulation of a rapidly rotating Sun within which buoyant flux structures arise naturally from wreaths of magnetism. The initial conditions of the TFTs are informed by rising flux structures identified in the dynamo simulation. We compare the trajectories of the dynamo-generated flux loops with those computed through the TFT approach. We also assess the nature of the relevant forces acting on both sets of flux structures, such as buoyancy, the Coriolis force, and external forces imparted by the surrounding convection. To achieve the fast <15 day rise of the buoyant flux structures, we must suppress the large retrograde flow established inside the TFTs which occurs due to a strong conservation of angular momentum as they move outward. This tendency is common in flux tube models in solar-like convection zones, but is not present to the same degree in the dynamo-generated flux loops. We discuss the mechanisms that may be responsible for suppressing the axial flow inside the flux tube, and consider the implications this has regarding the role of the Coriolis force in explaining sunspot latitudes and the observed Joy’s Law trend of active regions. Our work aims to provide constraints, and possible calibrations, on the traditional flux tube model as it pertains to the Sun and other spotted stars.

Magnetic and velocity fields in a dynamo operating at extremely small Ekman and magnetic Prandtl numbers

NASA Astrophysics Data System (ADS)

Šimkanin, Ján; Kyselica, Juraj

2017-12-01

Numerical simulations of the geodynamo are becoming more realistic because of advances in computer technology. Here, the geodynamo model is investigated numerically at the extremely low Ekman and magnetic Prandtl numbers using the PARODY dynamo code. These parameters are more realistic than those used in previous numerical studies of the geodynamo. Our model is based on the Boussinesq approximation and the temperature gradient between upper and lower boundaries is a source of convection. This study attempts to answer the question how realistic the geodynamo models are. Numerical results show that our dynamo belongs to the strong-field dynamos. The generated magnetic field is dipolar and large-scale while convection is small-scale and sheet-like flows (plumes) are preferred to a columnar convection. Scales of magnetic and velocity fields are separated, which enables hydromagnetic dynamos to maintain the magnetic field at the low magnetic Prandtl numbers. The inner core rotation rate is lower than that in previous geodynamo models. On the other hand, dimensional magnitudes of velocity and magnetic fields and those of the magnetic and viscous dissipation are larger than those expected in the Earth's core due to our parameter range chosen.

Magnetic fields on asteroid 4 Vesta recorded by the Millbillillie eucrite

NASA Astrophysics Data System (ADS)

Weiss, B. P.; Fu, R.

2011-12-01

The detection of past dynamo activity on the asteroid 4 Vesta would confirm the existence of a metallic core, placing important constraints on its accretional and thermal history. Knowledge of the strength and duration of a dynamo on 4 Vesta also has important implications for the theoretical understanding of dynamo generation in small bodies. Magnetic fields from a putative core dynamo may have been recorded as remanent magnetization in achondritic meteorites of the howardite-eucrite-diogenite (HED) clan, which are thought to originate from the asteroid. To search for evidence for past dynamo activity, we performed a paleomagnetic study of nine mutually oriented samples of the Millbillillie eucrite. We found that the magnitude and direction of the magnetization change systematically for samples progressively farther away from the fusion crust, indicating that the samples were not remagnetized on Earth and that the interior samples carry an extraterrestrial magnetization. The fusion crust is ~1000 times more magnetic per unit mass than the interior, which was likely a source of contamination in earlier studies of bulk samples from this meteorite. Two interior samples were subjected to alternating field (AF) demagnetization up to 290 mT. We found a high coercivity (HC) component of magnetization carried by grains with coercivities between 70 and 180 mT. The HC magnetization is approximately unidirectional in the subsamples. The AF demagnetization profile of this component is similar to that of an anhysteretic remanent magnetization (ARM), suggesting that it may represent a thermoremanent magnetization (TRM). Under this assumption, our ARM paleointensity experiments yield field strengths of 2-3 μT while our IRM paleointensities are between 5 and 8 μT. Ongoing analysis of additional samples will further test this result. The HC magnetization may record 1) transient impact-generated fields, 2) remanent crustal fields, or 3) dynamo fields. Case 1) is unlikely if the sample has a thermoremanence because stable magnetization over the wide coercivity range observed for the HC component requires a magnetic field stable for the duration of the cooling process. Furthermore, the characteristic coercivities of the HC magnetization are very high compared to typical values for shock remanent magnetization. In case 2), the strength of putative impact-generated crustal fields on the moon suggests that impacts on Vesta would have caused remanent crustal fields of < 2 μT strength, which is below our observed paleointensities. Remanent crustal fields stronger than ~2 μT require a different magnetizing source, such as an earlier dynamo. Together, these facts suggest that the HC magnetization is unlikely to be a result of meteoroid bombardment and more probably record dynamo fields or remanent crustal fields due to an earlier dynamo. We therefore regard our results as tentative evidence of a past dynamo on 4 Vesta

Generation of large-scale magnetic fields by small-scale dynamo in shear flows

DOE PAGES

Squire, J.; Bhattacharjee, A.

2015-10-20

We propose a new mechanism for a turbulent mean-field dynamo in which the magnetic fluctuations resulting from a small-scale dynamo drive the generation of large-scale magnetic fields. This is in stark contrast to the common idea that small-scale magnetic fields should be harmful to large-scale dynamo action. These dynamos occur in the presence of a large-scale velocity shear and do not require net helicity, resulting from off-diagonal components of the turbulent resistivity tensor as the magnetic analogue of the "shear-current" effect. Furthermore, given the inevitable existence of nonhelical small-scale magnetic fields in turbulent plasmas, as well as the generic naturemore » of velocity shear, the suggested mechanism may help explain the generation of large-scale magnetic fields across a wide range of astrophysical objects.« less

Generation of Large-Scale Magnetic Fields by Small-Scale Dynamo in Shear Flows.

PubMed

Squire, J; Bhattacharjee, A

2015-10-23

We propose a new mechanism for a turbulent mean-field dynamo in which the magnetic fluctuations resulting from a small-scale dynamo drive the generation of large-scale magnetic fields. This is in stark contrast to the common idea that small-scale magnetic fields should be harmful to large-scale dynamo action. These dynamos occur in the presence of a large-scale velocity shear and do not require net helicity, resulting from off-diagonal components of the turbulent resistivity tensor as the magnetic analogue of the "shear-current" effect. Given the inevitable existence of nonhelical small-scale magnetic fields in turbulent plasmas, as well as the generic nature of velocity shear, the suggested mechanism may help explain the generation of large-scale magnetic fields across a wide range of astrophysical objects.

Magnetic Signatures of Nectarian-Aged Lunar Basin-Forming Impacts: Probable Evidence for a Former Core Dynamo

NASA Astrophysics Data System (ADS)

Hood, Lon

2010-05-01

Previous analyses of Lunar Prospector magnetometer (MAG) and electron reflectometer (ER) data have shown that impact processes played an important role in producing the observed crustal magnetization. In particular, the largest areas of strong anomalies occur antipodal to the youngest large basins and correlative studies indicate that basin ejecta materials are important anomaly sources. Models suggest that transient fields generated by the expansion of impact vapor-melt clouds in the presence of an initial solar wind magnetic field are sufficient to explain the antipodal anomalies (Hood and Artemieva, Icarus, v. 193, p. 485, 2008). However, analyses of ER data have also shown that some anomalies are present within Nectarian-aged basins including Moscoviense, Mendel-Rydberg, and Crisium (Halekas et al., Meteorit. Planet. Sci., v. 38, p. 565, 2003). These latter anomalies could be due either to thermoremanence (TRM) in impact melt or to shock remanence in the central uplift. The former interpretation would require a long-lived, steady magnetizing field, consistent with a core dynamo, while the latter interpretation could in principle be explained by an impact-generated field. Here, LP MAG data are applied to produce more detailed regional maps of magnetic anomalies within selected Nectarian basins. Anomalies within the Crisium basin, in particular, are located inside the inner rim edges and are clearly genetically associated with the basin (rather than being due to ejecta from younger basins superposed on Crisium). An analysis of the vector field components shows that the directions of magnetization of the two main sources are close to parallel within the errors of the modeling. These anomalies are therefore most probably due to TRM of impact melt that cooled in a steady, large-scale field. In addition, the paleomagnetic pole position calculated for the strongest and most isolated anomaly lies close to the present rotational pole. Assuming no true polar wander since the Crisium impact and that the lunar dynamo behaved similarly to presently existing terrestrial planet dynamos, they are therefore consistent with the existence of a lunar dynamo field.

The lunar dynamo

NASA Astrophysics Data System (ADS)

Weiss, Benjamin P.; Tikoo, Sonia M.

2014-12-01

The inductive generation of magnetic fields in fluid planetary interiors is known as the dynamo process. Although the Moon today has no global magnetic field, it has been known since the Apollo era that the lunar rocks and crust are magnetized. Until recently, it was unclear whether this magnetization was the product of a core dynamo or fields generated externally to the Moon. New laboratory and spacecraft measurements strongly indicate that much of this magnetization is the product of an ancient core dynamo. The dynamo field persisted from at least 4.25 to 3.56 billion years ago (Ga), with an intensity reaching that of the present Earth. The field then declined by at least an order of magnitude by ∼3.3 Ga. The mechanisms for sustaining such an intense and long-lived dynamo are uncertain but may include mechanical stirring by the mantle and core crystallization.

Axial dipolar dynamo action in the Taylor-Green vortex.

PubMed

Krstulovic, Giorgio; Thorner, Gentien; Vest, Julien-Piera; Fauve, Stephan; Brachet, Marc

2011-12-01

We present a numerical study of the magnetic field generated by the Taylor-Green vortex. We show that periodic boundary conditions can be used to mimic realistic boundary conditions by prescribing the symmetries of the velocity and magnetic fields. This gives insight into some problems of central interest for dynamos: the possible effect of velocity fluctuations on the dynamo threshold, and the role of boundary conditions on the threshold and on the geometry of the magnetic field generated by dynamo action. In particular, we show that an axial dipolar dynamo similar to the one observed in a recent experiment can be obtained with an appropriate choice of the symmetries of the magnetic field. The nonlinear saturation is studied and a simple model explaining the magnetic Prandtl number dependence of the super- and subcritical nature of the dynamo transition is given.

Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic-field generation in shear flows.

PubMed

Herault, J; Rincon, F; Cossu, C; Lesur, G; Ogilvie, G I; Longaretti, P-Y

2011-09-01

The nature of dynamo action in shear flows prone to magnetohydrodynamc instabilities is investigated using the magnetorotational dynamo in Keplerian shear flow as a prototype problem. Using direct numerical simulations and Newton's method, we compute an exact time-periodic magnetorotational dynamo solution to three-dimensional dissipative incompressible magnetohydrodynamic equations with rotation and shear. We discuss the physical mechanism behind the cycle and show that it results from a combination of linear and nonlinear interactions between a large-scale axisymmetric toroidal magnetic field and nonaxisymmetric perturbations amplified by the magnetorotational instability. We demonstrate that this large-scale dynamo mechanism is overall intrinsically nonlinear and not reducible to the standard mean-field dynamo formalism. Our results therefore provide clear evidence for a generic nonlinear generation mechanism of time-dependent coherent large-scale magnetic fields in shear flows and call for new theoretical dynamo models. These findings may offer important clues to understanding the transitional and statistical properties of subcritical magnetorotational turbulence.

A study of the kinematic dynamo equation with time-dependent coefficients

NASA Technical Reports Server (NTRS)

Ko, Chung-Ming

1990-01-01

During an active star formation epoch the interstellar medium of a galaxy is in a hyperactive state, and the average turbulent velocity is higher than in the long periods between star formation epochs. The galactic magnetic field generated by dynamo action depends strongly on the turbulent velocity, so that generation of magnetic field should vary with star formation activity. This paper is a preliminary study of the kinematic dynamo equation with time-dependent coefficients simulating the time dependence of the star formation activities. Ko and Parker argued in a simple model that the thickness of the dynamo region is the most sensitive dynamo parameter. The present work shows that the effect of inflating the galactic disk suddenly is to transform a stationary magnetic field into a growing field while keeping the profile more or less intact. Plane wave solutions for a dynamo with power-law time-dependent parameters show that the field may decay first and then grow, and vice versa, which is quite different from a constant parameter dynamo.

Measurements of dynamo effect on double-CHI pulse ST plasmas on HIST

NASA Astrophysics Data System (ADS)

Ito, K.; Hanao, T.; Ishihara, M.; Matsumoto, K.; Higashi, T.; Kikuchi, Y.; Fukumoto, N.; Nagata, M.

2011-10-01

Coaxial Helicity injection (CHI) is an efficient current-drive method used in spheromak and spherical torus (ST) experiments. An anticipated issue for CHI is achieving good energy confinement, since it relies on the magnetic relaxation and dynamo. This is essentially because CHI cannot drive a dynamo directly inside a closed magnetic flux surface. Thus, it is an important issue to investigate dynamo effect to explore CHI current drive mechanisms in a new approach such as Multi-pulsing CHI method. To study the dynamo model with two-fluid Hall effects, we have started from the generalized Ohm law. We have measured each MHD dynamo term and Hall dynamo term separately by using Mach probe and Hall probe involving 3-axis magnetic pick-up coils. The result shows that the induced electric field due to MHD dynamo is large enough to sustain the mean toroidal current against resistive decay in the core region. In the other hand, the anti-dynamo effect in the MHD dynamo term is observed in the central open flux column (OFC) region. From the viewpoint of two-fluid theory, ion diamagnetic drift is opposite to the electron diamagnetic drift, maybe resulting in the anti-dynamo effect. Hall dynamo may arise from the fluctuating electron diamagnetic current due to high electron density gradient which is large in the OFC region.

Onset of a planetesimal dynamo

NASA Astrophysics Data System (ADS)

Wang, H.; Weiss, B. P.; Wang, J.; Chen-Wiegart, Y. C. K.; Downey, B. G.; Suavet, C. R.; Andrade Lima, E.; Zucolotto, M. E.

2014-12-01

The paleomagnetism of achondritic meteorites provides evidence for advecting metallic core dynamos and large-scale differentiation on their parent planetesimals. The small sizes of these bodies (~102 km) enable a new opportunity to understand the physics of dynamo generation in a size regime with distinct thermal evolution parameters that are more accessible to model than planets. One key unknown about planetesimal dynamos is their onset time. Theoretical studies have suggested that it might occur instantaneously after large-scale melting (Weiss et al. 2008, Elkins-Tanton et al. 2011) while others have argued that a dynamo could be delayed by ~6 My (Sterenborg and Crowley 2013) or longer. Here we present the first paleomagnetic study that has constrained the onset time of a planetesimal dynamo, which has key implications for the physics of core formation, planetary thermal evolution and dynamo generation mechanisms. Our study focused on angrites, a group of ancient basaltic achondrites from near the surface of an early differentiated planetesimal. With unshocked, unbrecciated textures and Pb/Pb ages ranging from only ~3-10 My younger than the formation of calcium aluminum inclusions (CAIs), they are among the oldest known and best preserved planetary igneous rocks. We used a new CO2 + H2 gas mixture system (Suavet et al. 2014) for controlled oxygen fugacity thermal paleointensity experiments on two of the oldest angrites (D'Orbigny and SAH 99555; 4564.4 Ma) and a younger angrite (Angra dos Reis; 4557.7 Ma). For D'Orbigny and SAH 99555, we found that the natural remanence (NRM) demagnetizes at much lower temperatures than lab-applied thermoremanence (TRM), indicating that their NRMs are dominantly overprints from the Earth's field and hand magnets. In contrast, the NRM of Angra dos Reis behaves similarly to a TRM, confirming its thermal origin. We estimate the paleointensities to be < 0.2 µT for D'Orbigny and SAH 99555 and ~10 µT for Angra dos Reis. This indicates that the angrite parent body dynamo originated between 3 and 10 My after CAI formation. Our results are consistent with planetesimal evolution models calling for dynamos delayed by mantle heating due to radiogenic 26Al. Furthermore, these data suggest that external nebular fields in the angrite parent body region had declined to < 0.2 μT at 3 My after CAI formation.

The lunar dynamo.

PubMed

Weiss, Benjamin P; Tikoo, Sonia M

2014-12-05

The inductive generation of magnetic fields in fluid planetary interiors is known as the dynamo process. Although the Moon today has no global magnetic field, it has been known since the Apollo era that the lunar rocks and crust are magnetized. Until recently, it was unclear whether this magnetization was the product of a core dynamo or fields generated externally to the Moon. New laboratory and spacecraft measurements strongly indicate that much of this magnetization is the product of an ancient core dynamo. The dynamo field persisted from at least 4.25 to 3.56 billion years ago (Ga), with an intensity reaching that of the present Earth. The field then declined by at least an order of magnitude by ∼3.3 Ga. The mechanisms for sustaining such an intense and long-lived dynamo are uncertain but may include mechanical stirring by the mantle and core crystallization. Copyright © 2014, American Association for the Advancement of Science.

Generation of a dynamo magnetic field in a protoplanetary accretion disk

NASA Technical Reports Server (NTRS)

Stepinski, T.; Levy, E. H.

1987-01-01

A new computational technique is developed that allows realistic calculations of dynamo magnetic field generation in disk geometries corresponding to protoplanetary and protostellar accretion disks. The approach is of sufficient generality to allow, in the future, a wide class of accretion disk problems to be solved. Here, basic modes of a disk dynamo are calculated. Spatially localized oscillatory states are found to occur in Keplerain disks. A physical interpretation is given that argues that spatially localized fields of the type found in these calculations constitute the basic modes of a Keplerian disk dynamo.

Bipolar Jets Launched by a Mean-field Accretion Disk Dynamo

NASA Astrophysics Data System (ADS)

Fendt, Christian; Gaßmann, Dennis

2018-03-01

By applying magnetohydrodynamic simulations, we investigate the launching of jets driven by a disk magnetic field generated by a mean-field disk dynamo. Extending our earlier studies, we explore the bipolar evolution of the disk α 2Ω-dynamo and the outflow. We confirm that a negative dynamo-α leads to a dipolar field geometry, whereas positive values generate quadrupolar fields. The latter remain mainly confined to the disk and cannot launch outflows. We investigate a parameter range for the dynamo-α ranging from a critical value below which field generation is negligible, {α }0,{crit}=-0.0005, to α 0 = ‑1.0. For weak | {α }0| ≤slant 0.07, two magnetic loop structures with opposite polarity may arise, which leads to reconnection and disturbs the field evolution and accretion-ejection process. For a strong dynamo-α, a higher poloidal magnetic energy is reached, roughly scaling with {E}mag}∼ | {α }0| , which also leads to higher accretion and ejection rates. The terminal jet speed is governed by the available magnetic energy and increases with the dynamo-α. We find jet velocities on the order of the inner disk Keplerian velocity. For a strong dynamo-α, oscillating dynamo modes may occur that can lead to a pulsed ejection. This is triggered by an oscillating mode in the toroidal field component. The oscillation period is comparable to the Keplerian timescale in the launching region, thus too short to be associated with the knots in observed jets. We find a hemispherically asymmetric evolution for the jet and counter-jet in the mass flux and field structure.

Inertial effects on thermochemically driven convection and hydromagnetic dynamos in a spherical shell

NASA Astrophysics Data System (ADS)

Šimkanin, Ján; Kyselica, Juraj; Guba, Peter

2018-03-01

We investigate the thermochemical convection and hydromagnetic dynamos in a spherical shell using the so-called codensity formulation with different buoyancy sources: the secular cooling from the mantle, the buoyancy sources due to the solidification at the inner core boundary and the combination of the two sources. Numerical simulations of the fully non-linear problem are performed using the PARODY code. In the thermochemical regime, we find that when the Prandtl numbers are lower than Ekman numbers, inertial convection is preferred, while the large-scale columnar convection is preferred otherwise. Unlike the large-scale convection, the inertial convection is found to be almost independent of the nature of driving buoyancy source. Moreover, the codensity field evolves to a new, radially symmetric stationary state. At the Ekman numbers much smaller than the Prandtl numbers, we have obtained the westward equatorial zonal flow in the chemically driven regime, while for the other cases zonal flows are eastward near the equator. In the dynamo regime, inertial convection is preferred when the Prandtl numbers are lower than Ekman numbers and the generated dipolar magnetic fields oscillate from the polar region to the mid-latitudes and back. In this case, the generated magnetic fields are independent of the type of buoyancy source. At the Prandtl numbers greater than Ekman numbers, both dipolar and hemispherical dynamos are found.

Dynamo magnetic-field generation in turbulent accretion disks

NASA Technical Reports Server (NTRS)

Stepinski, T. F.

1991-01-01

Magnetic fields can play important roles in the dynamics and evolution of accretion disks. The presence of strong differential rotation and vertical density gradients in turbulent disks allows the alpha-omega dynamo mechanism to offset the turbulent dissipation and maintain strong magnetic fields. It is found that MHD dynamo magnetic-field normal modes in an accretion disk are highly localized to restricted regions of a disk. Implications for the character of real, dynamically constrained magnetic fields in accretion disks are discussed. The magnetic stress due to the mean magnetic field is found to be of the order of a viscous stress. The dominant stress, however, is likely to come from small-scale fluctuating magnetic fields. These fields may also give rise to energetic flares above the disk surface, providing a possible explanation for the highly variable hard X-ray emission from objects like Cyg X-l.

On large-scale dynamo action at high magnetic Reynolds number

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cattaneo, F.; Tobias, S. M., E-mail: smt@maths.leeds.ac.uk

2014-07-01

We consider the generation of magnetic activity—dynamo waves—in the astrophysical limit of very large magnetic Reynolds number. We consider kinematic dynamo action for a system consisting of helical flow and large-scale shear. We demonstrate that large-scale dynamo waves persist at high Rm if the helical flow is characterized by a narrow band of spatial scales and the shear is large enough. However, for a wide band of scales the dynamo becomes small scale with a further increase of Rm, with dynamo waves re-emerging only if the shear is then increased. We show that at high Rm, the key effect ofmore » the shear is to suppress small-scale dynamo action, allowing large-scale dynamo action to be observed. We conjecture that this supports a general 'suppression principle'—large-scale dynamo action can only be observed if there is a mechanism that suppresses the small-scale fluctuations.« less

Experimental realization of dynamo action: present status and prospects

NASA Astrophysics Data System (ADS)

Giesecke, André; Stefani, Frank; Gundrum, Thomas; Gerbeth, Gunter; Nore, Caroline; Léorat, Jacques

2013-07-01

In the last decades, the experimental study of dynamo action has made great progress. However, after the dynamo experiments in Karlsruhe and Riga, the von-Kármán-Sodium (VKS) dynamo is only the third facility that has been able to demonstrate fluid flow driven self-generation of magnetic fields in a laboratory experiment. Further progress in the experimental examination of dynamo action is expected from the planned precession driven dynamo experiment that will be designed in the framework of the liquid sodium facility DRESDYN (DREsden Sodium facility for DYNamo and thermohydraulic studies). In this paper, we briefly present numerical models of the VKS dynamo that demonstrate the close relation between the axisymmetric field observed in that experiment and the soft iron material used for the flow driving impellers. We further show recent results of preparatory water experiments and design studies related to the precession dynamo and delineate the scientific prospects for the final set-up.

Magnetic field amplification via protostellar disc dynamos

NASA Astrophysics Data System (ADS)

Dyda, S.; Lovelace, R. V. E.; Ustyugova, G. V.; Koldoba, A. V.; Wasserman, I.

2018-06-01

We numerically investigate the generation of a magnetic field in a protostellar disc via an αΩ-dynamo and the resulting magnetohydrodynamic (MHD) driven outflows. We find that for small values of the dimensionless dynamo parameter αd, the poloidal field grows exponentially at a rate σ ∝ Ω _K √{α _d}, before saturating to a value ∝ √{α _d}. The dynamo excites dipole and octupole modes, but quadrupole modes are suppressed, because of the symmetries of the seed field. Initial seed fields too weak to launch MHD outflows are found to grow sufficiently to launch winds with observationally relevant mass fluxes of the order of 10^{-9} M_{⊙} yr^{-1} for T Tauri stars. This suggests that αΩ-dynamos may be responsible for generating magnetic fields strong enough to launch observed outflows.

Dynamo magnetic field modes in thin astrophysical disks - An adiabatic computational approximation

NASA Technical Reports Server (NTRS)

Stepinski, T. F.; Levy, E. H.

1991-01-01

An adiabatic approximation is applied to the calculation of turbulent MHD dynamo magnetic fields in thin disks. The adiabatic method is employed to investigate conditions under which magnetic fields generated by disk dynamos permeate the entire disk or are localized to restricted regions of a disk. Two specific cases of Keplerian disks are considered. In the first, magnetic field diffusion is assumed to be dominated by turbulent mixing leading to a dynamo number independent of distance from the center of the disk. In the second, the dynamo number is allowed to vary with distance from the disk's center. Localization of dynamo magnetic field structures is found to be a general feature of disk dynamos, except in the special case of stationary modes in dynamos with constant dynamo number. The implications for the dynamical behavior of dynamo magnetized accretion disks are discussed and the results of these exploratory calculations are examined in the context of the protosolar nebula and accretion disks around compact objects.

Recovery from Maunder-like Grand Minima in a Babcock–Leighton Solar Dynamo Model

NASA Astrophysics Data System (ADS)

Karak, Bidya Binay; Miesch, Mark

2018-06-01

The Sun occasionally goes through Maunder-like extended grand minima when its magnetic activity drops considerably from the normal activity level for several decades. Many possible theories have been proposed to explain the origin of these minima. However, how the Sun managed to recover from such inactive phases every time is even more enigmatic. The Babcock–Leighton type dynamos, which are successful in explaining many features of the solar cycle remarkably well, are not expected to operate during grand minima due to the lack of a sufficient number of sunspots. In this Letter, we explore the question of how the Sun could recover from grand minima through the Babcock–Leighton dynamo. In our three-dimensional dynamo model, grand minima are produced spontaneously as a result of random variations in the tilt angle of emerging active regions. We find that the Babcock–Leighton process can still operate during grand minima with only a minimal number of sunspots, and that the model can emerge from such phases without the need for an additional generation mechanism for the poloidal field. The essential ingredient in our model is a downward magnetic pumping, which inhibits the diffusion of the magnetic flux across the solar surface.

Properties of Nonlinear Dynamo Waves

NASA Technical Reports Server (NTRS)

Tobias, S. M.

1997-01-01

Dynamo theory offers the most promising explanation of the generation of the sun's magnetic cycle. Mean field electrodynamics has provided the platform for linear and nonlinear models of solar dynamos. However, the nonlinearities included are (necessarily) arbitrarily imposed in these models. This paper conducts a systematic survey of the role of nonlinearities in the dynamo process, by considering the behaviour of dynamo waves in the nonlinear regime. It is demonstrated that only by considering realistic nonlinearities that are non-local in space and time can modulation of the basic dynamo wave he achieved. Moreover, this modulation is greatest when there is a large separation of timescales provided by including a low magnetic Prandtl number in the equation for the velocity perturbations.

Precessionally driven dynamos in ellipsoidal geometry

NASA Astrophysics Data System (ADS)

Ernst-Hullermann, J.; Harder, H.; Hansen, U.

2013-12-01

Precession was suggested as an alternative driving mechanism for Earth's and planetary magnetic fields by Bullard in 1949. Recent estimates of the thermal and electrical conductivity of Earth's core even show that the energy budget for buoyancy driven dynamos might be very tight. Therefore it seems worth to consider precession at least as an additional if not the only source of energy for the geodynamo. We are going to investigate precessionally driven dynamos by the use of a Finite Volume code. As precession drives a flow only due to the movement of the boundaries the shape of the container is essential for the character of the flow. In planets, it is much more effective to drive a precessional flow by the pressure differences induced by the topography of the precessing body rather than by viscous coupling to the walls. Numerical simulations are the only method offering the possibility to investigate the influence of the topography since laboratory experiments normally are constrained by the predetermined geometry of the vessel. We discuss how ellipticity of the planets can be included in our simulations by the use of a non-orthogonal grid. We will show that even laminar precession-driven flows are capable to generate a magnetic field. Most of the magnetic energy of this dynamos resides in the outer viscous boundary layer. While at lower Ekman number the kinematic dynamos also have magnetic fields located in the bulk, these diminish in the full magneto-hydrodynamic case. The laminar dynamos may not scale to Earth-like parameters. Nevertheless, with our new method we have the possibility to explore the parameter space much more systematically.

Modeling MHD accretion-ejection: episodic ejections of jets triggered by a mean-field disk dynamo

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stepanovs, Deniss; Fendt, Christian; Sheikhnezami, Somayeh, E-mail: deniss@stepanovs.org, E-mail: fendt@mpia.de

2014-11-20

We present MHD simulations exploring the launching, acceleration, and collimation of jets and disk winds. The evolution of the disk structure is consistently taken into account. Extending our earlier studies, we now consider the self-generation of the magnetic field by an α{sup 2}Ω mean-field dynamo. The disk magnetization remains on a rather low level, which helps to evolve the simulations for T > 10, 000 dynamical time steps on a domain extending 1500 inner disk radii. We find the magnetic field of the inner disk to be similar to the commonly found open field structure, favoring magneto-centrifugal launching. The outermore » disk field is highly inclined and predominantly radial. Here, differential rotation induces a strong toroidal component, which plays a key role in outflow launching. These outflows from the outer disk are slower, denser, and less collimated. If the dynamo action is not quenched, magnetic flux is continuously generated, diffuses outward through the disk, and fills the entire disk. We have invented a toy model triggering a time-dependent mean-field dynamo. The duty cycles of this dynamo lead to episodic ejections on similar timescales. When the dynamo is suppressed as the magnetization falls below a critical value, the generation of the outflows and also accretion is inhibited. The general result is that we can steer episodic ejection and large-scale jet knots by a disk-intrinsic dynamo that is time-dependent and regenerates the jet-launching magnetic field.« less

Coherent nonhelical shear dynamos driven by magnetic fluctuations at low Reynolds numbers

DOE PAGES

Squire, J.; Bhattacharjee, A.

2015-10-28

Nonhelical shear dynamos are studied with a particular focus on the possibility of coherent dynamo action. The primary results—serving as a follow up to the results of Squire & Bhattacharjee—pertain to the "magnetic shear-current effect" as a viable mechanism to drive large-scale magnetic field generation. This effect raises the interesting possibility that the saturated state of the small-scale dynamo could drive large-scale dynamo action, and is likely to be important in the unstratified regions of accretion disk turbulence. In this paper, the effect is studied at low Reynolds numbers, removing the complications of small-scale dynamo excitation and aiding analysis bymore » enabling the use of quasi-linear statistical simulation methods. In addition to the magnetically driven dynamo, new results on the kinematic nonhelical shear dynamo are presented. Furthermore, these illustrate the relationship between coherent and incoherent driving in such dynamos, demonstrating the importance of rotation in determining the relative dominance of each mechanism.« less

COHERENT NONHELICAL SHEAR DYNAMOS DRIVEN BY MAGNETIC FLUCTUATIONS AT LOW REYNOLDS NUMBERS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Squire, J.; Bhattacharjee, A., E-mail: jsquire@caltech.edu

2015-11-01

Nonhelical shear dynamos are studied with a particular focus on the possibility of coherent dynamo action. The primary results—serving as a follow up to the results of Squire and Bhattacharjee—pertain to the “magnetic shear-current effect” as a viable mechanism to drive large-scale magnetic field generation. This effect raises the interesting possibility that the saturated state of the small-scale dynamo could drive large-scale dynamo action, and is likely to be important in the unstratified regions of accretion disk turbulence. In this paper, the effect is studied at low Reynolds numbers, removing the complications of small-scale dynamo excitation and aiding analysis bymore » enabling the use of quasi-linear statistical simulation methods. In addition to the magnetically driven dynamo, new results on the kinematic nonhelical shear dynamo are presented. These illustrate the relationship between coherent and incoherent driving in such dynamos, demonstrating the importance of rotation in determining the relative dominance of each mechanism.« less

An Elementary Introduction to Solar Dynamo Theory

NASA Astrophysics Data System (ADS)

Choudhuri, Arnab Rai

2007-07-01

The cyclically varying magnetic field of the Sun is believed to be produced by the hydromagnetic dynamo process. We first summarize the relevant observational data pertaining to sunspots and solar cycle. Then we review the basic principles of MHD needed to develop the dynamo theory. This is followed by a discussion how bipolar sunspots form due to magnetic buoyancy of flux tubes formed at the base of the solar convection zone. Following this, we come to the heart of dynamo theory. After summarizing the basic ideas of a turbulent dynamo and the basic principles of its mean field formulation, we present the famous dynamo wave solution, which was supposed to provide a model for the solar cycle. Finally we point out how a flux transport dynamo can circumvent some of the difficulties associated with the older dynamo models.

The Solar Dynamo

NASA Technical Reports Server (NTRS)

Hathaway, David H.

1998-01-01

The solar dynamo is the process by which the Sun's magnetic field is generated through the interaction of the field with convection and rotation. In this, it is kin to planetary dynamos and other stellar dynamos. Although the precise mechanism by which the Sun generates its field remains poorly understood despite decades of theoretical and observational work, recent advances suggest that solutions to this solar dynamo problem may be forthcoming. Two basic processes are involved in dynamo activity. When the fluid stresses dominate the magnetic stresses (high plasma beta = 8(pi)rho/B(sup 2)), shear flows can stretch magnetic field lines in the direction of the shear (the "alpha effect") and helical flows can lift and twist field lines into orthogonal planes (the "alpha effect"). These two processes can be active anywhere in the solar convection zone but with different results depending upon their relative strengths and signs. Little is known about how and where these processes occur. Other processes, such as magnetic diffusion and the effects of the fine scale structure of the solar magnetic field, pose additional problems.

Origin of Magnetar-Scale Crustal Field in PSR J1852+0040 and 'Frozen' Magnetars

NASA Astrophysics Data System (ADS)

Popov, S. B.

2013-08-01

We discuss the origin of strong crustal magnetic field in one of central compact objects (CCOs)-a neutron star PSR J1852+0040 in the supernova remnant Kes 79. Taking into account its relatively long present day spin period we conclude that the field could not be generated via a dynamo mechanism. If this neutron star indeed is a magnetar with field submerged during a strong fall-back episode, then it argues against the dynamo field origin in magnetars. Otherwise, Kes 79 is not a close relative of normal magnetars. A discovery of an anti-magnetar with a millisecond period and strong crustal field identifiable, for example, due to large pulse fraction, would be the proof of the dynamo field origin. Existence of such sources is in correspondence with the present standard picture of neutron star unification. However, the fraction of magnetars with submerged fields can be small-few percent of the total number of CCOs.

IS THE SMALL-SCALE MAGNETIC FIELD CORRELATED WITH THE DYNAMO CYCLE?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Karak, Bidya Binay; Brandenburg, Axel, E-mail: bbkarak@nordita.org

2016-01-01

The small-scale magnetic field is ubiquitous at the solar surface—even at high latitudes. From observations we know that this field is uncorrelated (or perhaps even weakly anticorrelated) with the global sunspot cycle. Our aim is to explore the origin, and particularly the cycle dependence, of such a phenomenon using three-dimensional dynamo simulations. We adopt a simple model of a turbulent dynamo in a shearing box driven by helically forced turbulence. Depending on the dynamo parameters, large-scale (global) and small-scale (local) dynamos can be excited independently in this model. Based on simulations in different parameter regimes, we find that, when onlymore » the large-scale dynamo is operating in the system, the small-scale magnetic field generated through shredding and tangling of the large-scale magnetic field is positively correlated with the global magnetic cycle. However, when both dynamos are operating, the small-scale field is produced from both the small-scale dynamo and the tangling of the large-scale field. In this situation, when the large-scale field is weaker than the equipartition value of the turbulence, the small-scale field is almost uncorrelated with the large-scale magnetic cycle. On the other hand, when the large-scale field is stronger than the equipartition value, we observe an anticorrelation between the small-scale field and the large-scale magnetic cycle. This anticorrelation can be interpreted as a suppression of the small-scale dynamo. Based on our studies we conclude that the observed small-scale magnetic field in the Sun is generated by the combined mechanisms of a small-scale dynamo and tangling of the large-scale field.« less

Persistence and origin of the lunar core dynamo

PubMed Central

Suavet, Clément; Weiss, Benjamin P.; Cassata, William S.; Shuster, David L.; Gattacceca, Jérôme; Chan, Lindsey; Garrick-Bethell, Ian; Head, James W.; Grove, Timothy L.; Fuller, Michael D.

2013-01-01

The lifetime of the ancient lunar core dynamo has implications for its power source and the mechanism of field generation. Here, we report analyses of two 3.56-Gy-old mare basalts demonstrating that they were magnetized in a stable and surprisingly intense dynamo magnetic field of at least ∼13 μT. These data extend the known lifetime of the lunar dynamo by ∼160 My and indicate that the field was likely continuously active until well after the final large basin-forming impact. This likely excludes impact-driven changes in rotation rate as the source of the dynamo at this time in lunar history. Rather, our results require a persistent power source like precession of the lunar mantle or a compositional convection dynamo. PMID:23650386

Persistence and origin of the lunar core dynamo.

PubMed

Suavet, Clément; Weiss, Benjamin P; Cassata, William S; Shuster, David L; Gattacceca, Jérôme; Chan, Lindsey; Garrick-Bethell, Ian; Head, James W; Grove, Timothy L; Fuller, Michael D

2013-05-21

The lifetime of the ancient lunar core dynamo has implications for its power source and the mechanism of field generation. Here, we report analyses of two 3.56-Gy-old mare basalts demonstrating that they were magnetized in a stable and surprisingly intense dynamo magnetic field of at least ~13 μT. These data extend the known lifetime of the lunar dynamo by ~160 My and indicate that the field was likely continuously active until well after the final large basin-forming impact. This likely excludes impact-driven changes in rotation rate as the source of the dynamo at this time in lunar history. Rather, our results require a persistent power source like precession of the lunar mantle or a compositional convection dynamo.

Time-Resolved Records of Magnetic Activity on the Pallasite Parent Body and Psyche

NASA Astrophysics Data System (ADS)

Bryson, J. F. J.; Nichols, C. I. O.; Herrero-Albillos, J.; Kronast, F.; Kasama, T.; Alimadadi, H.; van der Laan, G.; Nimmo, F.; Harrison, R. J.

2014-12-01

Although many small bodies apparently generated dynamo fields in the early solar system, the nature and temporal evolution of these fields has remained enigmatic. Time-resolved records of the Earth's planetary field have been essential in understanding the dynamic history of our planet, and equivalent information from asteroids could provide a unique insight into the development of the solar system. Here we present time-resolved records of magnetic activity on the main-group pallasite parent body and (16) Psyche, obtained using newly-developed nanomagnetic imaging techniques. For the pallasite parent body, the inferred field direction remained relatively constant and the intensity was initially stable at ~100 μT before it decreased in two discrete steps down to 0 μT. We interpret this behaviour as due to vigorous dynamo activity driven by compositional convection in the core, ultimately transitioning from a dipolar to multipolar field as the inner core grew from the bottom-up. For Psyche (measured from IVA iron meteorites), the inferred field direction reversed, while the intensity remained stable at >50 μT. Psyche cooled rapidly as an unmantled core, although the resulting thermal convection alone cannot explain these observations. Instead, this behaviour required top-down core solidification, and is attributed either to compositional convection (if the core also solidified from the bottom-up) or convection generated directly by top-down solidification (e.g. Fe-snow). The mechanism governing convection in small body cores is an open question (due partly to uncertainties in the direction of core solidification), and these observations suggest that unconventional (i.e. not thermal) mechanisms acted in the early solar system. These mechanisms are very efficient at generating convection, implying a long-lasting and widespread epoch of dynamo activity among small bodies in the early solar system.

Facilitating dynamo action via control of large-scale turbulence.

PubMed

Limone, A; Hatch, D R; Forest, C B; Jenko, F

2012-12-01

The magnetohydrodynamic dynamo effect is considered to be the major cause of magnetic field generation in geo- and astrophysical systems. Recent experimental and numerical results show that turbulence constitutes an obstacle to dynamos; yet its role in this context is not totally clear. Via numerical simulations, we identify large-scale turbulent vortices with a detrimental effect on the amplification of the magnetic field in a geometry of experimental interest and propose a strategy for facilitating the dynamo instability by manipulating these detrimental "hidden" dynamics.

A deep dynamo generating Mercury's magnetic field.

PubMed

Christensen, Ulrich R

2006-12-21

Mercury has a global magnetic field of internal origin and it is thought that a dynamo operating in the fluid part of Mercury's large iron core is the most probable cause. However, the low intensity of Mercury's magnetic field--about 1% the strength of the Earth's field--cannot be reconciled with an Earth-like dynamo. With the common assumption that Coriolis and Lorentz forces balance in planetary dynamos, a field thirty times stronger is expected. Here I present a numerical model of a dynamo driven by thermo-compositional convection associated with inner core solidification. The thermal gradient at the core-mantle boundary is subadiabatic, and hence the outer region of the liquid core is stably stratified with the dynamo operating only at depth, where a strong field is generated. Because of the planet's slow rotation the resulting magnetic field is dominated by small-scale components that fluctuate rapidly with time. The dynamo field diffuses through the stable conducting region, where rapidly varying parts are strongly attenuated by the skin effect, while the slowly varying dipole and quadrupole components pass to some degree. The model explains the observed structure and strength of Mercury's surface magnetic field and makes predictions that are testable with space missions both presently flying and planned.

Mars' paleomagnetic field as the result of a single-hemisphere dynamo.

PubMed

Stanley, Sabine; Elkins-Tanton, Linda; Zuber, Maria T; Parmentier, E Marc

2008-09-26

Mars' crustal magnetic field was most likely generated by dynamo action in the planet's early history. Unexplained characteristics of the field include its strength, concentration in the southern hemisphere, and lack of correlation with any surface features except for the hemispheric crustal dichotomy. We used numerical dynamo modeling to demonstrate that the mechanisms proposed to explain crustal dichotomy formation can result in a single-hemisphere dynamo. This dynamo produces strong magnetic fields in only the southern hemisphere. This magnetic field morphology can explain why Mars' crustal magnetic field intensities are substantially stronger in the southern hemisphere without relying on any postdynamo mechanisms.

A THREE-DIMENSIONAL BABCOCK-LEIGHTON SOLAR DYNAMO MODEL

DOE Office of Scientific and Technical Information (OSTI.GOV)

Miesch, Mark S.; Dikpati, Mausumi, E-mail: miesch@ucar.edu

We present a three-dimensional (3D) kinematic solar dynamo model in which poloidal field is generated by the emergence and dispersal of tilted sunspot pairs (more generally bipolar magnetic regions, or BMRs). The axisymmetric component of this model functions similarly to previous 2.5 dimensional (2.5D, axisymmetric) Babcock-Leighton (BL) dynamo models that employ a double-ring prescription for poloidal field generation but we generalize this prescription into a 3D flux emergence algorithm that places BMRs on the surface in response to the dynamo-generated toroidal field. In this way, the model can be regarded as a unification of BL dynamo models (2.5D in radius/latitude)more » and surface flux transport models (2.5D in latitude/longitude) into a more self-consistent framework that builds on the successes of each while capturing the full 3D structure of the evolving magnetic field. The model reproduces some basic features of the solar cycle including an 11 yr periodicity, equatorward migration of toroidal flux in the deep convection zone, and poleward propagation of poloidal flux at the surface. The poleward-propagating surface flux originates as trailing flux in BMRs, migrates poleward in multiple non-axisymmetric streams (made axisymmetric by differential rotation and turbulent diffusion), and eventually reverses the polar field, thus sustaining the dynamo. In this Letter we briefly describe the model, initial results, and future plans.« less

Magnetic Flux Concentrations in Stratified Turbulent Plasma Due to Negative Effective Magnetic Pressure Instability

NASA Astrophysics Data System (ADS)

Jabbari, S.; Brandenburg, A.

2014-12-01

Recent studies have suggested a new mechanism that can be used to explain the formation of magnetic spots or bipolar regions in highly stratified turbulent plasmas. According to this model, a large-scale magnetic field suppresses the turbulent pressure, which leads to a negative contribution of turbulence to the effective magnetic pressure. Direct numerical simulations (DNS) have confirmed that the negative contribution is large enough so that the effective magnetic pressure becomes negative and leads to a large-scale instability, which we refer to as negative effective magnetic pressure Instability (NEMPI). NEMPI was used to explain the formation of active regions and sunspots on the solar surface. One step toward improving this model was to combine dynamo in- stability with NEMPI. The dynamo is known to be responsible for the solar large-scale magnetic field and to play a role in solar activity. In this context, we studied stratified turbulent plasmas in spherical geometry, where the background field was generated by alpha squared dynamo. For NEMPI to be excited, the initial magnetic field should be in a proper range, so we used quenching function for alpha. Using the Pencil Code and mean field simulations (MFS), we showed that in the presence of dynamo-generated magnetic fields, we deal with a coupled system, where both instabilities, dynamo and NEMPI, work together and lead to the formation of magnetic structures (Jabbari et al. 2013). We also studied a similar system in plane geometry in the presence of rotation and confirmed that for slow rotation NEMPI works, but as the Coriolis number increases, the rotation suppresses NEMPI. By increasing the Coriolis number even further, the combination of fast rotation and high stratification excites a dynamo, which leads again to a coupled system of dynamo and NEMPI (Jabbari et al. 2014). Another important finding concerning NEMPI is the case where the instability is excited by a vertical magnetic field (Brandenburg et al. 2013). When the field is vertical, the resulting magnetic flux concentrations lead to the magnetic spots and can be of equipartition field strength. DNS, MFS, and implicit large eddy simulations (ILES) confirm that in a proper parameter regime, vertical imposed fields lead to the formation of circular magnetic spots (Brandenburg et al. 2014).

Waldmeier's Rules in the Solar and Stellar Dynamos

NASA Astrophysics Data System (ADS)

Pipin, Valery; Kosovichev, Alexander

2015-08-01

The Waldmeier's rules [1] establish important empirical relations between the general parameters of magnetic cycles (such as the amplitude, period, growth rate and time profile) on the Sun and solar-type stars [2]. Variations of the magnetic cycle parameters depend on properties of the global dynamo processes operating in the stellar convection zones. We employ nonlinear mean-field axisymmetric dynamo models [3] and calculate of the magnetic cycle parameters, such as the dynamo cycle period, total magnetic and Poynting fluxes for the Sun and solar-type stars with rotational periods from 15 to 30 days. We consider two types of the dynamo models: 1) distributed (D-type) models employing the standard α - effect distributed in the whole convection zone, and 2) Babcock-Leighton (BL-type) models with a non-local α - effect. The dynamo models take into account the principal mechanisms of the nonlinear dynamo generation and saturation, including the magnetic helicity conservation, magnetic buoyancy effects, and the feedback on the angular momentum balance inside the convection zones. Both types of models show that the dynamo generated magnetic flux increases with the increase of the rotation rate. This corresponds to stronger brightness variations. The distributed dynamo model reproduces the observed dependence of the cycle period on the rotation rate for the Sun analogs better than the BL-type model. For the solar-type stars rotating more rapidly than the Sun we find dynamo regimes with multiple periods. Such stars with multiple cycles form a separate branch in the variability-rotation diagram.1. Waldmeier, M., Prognose für das nächste Sonnenfleckenmaximum, 1936, Astron. Nachrichten, 259,262. Soon,W.H., Baliunas,S.L., Zhang,Q.,An interpretation of cycle periods of stellar chromospheric activity, 1993, ApJ, 414,333. Pipin,V.V., Dependence of magnetic cycle parameters on period of rotation in nonlinear solar-type dynamos, 2015, astro-ph: 14125284

The Case Against an Early Lunar Dynamo Powered by Core Convection

NASA Astrophysics Data System (ADS)

Evans, Alexander J.; Tikoo, Sonia M.; Andrews-Hanna, Jeffrey C.

2018-01-01

Paleomagnetic analyses of lunar samples indicate that the Moon had a dynamo-generated magnetic field with 50 μT surface field intensities between 3.85 and 3.56 Ga followed by a period of much lower (≤ 5 μT) intensities that persisted beyond 2.5 Ga. However, we determine herein that there is insufficient energy associated with core convection—the process commonly recognized to generate long-lived magnetic fields in planetary bodies—to sustain a lunar dynamo for the duration and intensities indicated. We find that a lunar surface field of ≤1.9 μT could have persisted until 200 Ma, but the 50 μT paleointensities recorded by lunar samples between 3.85 and 3.56 Ga could not have been sustained by a convective dynamo for more than 28 Myr. Thus, for a continuously operating, convective dynamo to be consistent with the early lunar paleomagnetic record, either an exotic mechanism or unknown energy source must be primarily responsible for the ancient lunar magnetic field.

View forward to aft of dynamo room (compartment A21) showing ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

View forward to aft of dynamo room (compartment A-21) showing port ventilation fan; electrical generator is at left center of photograph. Platform for generator is at bottom center of photograph. Hatch for passing powder up from magazine is located just above the generator base. Frames support armored protective deck. (018) - USS Olympia, Penn's Landing, 211 South Columbus Boulevard, Philadelphia, Philadelphia County, PA

Dynamo action in stratified convection with overshoot

NASA Technical Reports Server (NTRS)

Nordlund, Ake; Brandenburg, Axel; Jennings, Richard L.; Rieutord, Michel; Ruokolainen, Juha; Stein, Robert F.; Tuominen, Ilkka

1992-01-01

Results are presented from direct simulations of turbulent compressible hydromagnetic convection above a stable overshoot layer. Spontaneous dynamo action occurs followed by saturation, with most of the generated magnetic field appearing as coherent flux tubes in the vicinity of strong downdrafts, where both the generation and destruction of magnetic field is most vigorous. Whether or not this field is amplified depends on the sizes of the magnetic Reynolds and magnetic Prandtl numbers. Joule dissipation is balanced mainly by the work done against the magnetic curvature force. It is this curvature force which is also responsible for the saturation of the dynamo.

Numerical simulations of current generation and dynamo excitation in a mechanically forced turbulent flow.

PubMed

Bayliss, R A; Forest, C B; Nornberg, M D; Spence, E J; Terry, P W

2007-02-01

The role of turbulence in current generation and self-excitation of magnetic fields has been studied in the geometry of a mechanically driven, spherical dynamo experiment, using a three-dimensional numerical computation. A simple impeller model drives a flow that can generate a growing magnetic field, depending on the magnetic Reynolds number Rm=micro0sigmaVa and the fluid Reynolds number Re=Vanu of the flow. For Re<420, the flow is laminar and the dynamo transition is governed by a threshold of Rmcrit=100, above which a growing magnetic eigenmode is observed that is primarily a dipole field transverse to the axis of symmetry of the flow. In saturation, the Lorentz force slows the flow such that the magnetic eigenmode becomes marginally stable. For Re>420 and Rm approximately 100 the flow becomes turbulent and the dynamo eigenmode is suppressed. The mechanism of suppression is a combination of a time varying large-scale field and the presence of fluctuation driven currents (such as those predicted by the mean-field theory), which effectively enhance the magnetic diffusivity. For higher Rm, a dynamo reappears; however, the structure of the magnetic field is often different from the laminar dynamo. It is dominated by a dipolar magnetic field aligned with the axis of symmetry of the mean-flow, which is apparently generated by fluctuation-driven currents. The magnitude and structure of the fluctuation-driven currents have been studied by applying a weak, axisymmetric seed magnetic field to laminar and turbulent flows. An Ohm's law analysis of the axisymmetric currents allows the fluctuation-driven currents to be identified. The magnetic fields generated by the fluctuations are significant: a dipole moment aligned with the symmetry axis of the mean-flow is generated similar to those observed in the experiment, and both toroidal and poloidal flux expulsion are observed.

Nonlinear dynamo action in a precessing cylindrical container.

PubMed

Nore, C; Léorat, J; Guermond, J-L; Luddens, F

2011-07-01

It is numerically demonstrated by means of a magnetohydrodynamics code that precession can trigger the dynamo effect in a cylindrical container. When the Reynolds number, based on the radius of the cylinder and its angular velocity, increases, the flow, which is initially centrosymmetric, loses its stability and bifurcates to a quasiperiodic motion. This unsteady and asymmetric flow is shown to be capable of sustaining dynamo action in the linear and nonlinear regimes. The magnetic field thus generated is unsteady and quadrupolar. These numerical evidences of dynamo action in a precessing cylindrical container may be useful for an experiment now planned at the Dresden sodium facility for dynamo and thermohydraulic studies in Germany.

Dynamo room (compartment A21) with view of port side electrical ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

Dynamo room (compartment A-21) with view of port side electrical generator and ventilation ducting. (013) - USS Olympia, Penn's Landing, 211 South Columbus Boulevard, Philadelphia, Philadelphia County, PA

Magnetic dynamo action at low magnetic Prandtl numbers.

PubMed

Malyshkin, Leonid M; Boldyrev, Stanislav

2010-11-19

Amplification of magnetic field due to kinematic turbulent dynamo action is studied in the regime of small magnetic Prandtl numbers. Such a regime is relevant for planets and stars interiors, as well as for liquid-metal laboratory experiments. A comprehensive analysis based on the Kazantsev-Kraichnan model is reported, which establishes the dynamo threshold and the dynamo growth rates for varying kinetic helicity of turbulent fluctuations. It is proposed that in contrast with the case of large magnetic Prandtl numbers, the kinematic dynamo action at small magnetic Prandtl numbers is significantly affected by kinetic helicity, and it can be made quite efficient with an appropriate choice of the helicity spectrum.

Quasi-geostrophic dynamo theory

NASA Astrophysics Data System (ADS)

Calkins, Michael A.

2018-03-01

The asymptotic theory of rapidly rotating, convection-driven dynamos in a plane layer is discussed. A key characteristic of these quasi-geostrophic dynamos is that the Lorentz force is comparable in magnitude to the ageostrophic component of the Coriolis force, rather than the leading order component that yields geostrophy. This characteristic is consistent with both observations of planetary dynamos and numerical dynamo investigations, where the traditional Elssasser number, ΛT = O (1) . Thus, while numerical dynamo simulations currently cannot access the strongly turbulent flows that are thought to be characteristic of planetary interiors, it is argued that they are in the appropriate geostrophically balanced regime provided that inertial and viscous forces are both small relative to the leading order Coriolis force. Four distinct quasi-geostrophic dynamo regimes are discussed, with each regime characterized by a unique magnetic to kinetic energy density ratio and differing dynamics. The axial torque due to the Lorentz force is shown to be asymptotically small for such quasi-geostrophic dynamos, suggesting that 'Taylor's constraint' represents an ambiguous measure of the primary force balance in a rapidly rotating dynamo.

A two-billion-year history for the lunar dynamo.

PubMed

Tikoo, Sonia M; Weiss, Benjamin P; Shuster, David L; Suavet, Clément; Wang, Huapei; Grove, Timothy L

2017-08-01

Magnetic studies of lunar rocks indicate that the Moon generated a core dynamo with surface field intensities of ~20 to 110 μT between at least 4.25 and 3.56 billion years ago (Ga). The field subsequently declined to <~4 μT by 3.19 Ga, but it has been unclear whether the dynamo had terminated by this time or just greatly weakened in intensity. We present analyses that demonstrate that the melt glass matrix of a young regolith breccia was magnetized in a ~5 ± 2 μT dynamo field at ~1 to ~2.5 Ga. These data extend the known lifetime of the lunar dynamo by at least 1 billion years. Such a protracted history requires an extraordinarily long-lived power source like core crystallization or precession. No single dynamo mechanism proposed thus far can explain the strong fields inferred for the period before 3.56 Ga while also allowing the dynamo to persist in such a weakened state beyond ~2.5 Ga. Therefore, our results suggest that the dynamo was powered by at least two distinct mechanisms operating during early and late lunar history.

A two-billion-year history for the lunar dynamo

PubMed Central

Tikoo, Sonia M.; Weiss, Benjamin P.; Shuster, David L.; Suavet, Clément; Wang, Huapei; Grove, Timothy L.

2017-01-01

Magnetic studies of lunar rocks indicate that the Moon generated a core dynamo with surface field intensities of ~20 to 110 μT between at least 4.25 and 3.56 billion years ago (Ga). The field subsequently declined to <~4 μT by 3.19 Ga, but it has been unclear whether the dynamo had terminated by this time or just greatly weakened in intensity. We present analyses that demonstrate that the melt glass matrix of a young regolith breccia was magnetized in a ~5 ± 2 μT dynamo field at ~1 to ~2.5 Ga. These data extend the known lifetime of the lunar dynamo by at least 1 billion years. Such a protracted history requires an extraordinarily long-lived power source like core crystallization or precession. No single dynamo mechanism proposed thus far can explain the strong fields inferred for the period before 3.56 Ga while also allowing the dynamo to persist in such a weakened state beyond ~2.5 Ga. Therefore, our results suggest that the dynamo was powered by at least two distinct mechanisms operating during early and late lunar history. PMID:28808679

Simulations of plasma dynamo in cylindrical and spherical geometries

NASA Astrophysics Data System (ADS)

Khalzov, Ivan; Forest, Cary; Schnack, Dalton; Ebrahimi, Fatima

2010-11-01

We have performed the numerical investigation of plasma flow and possibility of dynamo effect in Madison Plasma Couette Experiment (MPCX) and Madison Plasma Dynamo Experiment (MPDX), which are being installed at the University of Wisconsin- Madison. Using the extended MHD code, NIMROD, we have studied several types of plasma flows appropriate for dynamo excitation. Calculations are done for isothermal compressible plasma model including two-fluid effects (Hall term), which is beyond the standard incompressible MHD picture. It is found that for magnetic Reynolds numbers exceeding the critical one the counter-rotating Von Karman flow (in cylinder) and Dudley- James flow (in sphere) result in self-generation of magnetic field. Depending on geometry and plasma parameters this field can either saturate at certain amplitude corresponding to a new stable equilibrium (laminar dynamo) or lead to turbulent dynamo. It is shown that plasma compressibility results in increase of the critical magnetic Reynolds number while two- fluid effects change the level of saturated dynamo field. The work is supported by NSF.

Multiple periodicities in the solar magnetic field - Possible origin in a multiple-mode solar dynamo

NASA Technical Reports Server (NTRS)

Boyer, D. W.; Levy, E. H.

1992-01-01

The solar magnetic field is generated in an oscillatory mode with a 22 yr full period and gives rise to the 11 yr sunspot cycle. However, analyses of contemporary solar records, as well as other surrogate indicators of solar activity, suggest the presence also of longer term periodicities in the solar magnetic cycle. This paper suggests that the solar dynamo can operate in a multiply periodic state, with several periodicites being generated simultaneously at different depths in the convection zone. A simple two-layer model of the solar convection zone is used to illustrate the physical mechanism of spatially localized, multiple-periodicity-mode dynamo regeneration. The two layers are characterized by differences in their respective turbulent magnetic diffusivities. Although the magnetic modes interact with one another, each mode is produced large in one layer or the other, and has an oscillation period approximately equal to the time characteristic of magnetic diffusion across the layer. The observed complicated periodicity pattern in the solar magnetic field could be a combination of two (or more) dynamo modes generated in this manner. The calculations are carried out using a differential rotation model consistent with recent helioseismological measurements, illustrating the challenge to dynamo theory raised by those observational results.

Integral equation approach to time-dependent kinematic dynamos in finite domains

NASA Astrophysics Data System (ADS)

Xu, Mingtian; Stefani, Frank; Gerbeth, Gunter

2004-11-01

The homogeneous dynamo effect is at the root of cosmic magnetic field generation. With only a very few exceptions, the numerical treatment of homogeneous dynamos is carried out in the framework of the differential equation approach. The present paper tries to facilitate the use of integral equations in dynamo research. Apart from the pedagogical value to illustrate dynamo action within the well-known picture of the Biot-Savart law, the integral equation approach has a number of practical advantages. The first advantage is its proven numerical robustness and stability. The second and perhaps most important advantage is its applicability to dynamos in arbitrary geometries. The third advantage is its intimate connection to inverse problems relevant not only for dynamos but also for technical applications of magnetohydrodynamics. The paper provides the first general formulation and application of the integral equation approach to time-dependent kinematic dynamos, with stationary dynamo sources, in finite domains. The time dependence is restricted to the magnetic field, whereas the velocity or corresponding mean-field sources of dynamo action are supposed to be stationary. For the spherically symmetric α2 dynamo model it is shown how the general formulation is reduced to a coupled system of two radial integral equations for the defining scalars of the poloidal and toroidal field components. The integral equation formulation for spherical dynamos with general stationary velocity fields is also derived. Two numerical examples—the α2 dynamo model with radially varying α and the Bullard-Gellman model—illustrate the equivalence of the approach with the usual differential equation method. The main advantage of the method is exemplified by the treatment of an α2 dynamo in rectangular domains.

THE COMBINED EFFECT OF PRECESSION AND CONVECTION ON THE DYNAMO ACTION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wei, Xing, E-mail: xing.wei@sjtu.edu.cn; Princeton University Observatory, Princeton, NJ 08544

2016-08-20

To understand the generation of the Earth’s magnetic field and those of other planets, we numerically investigate the combined effect of precession and convection on dynamo action in a spherical shell. Convection alone, precession alone, and the combined effect of convection and precession are studied at the low Ekman number at which the precessing flow is already unstable. The key result is that although precession or convection alone are not strong enough to support the dynamo action, the combined effect of precession and convection can support the dynamo action because of the resonance of precessional and convective instabilities. This resultmore » may explain why the geodynamo has been maintained for such a long time compared to the Martian dynamo.« less

Nonlinear restrictions on dynamo action. [in magnetic fields of astrophysical objects

NASA Technical Reports Server (NTRS)

Vainshtein, Samuel I.; Cattaneo, Fausto

1992-01-01

Astrophysical dynamos operate in the limit of small magnetic diffusivity. In order for magnetic reconnection to occur, very small magnetic structures must form so that diffusion becomes effective. The formation of small-scale fields is accompanied by the stretching of the field lines and therefore by an amplification of the magnetic field strength. The back reaction of the magnetic field on the motions leads to the eventual saturation of the dynamo process, thus posing a constraint on the amount of magnetic flux that can be generated by dynamo action, It is argued that in the limit of small diffusivity only a small amount of flux, many orders of magnitude less than the observed fluxes, can be created by dynamo processes.

Instrumental Implementation of an Experiment to Demonstrate αω -dynamos in Accretion Disks

NASA Astrophysics Data System (ADS)

Si, Jiahe; Sonnenfeld, Richard; Colgate, Art; Li, Hui; Nornberg, Mark

2016-10-01

The New Mexico Liquid Metal αω -dynamo experiment is aimed to demonstrate a galactic dynamo. Our goal is to generate the ω-effect and α-effect by two semi-coherent flows in laboratory. Two coaxial cylinders are used to generate Taylor-Couette flows to simulate the differential rotation of accretion disks. Plumes induced by jets injected into the Couette flows are expected to produce helicities necessary for the α-effect. We have demonstrated an 8-fold poloidal-to-toroidal flux amplification from differential rotation (the ω-effect) by minimizing turbulence in our apparatus. To demonstrate the α-effect, the experimental apparatus is undergoing significant upgrade. We have constructed a helicity injection facility, and are also designing and testing a new data acquisition system capable of transmitting data in a high speed rotating frame. Additional magnetic field diagnostics will also be included. The upgrade is intended to answer the question of whether a self-sustaining αω -dynamo can be constructed with a realistic fluid flow field, as well as to obtain more details to understand dynamo action in highly turbulent Couette flow.

Generation of magnetic fields by chaotic fluid convection - The fast dynamo problem

NASA Technical Reports Server (NTRS)

Finn, John M.

1992-01-01

In the kinematic fast dynamo problem, the underlying nonlinear dynamics of the flow play a critical role in the behavior of a dynamo field. It is presently noted that the two important facets of the problem are the approximately lognormal distribution of vector lengths, and the presence of partial cancellation. It is suggested that these features may be reflected in the magnetic fields observed on the sun.

Faraday's first dynamo: A retrospective

NASA Astrophysics Data System (ADS)

Smith, Glenn S.

2013-12-01

In the early 1830s, Michael Faraday performed his seminal experimental research on electromagnetic induction, in which he created the first electric dynamo—a machine for continuously converting rotational mechanical energy into electrical energy. His machine was a conducting disc, rotating between the poles of a permanent magnet, with the voltage/current obtained from brushes contacting the disc. In his first dynamo, the magnetic field was asymmetric with respect to the axis of the disc. This is to be contrasted with some of his later symmetric designs, which are the ones almost invariably discussed in textbooks on electromagnetism. In this paper, a theoretical analysis is developed for Faraday's first dynamo. From this analysis, the eddy currents in the disc and the open-circuit voltage for arbitrary positioning of the brushes are determined. The approximate analysis is verified by comparing theoretical results with measurements made on an experimental recreation of the dynamo. Quantitative results from the analysis are used to elucidate Faraday's qualitative observations, from which he learned so much about electromagnetic induction. For the asymmetric design, the eddy currents in the disc dissipate energy that makes the dynamo inefficient, prohibiting its use as a practical generator of electric power. Faraday's experiments with his first dynamo provided valuable insight into electromagnetic induction, and this insight was quickly used by others to design practical generators.

Convection and Dynamo Action in Ice Giant Dynamo Models with Electrical Conductivity Stratification

NASA Astrophysics Data System (ADS)

Soderlund, K. M.; Featherstone, N. A.; Heimpel, M. H.; Aurnou, J. M.

2017-12-01

Uranus and Neptune are relatively unexplored, yet critical for understanding the physical and chemical processes that control the behavior and evolution of giant planets. Because their multipolar magnetic fields, three-jet zonal winds, and extreme energy balances are distinct from other planets in our Solar System, the ice giants provide a unique opportunity to test hypotheses for internal dynamics and magnetic field generation. While it is generally agreed that dynamo action in the ionic ocean generates their magnetic fields, the mechanisms that control the morphology, strength, and evolution of the dynamos - which are likely distinct from those in the gas giants and terrestrial planets - are not well understood. We hypothesize that the dynamos and zonal winds are dynamically coupled and argue that their characteristics are a consequence of quasi-three-dimensional turbulence in their interiors. Here, we will present new dynamo simulations with an inner electrically conducting region and outer electrically insulating layer to self-consistently couple the ionic oceans and molecular envelopes of these planets. For each simulation, the magnetic field morphology and amplitude, zonal flow profile, and internal heat flux pattern will be compared against corresponding observations of Uranus and Neptune. We will also highlight how these simulations will both contribute to and benefit from a future ice giant mission.

Solar Nebula Magnetohydrodynamic Dynamos: Kinematic Theory, Dynamical Constraints, and Magnetic Transport of Angular Momentum

NASA Technical Reports Server (NTRS)

Stepinski, Tomasz F.; Reyes-Ruiz, Mauricio; Vanhala, Harri A. T.

1993-01-01

A hydromagnetic dynamo provides the best mechanism for contemporaneously producing magnetic fields in a turbulent solar nebula. We investigate the solar nebula in the framework of a steady-state accretion disk model and establish the criteria for a viable nebular dynamo. We have found that typically a magnetic gap exists in the nebula, the region where the degree of ionization is too small for the magnetic field to couple to the gas. The location and width of this gap depend on the particular model; the supposition is that gaps cover different parts of the nebula at different evolutionary stages. We have found, from several dynamical constraints, that the generated magnetic field is likely to saturate at a strength equal to equipartition with the kinetic energy of turbulence. Maxwell stress arising from a large-scale magnetic field may significantly influence nebular structure, and Maxwell stress due to small-scale fields can actually dominate other stresses in the inner parts of the nebula. We also argue that the bulk of nebular gas, within the scale height from the midplane, is stable against Balbus-Hawley instability.

Relativistic Dynamos in Magnetospheres of Rotating Compact Objects

NASA Astrophysics Data System (ADS)

Tomimatsu, Akira

2000-01-01

The kinematic evolution of axisymmetric magnetic fields in rotating magnetospheres of relativistic compact objects is analytically studied, based on relativistic Ohm's law in stationary axisymmetric geometry. By neglecting the poloidal flows of plasma in simplified magnetospheric models, we discuss a self-excited dynamo due to the frame-dragging effect (originally pointed out by Khanna & Camenzind) and propose alternative processes to generate axisymmetric magnetic fields against ohmic dissipation. The first process (which may be called ``induced excitation'') is caused by the help of a background uniform magnetic field in addition to the dragging of inertial frames. It is shown that excited multipolar components of poloidal and azimuthal fields are sustained as stationary modes, and outgoing Poynting flux converges toward the rotation axis. The second process is a self-excited dynamo through azimuthal convection current, which is found to be effective if plasma rotation becomes highly relativistic with a sharp gradient in the angular velocity. In this case, no frame-dragging effect is needed, and the coupling between charge separation and plasma rotation becomes important. We discuss briefly the results in relation to active phenomena in the relativistic magnetospheres.

The kinematic dynamo problem, part I: analytical treatment with the Bullard-Gellman formalism

NASA Astrophysics Data System (ADS)

Glane, Sebastian; Reich, Felix A.; Müller, Wolfgang H.

2018-03-01

This paper is dedicated to the description of kinematic dynamo action in a sphere and its analytical treatment with the uc(Bullard)-uc(Gellman) formalism. One goal of dynamo theory is to answer the question: Can magnetic fields of stellar objects be generated or sustained due to (fluid) motion in the interior? uc(Bullard) and uc(Gellman) were among the first to study this question, leading the way for many subsequent studies, cf. Bullard (Philos Trans R Soc A 247(928):213-278, 1954). In their publication the differential equations resulting from a toroidal-poloidal decomposition of the velocity and magnetic field are stated without an in-depth discussion of the employed methods and computation steps. This study derives the necessary formalism in a compact and concise manner by using an operator-based approach. The focus lies on the mathematical steps and necessary properties of the considered formalism. Prior to that a derivation of the induction equation is presented based on rational continuum electrodynamics. As an example of the formalism the decay of two magnetic fields is analyzed.

Confinement of the solar tachocline by a cyclic dynamo magnetic field

NASA Astrophysics Data System (ADS)

Barnabé, Roxane; Strugarek, Antoine; Charbonneau, Paul; Brun, Allan Sacha; Zahn, Jean-Paul

2017-05-01

Context. The surprising thinness of the solar tachocline is still not understood with certainty today. Among the numerous possible scenarios suggested to explain its radial confinement, one hypothesis is based on Maxwell stresses that are exerted by the cyclic dynamo magnetic field of the Sun penetrating over a skin depth below the turbulent convection zone. Aims: Our goal is to assess under which conditions (turbulence level in the tachocline, strength of the dynamo-generated field, spreading mechanism) this scenario can be realized in the solar tachocline. Methods: We develop a simplified 1D model of the upper tachocline under the influence of an oscillating magnetic field imposed from above. The turbulent transport is parametrized with enhanced turbulent diffusion (or anti-diffusion) coefficients. Two main processes that thicken the tachocline are considered; either turbulent viscous spreading or radiative spreading. An extensive parameter study is carried out to establish the physical parameter regimes under which magnetic confinement of the tachocline that is due to a surface dynamo field can be realized. Results: We have explored a large range of magnetic field amplitudes, viscosities, ohmic diffusivities and thermal diffusivities. We find that, for large but still realistic magnetic field strengths, the differential rotation can be suppressed in the upper radiative zone (and hence the tachocline confined) if weak turbulence is present (with an enhanced ohmic diffusivity of η> 107-8 cm2/ s), even in the presence of radiative spreading. Conclusions: Our results show that a dynamo magnetic field can, in the presence of weak turbulence, prevent the inward burrowing of a tachocline subject to viscous diffusion or radiative spreading.

Chinks in Solar Dynamo Theory: Turbulent Diffusion, Dynamo Waves and Magnetic Helicity

NASA Technical Reports Server (NTRS)

DeLuca, E. E.; Wagner, William J. (Technical Monitor)

2001-01-01

We have investigated the generation of magnetic fields in the Sun using two-dimensional and three-dimensional numerical simulations. The results of our investigations have been presented at scientific meetings and published.

The precession dynamo experiment at HZDR

NASA Astrophysics Data System (ADS)

Giesecke, A.; Albrecht, T.; Gerbeth, G.; Gundrum, T.; Nore, C.; Stefani, F.; Steglich, C.

2013-12-01

Most planets of the solar system are accompanied by a magnetic field with a large scale structure. These fields are generated by the dynamo effect, the process that provides for the transfer of kinetic energy from a flow of a conducting fluid into magnetic energy. In case of planetary dynamos it is generally assumed that these flows are driven by thermal and/or chemical convection but other driving sources like libration, tidal forcing or precession are possible as well. Precessional forcing, in particular, has been discussed since long as an at least additional power source for the geodynamo. A fluid flow of liquid sodium, solely driven by precession, will be the source for magnetic field generation in the next generation dynamo experiment currently under development at the Helmholz-Zentrum Dresden-Rossendorf (HZDR). In contrast to previous dynamo experiments no internal blades, propellers or complex systems of guiding tubes will be used for the optimization of the flow properties. However, in order to reach sufficiently high magnetic Reynolds numbers required for the onset of dynamo action rather large dimensions of the container are necessary making the construction of the experiment a challenge. At present state a small scale water experiment is running in order to estimate the hydrodynamic flow properties in dependence of precession angle and precession rate. The measurements are utilized in combination with numerical simulations of the hydrodynamic case as input data for kinematic simulations of the induction equation. The resulting growth rates and the corresponding critical magnetic Reynolds numbers will provide a restriction of the useful parameter regime and will allow an optimization of the experimental configuration.

Oscillating dynamo in the presence of a fossil magnetic field - The solar cycle

NASA Technical Reports Server (NTRS)

Levy, E. H.; Boyer, D.

1982-01-01

Hydromagnetic dynamo generation of oscillating magnetic fields in the presence of an external, ambient magnetic field introduces a marked polarity asymmetry between the two halves of the magnetic cycle. The principle of oscillating dynamo interaction with external fields is developed, and a tentative application to the sun is described. In the sun a dipole moment associated with the stable fluid beneath the convection zone would produce an asymmetrical solar cycle.

Ambipolar diffusion drifts and dynamos in turbulent gases

NASA Technical Reports Server (NTRS)

Zweibel, Ellen G.

1988-01-01

Ambipolar drift in turbulent fluids are considered. Using mean-field electrodynamics, a two-scale theory originally used to study hydromagnetic dynamos, it is shown that magnetic fields can be advected by small-scale magnetosonic (compressional) turbulence or generated by Alfvenic (helical) turbulence. A simple dynamo theory is made and is compared with standard theories in which dissipation is caused by turbulent diffusion. The redistribution of magnetic flux in interstellar clouds is also discussed.

Multiscale Analysis of Rapidly Rotating Dynamo Simulations

NASA Astrophysics Data System (ADS)

Orvedahl, R.; Calkins, M. A.; Featherstone, N. A.

2017-12-01

The magnetic field of the planets and stars are generated by dynamo action in their electrically conducting fluid interiors. Numerical models of this process solve the fundamental equations of magnetohydrodynamics driven by convection in a rotating spherical shell. Rotation plays an important role in modifying the resulting convective flows and the self-generated magnetic field. We present results of simulating rapidly rotating systems that are unstable to dynamo action. We use the pseudo-spectral code Rayleigh to generate a suite of direct numerical simulations. Each simulation uses the Boussinesq approximation and is characterized by an Ekman number (Ek=ν /Ω L2) of 10-5. We vary the degree of convective forcing to obtain a range of convective Rossby numbers. The resulting flows and magnetic structures are analyzed using a Reynolds decomposition. We determine the relative importance of each term in the scale-separated governing equations and estimate the relevant spatial scales responsible for generating the mean magnetic field.

Generation of field-aligned current (FAC) and convection through the formation of pressure regimes: Correction for the concept of Dungey's convection

NASA Astrophysics Data System (ADS)

Tanaka, T.; Watanabe, M.; Den, M.; Fujita, S.; Ebihara, Y.; Kikuchi, T.; Hashimoto, K. K.; Kataoka, R.

2016-09-01

In this paper, we try to elucidate the generation mechanism of the field-aligned current (FAC) and coexisting convection. From the comparison between the theoretical prediction and the state of numerical solution from the high-resolution global simulation, we obtain the following conclusions about the distribution of dynamo, the magnetic field structure along the flow path that diverges Poynting flux, and energy conversion promoting the generation of electromagnetic energy. The dynamo for the region 1 FAC, which is in the high-latitude-side cusp-mantle region, has a structure in which magnetic field is compressed along the convection path by the slow mode motion. The dynamo for the region 2 FAC is in the ring current region at the inner edge of the plasma sheet, and has a structure in which magnetic field is curved outward along the convection path. Under these structures, electromagnetic energy is generated from the work done by pressure gradient force, in both dynamos for the region 1 and region 2 FACs. In these generation processes of the FACs, the excitation of convection and the formation of pressure regimes occur as interdependent processes. This structure leads to a modification in the way of understanding the Dungey's convection. Generation of the FAC through the formation of pressure regimes is essential even for the case of substorm onset.

Dynamo action with wave motion.

PubMed

Tilgner, A

2008-03-28

It is shown that time dependent velocity fields in a fluid conductor can act as dynamos even when the same velocity fields frozen in at any particular time cannot. This effect is observed in propagating waves in which the time dependence is simply a steady drift of a fixed velocity pattern. The effect contributes to magnetic field generation in numerical models of planetary dynamos and relies on the property that eigenmodes of the induction equation are not all orthogonal to each other.

Nonlinear generation of large-scale magnetic fields in forced spherical shell dynamos

DOE Office of Scientific and Technical Information (OSTI.GOV)

Livermore, P. W.; Hughes, D. W.; Tobias, S. M.

2010-03-15

In an earlier paper [P. W. Livermore, D. W. Hughes, and S. M. Tobias, ''The role of helicity and stretching in forced kinematic dynamos in a spherical shell'', Phys. Fluids 19, 057101 (2007)], we considered the kinematic dynamo action resulting from a forced helical flow in a spherical shell. Although mean field electrodynamics suggests that the resulting magnetic field should have a significant mean (axisymmetric) component, we found no evidence for this; the dynamo action was distinctly small scale. Here we extend our investigation into the nonlinear regime in which the magnetic field reacts back on the velocity via themore » Lorentz force. Our main result is somewhat surprising, namely, that nonlinear effects lead to a considerable change in the structure of the magnetic field, its final state having a significant mean component. By investigating the dominant flow-field interactions, we isolate the dynamo mechanism and show schematically how the generation process differs between the kinematic and nonlinear regimes. In addition, we are able to calculate some components of the transport coefficient {alpha} and thus discuss our results within the context of mean field electrodynamics.« less

Laminar and Turbulent Dynamos in Chiral Magnetohydrodynamics. I. Theory

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rogachevskii, Igor; Kleeorin, Nathan; Ruchayskiy, Oleg

2017-09-10

The magnetohydrodynamic (MHD) description of plasmas with relativistic particles necessarily includes an additional new field, the chiral chemical potential associated with the axial charge (i.e., the number difference between right- and left-handed relativistic fermions). This chiral chemical potential gives rise to a contribution to the electric current density of the plasma ( chiral magnetic effect ). We present a self-consistent treatment of the chiral MHD equations , which include the back-reaction of the magnetic field on a chiral chemical potential and its interaction with the plasma velocity field. A number of novel phenomena are exhibited. First, we show that themore » chiral magnetic effect decreases the frequency of the Alfvén wave for incompressible flows, increases the frequencies of the Alfvén wave and of the fast magnetosonic wave for compressible flows, and decreases the frequency of the slow magnetosonic wave. Second, we show that, in addition to the well-known laminar chiral dynamo effect, which is not related to fluid motions, there is a dynamo caused by the joint action of velocity shear and chiral magnetic effect. In the presence of turbulence with vanishing mean kinetic helicity, the derived mean-field chiral MHD equations describe turbulent large-scale dynamos caused by the chiral alpha effect, which is dominant for large fluid and magnetic Reynolds numbers. The chiral alpha effect is due to an interaction of the chiral magnetic effect and fluctuations of the small-scale current produced by tangling magnetic fluctuations (which are generated by tangling of the large-scale magnetic field by sheared velocity fluctuations). These dynamo effects may have interesting consequences in the dynamics of the early universe, neutron stars, and the quark–gluon plasma.« less

Paleomagnetic evidence for dynamo activity driven by inward crystallisation of a metallic asteroid

NASA Astrophysics Data System (ADS)

Bryson, James F. J.; Weiss, Benjamin P.; Harrison, Richard J.; Herrero-Albillos, Julia; Kronast, Florian

2017-08-01

The direction in which a planetary core solidifies has fundamental implications for the feasibility and nature of dynamo generation. Although Earth's core is outwardly solidifying, the cores of certain smaller planetary bodies have been proposed to inwardly solidify due to their lower central pressures. However, there have been no unambiguous observations of inwardly solidified cores or the relationship between this solidification regime and planetary magnetic activity. To address this gap, we present the results of complimentary paleomagnetic techniques applied to the matrix metal and silicate inclusions within the IVA iron meteorites. This family of meteorites has been suggested to originate from a planetary core that had its overlaying silicate mantle removed by collisions during the early solar system. This process is thought to have produced a molten ball of metal that cooled rapidly and has been proposed to have inwardly solidified. Recent thermal evolution models of such a body predict that it should have generated an intense, multipolar and time-varying dynamo field. This field could have been recorded as a remanent magnetisation in the outer, cool layers of a solid crust on the IVA parent core. We find that the different components in the IVA iron meteorites display a range of paleomagnetic fidelities, depending crucially on the cooling rate of the meteorite. In particular, silicate inclusions in the quickly cooled São João Nepomuceno meteorite are poor paleomagnetic recorders. On the other hand, the matrix metal and some silicate subsamples from the relatively slowly cooled Steinbach meteorite are far better paleomagnetic recorders and provide evidence of an intense (≳100 μT) and directionally varying (exhibiting significant changes on a timescale ≲200 kyr) magnetic field. This is the first demonstration that some iron meteorites record ancient planetary magnetic fields. Furthermore, the observed field intensity, temporal variability and dynamo lifetime are consistent with thermal evolution models of the IVA parent core. Because the acquisition of remanent magnetisation by some IVA iron meteorites require that they cooled below their Curie temperature during the period of dynamo activity, the magnetisation carried by Steinbach also provides strong evidence favouring the inward solidification of its parent core.

The solar magnetic field: from complexity to simplicity (and back)

NASA Astrophysics Data System (ADS)

Schüssler, Manfred

2017-06-01

The Sun is the only astrophysical object that permits a detailed study of the basic processes governing its magnetic field. Observations reveal stunning complexity due to the interaction with turbulent convection. Numerical simulations and observations strongly suggest that most of the small-scale field is generated by a process called small-scale dynamo action. The fundamental nature of this process makes it a candidate for magnetic field generation in a broad variety of astrophysical settings.On the other hand, the global nature of the 11-year cycle (as exhibited, for instance, by the polarity laws of sunspot groups and the regularly reversing axial dipole field) reveals a surprising simplicity. This suggests a description of the global dynamo process underlying the solar cycle in terms of relatively simple concepts. Insufficient knowledge about the structure of magnetic field and flows in the convection zone requires the introduction of a variety of free parameters (or even free functions), which severely impairs the explanatory power of most such models. However, during the last decades, surface observations of plasma flows and magnetic flux emergence, together with studies of magnetic flux transport, provided crucial information aboutthe workings of the dynamo process. They confirm the visionary approach proposed already in the 1960s by Babcock and Leighton. A recent update of their model permits a full study of the space spanned by the few remaining parameters in order to identify the regions with solar-like solutions.Observations of other cool stars show that the magnetic activity level decreases strongly with stellar rotation rate. The relatively slow rotation of the Sun puts it near to the threshold at which global dynamo action ceases. This suggests a further simplification of the dynamo model in terms of a generic normal form for a weakly nonlinear system. Including the inherent randomness brought about by the flux emergence process leads to a stochastic model whose parameters are fixed by observations. The model results explain the variability of the solar cycle amplitudes from decadal to millennial time scales.

Nonlinear quenching of current fluctuations in a self-exciting homopolar dynamo

NASA Astrophysics Data System (ADS)

Hide, R.

In the interpretation of geomagnetic polarity reversals with their highly variable frequency over geological time it is necessary, as with other irregularly fluctuating geophysical phenomena, to consider the relative importance of forced contributions associated with changing boundary conditions and of free contributions characteristic of the behaviour of nonlinear systems operating under fixed boundary conditions. New evidence -albeit indirect- in favour of the likely predominance of forced contributions is provided by the discovery reported here of the possibility of complete quenching by nonlineax effects of current fluctuations in a self-exciting homopolar dynamo with its single Faraday disk driven into rotation with angular speed y(τ) (where τ denotes time) by a steady applied couple. The armature of an electric motor connected in series with the coil of the dynamo is driven into rotation' with angular speed z(τ) by a torque xf (x) due to Lorentz forces associated with the electric current x(τ) in the system (just as certain parts of the spectrum of eddies within the liquid outer core are generated largely by Lorentz forces associated with currents generated by the self-exciting magnetohydrodynamic (MHD) geodynamo). The discovery is based on bifurcation analysis supported by computational studies of the following (mathematically novel) autonomous set of nonlinear ordinary differential equations: dx/dt = x(y - 1) - βzf(x), dy/dt = α(1 - x²) - κy, dz/dt = xf (x) -λz, where f (x) = 1 - ɛ + ɛσx, in cases when the dimensionless parameters (α, β, κ, λ, σ) are all positive and 0 ≤ ɛ ≤ 1. Within those regions of (α, β, κ, λ, σ) parameter space where the applied couple, as measured by α, is strong enough for persistent dynamo action (i.e. x ≠ 0) to occur at all, there are in general extensive regions where x(τ) exhibits large amplitude regular or irregular (chaotic) fluctuations. But these fluctuating régimes shrink in size as increases from zero, and they disappear altogether when ɛ = 1, leaving only steady régimes of dynamo action.

An MHD Dynamo Experiment.

NASA Astrophysics Data System (ADS)

O'Connell, R.; Forest, C. B.; Plard, F.; Kendrick, R.; Lovell, T.; Thomas, M.; Bonazza, R.; Jensen, T.; Politzer, P.; Gerritsen, W.; McDowell, M.

1997-11-01

A MHD experiment is being constructed which will have the possibility of showing dynamo action: the self--generation of currents from fluid motion. The design allows sufficient experimental flexibility and diagnostic access to study a variety of issues central to dynamo theory, including mean--field electrodynamics and saturation (backreaction physics). Initially, helical flows required for dynamo action will be driven by propellers embedded in liquid sodium. The flow fields will first be measured using laser doppler velocimetry in a water experiment with an identical fluid Reynolds number. The magnetic field evolution will then be predicted using a MHD code, replacing the water with sodium; if growing magnetic fields are found, the experiment will be repeated with sodium.

Self-excitation in a helical liquid metal flow: the Riga dynamo experiments

NASA Astrophysics Data System (ADS)

Gailitis, A.; Gerbeth, G.; Gundrum, Th.; Lielausis, O.; Lipsbergs, G.; Platacis, E.

2018-06-01

The homogeneous dynamo effect is at the root of magnetic field generation in cosmic bodies, including planets, stars and galaxies. While the underlying theory had increasingly flourished since the middle of the 20th century, hydromagnetic dynamos were not realized in the laboratory until 1999. On 11 November 1999, this situation changed with the first observation of a kinematic dynamo in the Riga experiment. Since that time, a series of experimental campaigns has provided a wealth of data on the kinematic and the saturated regime. This paper is intended to give a comprehensive survey about these experiments, to summarize their main results and to compare them with numerical simulations.

Electrically driving large magnetic Reynolds number flows on the Madison plasma dynamo experiment

NASA Astrophysics Data System (ADS)

Weisberg, David; Wallace, John; Peterson, Ethan; Endrezzi, Douglass; Forest, Cary B.; Desangles, Victor

2015-11-01

Electrically-driven plasma flows, predicted to excite a large-scale dynamo instability, have been generated in the Madison plasma dynamo experiment (MPDX), at the Wisconsin Plasma Astrophysics Laboratory. Numerical simulations show that certain topologies of these simply-connected flows may be optimal for creating a plasma dynamo and predict critical thresholds as low as Rmcrit =μ0 σLV = 250 . MPDX plasmas are shown to exceed this critical Rm , generating large (L = 1 . 4 m), warm (Te > 10 eV), unmagnetized (MA > 1) plasmas where Rm < 600 . Plasma flow is driven using ten thermally emissive LaB6 cathodes which generate a J × B torque in Helium plasmas. Detailed Mach probe measurements of plasma velocity for two flow topologies will be presented: edge-localized drive using the multi-cusp boundary field, and volumetric drive using an axial Helmholtz field. Radial velocity profiles show that edge-driven flow is established via ion viscosity but is limited by a volumetric neutral drag force (χ ~ 1 / (ντin)), and measurements of velocity shear compare favorably to Braginskii transport theory. Volumetric flow drive is shown to produce stronger velocity shear, and is characterized by the radial potential gradient as determined by global charge balance.

Implications of Using the GAD Hypothesis in Paleopole Studies for the Moon

NASA Astrophysics Data System (ADS)

Powell, J.; Stanley, S.

2017-12-01

The Moon does not currently have a dynamo-generated magnetic field, however, observations of crustal magnetism and paleomagnetic analyses of Apollo samples have demonstrated that the Moon did possess a dynamo-generated field in the past. Several studies have attempted to use magnetic paleopole analyses to determine the previous rotation poles of the Moon and thereby infer lunar true polar wander. However, these studies all assumed that the Geocentric Axial Dipole (GAD) hypothesis is valid for the Moon. In this study we perform a paleopole analysis of dynamo simulations relevant to the ancient Moon to show the biases inherent in assuming the GAD hypothesis for the Moon. The results of this research have implications for studies of lunar true polar wander.

Flow and dynamo measurements during the coaxial helicity injection on HIST

NASA Astrophysics Data System (ADS)

Ando, K.; Higashi, T.; Nakatsuka, M.; Kikuchi, Y.; Fukumoto, N.; Nagata, M.

2009-11-01

The current drive by Coaxial Helicity Injection (CHI-CD) was performed on HIST in a wide range of configurations from high-q ST to low-q ST and spheromak generated by the utilization of the toroidal field. It is a key issue to investigate the dynamo mechanism required to maintain each configuration. To identify the detail mechanisms, it is needed to manifest a role of plasma flows in the CHI-CD. For this purpose, we have measured the ion flow and the dynamo electric field using an ion Doppler spectrometer (IDS) system, a Mach probe and a dynamo probe. The new dynamo probe consists of 3-axis Mach probes and magnetic pick-up coils. The flow measurements have shown that the intermittent generation of the flow is correlated to the fluctuation seen on the electron density and current signals during the driven phase. At this time, the toroidal direction of the ion flow in the central open flux column is opposite to that of the toroidal current there, i.e. the same direction as electrons. After the plasma enters to the resistive decay phase, the toroidal flow tends to reverse to the same direction as the toroidal current. The results are consistent with the model of the repetitive plasmoid ejection and coalescence proposed for CHI-CD. The plasma jet emanating from the gun source and magnetic field generations through reconnection during the driven phase is well reflected in the 3D MHD simulation.

Solar-type dynamo behaviour in fully convective stars without a tachocline.

PubMed

Wright, Nicholas J; Drake, Jeremy J

2016-07-28

In solar-type stars (with radiative cores and convective envelopes like our Sun), the magnetic field powers star spots, flares and other solar phenomena, as well as chromospheric and coronal emission at ultraviolet to X-ray wavelengths. The dynamo responsible for generating the field depends on the shearing of internal magnetic fields by differential rotation. The shearing has long been thought to take place in a boundary layer known as the tachocline between the radiative core and the convective envelope. Fully convective stars do not have a tachocline and their dynamo mechanism is expected to be very different, although its exact form and physical dependencies are not known. Here we report observations of four fully convective stars whose X-ray emission correlates with their rotation periods in the same way as in solar-type stars. As the X-ray activity-rotation relationship is a well-established proxy for the behaviour of the magnetic dynamo, these results imply that fully convective stars also operate a solar-type dynamo. The lack of a tachocline in fully convective stars therefore suggests that this is not a critical ingredient in the solar dynamo and supports models in which the dynamo originates throughout the convection zone.

Global Simulations of Dynamo and Magnetorotational Instability in Madison Plasma Experiments and Astrophysical Disks

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ebrahimi, Fatima

2014-07-31

Large-scale magnetic fields have been observed in widely different types of astrophysical objects. These magnetic fields are believed to be caused by the so-called dynamo effect. Could a large-scale magnetic field grow out of turbulence (i.e. the alpha dynamo effect)? How could the topological properties and the complexity of magnetic field as a global quantity, the so called magnetic helicity, be important in the dynamo effect? In addition to understanding the dynamo mechanism in astrophysical accretion disks, anomalous angular momentum transport has also been a longstanding problem in accretion disks and laboratory plasmas. To investigate both dynamo and momentum transport,more » we have performed both numerical modeling of laboratory experiments that are intended to simulate nature and modeling of configurations with direct relevance to astrophysical disks. Our simulations use fluid approximations (Magnetohydrodynamics - MHD model), where plasma is treated as a single fluid, or two fluids, in the presence of electromagnetic forces. Our major physics objective is to study the possibility of magnetic field generation (so called MRI small-scale and large-scale dynamos) and its role in Magneto-rotational Instability (MRI) saturation through nonlinear simulations in both MHD and Hall regimes.« less

Multiscale Analysis of Rapidly Rotating Dynamo Simulations

NASA Astrophysics Data System (ADS)

Orvedahl, Ryan; Calkins, Michael; Featherstone, Nicholas

2017-11-01

The magnetic field of the planets and stars are generated by dynamo action in their electrically conducting fluid interiors. Numerical models of this process solve the fundamental equations of magnetohydrodynamics driven by convection in a rotating spherical shell. Rotation plays an important role in modifying the resulting convective flows and the self-generated magnetic field. We present results of simulating rapidly rotating systems that are unstable to dynamo action. We use the pseudo-spectral code Rayleigh to generate a suite of direct numerical simulations. Each simulation uses the Boussinesq approximation and is characterized by an Ekman number (Ek = ν / ΩL2) of 10-5. We vary the degree of convective forcing to obtain a range of convective Rossby numbers. The resulting flows and magnetic structures are analyzed using a Reynolds decomposition. We determine the relative importance of each term in the scale-separated governing equations and estimate the relevant spatial scales responsible for generating the mean magnetic field.

A PROPOSED PARADIGM FOR SOLAR CYCLE DYNAMICS MEDIATED VIA TURBULENT PUMPING OF MAGNETIC FLUX IN BABCOCK–LEIGHTON-TYPE SOLAR DYNAMOS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hazra, Soumitra; Nandy, Dibyendu

At present, the Babcock–Leighton flux transport solar dynamo models appear to be the most promising models for explaining diverse observational aspects of the sunspot cycle. The success of these flux transport dynamo models is largely dependent upon a single-cell meridional circulation with a deep equatorward component at the base of the Sun’s convection zone. However, recent observations suggest that the meridional flow may in fact be very shallow (confined to the top 10% of the Sun) and more complex than previously thought. Taken together, these observations raise serious concerns on the validity of the flux transport paradigm. By accounting formore » the turbulent pumping of magnetic flux, as evidenced in magnetohydrodynamic simulations of solar convection, we demonstrate that flux transport dynamo models can generate solar-like magnetic cycles even if the meridional flow is shallow. Solar-like periodic reversals are recovered even when meridional circulation is altogether absent. However, in this case, the solar surface magnetic field dynamics does not extend all the way to the polar regions. Very importantly, our results demonstrate that the Parker–Yoshimura sign rule for dynamo wave propagation can be circumvented in Babcock–Leighton dynamo models by the latitudinal component of turbulent pumping, which can generate equatorward propagating sunspot belts in the absence of a deep, equatorward meridional flow. We also show that variations in turbulent pumping coefficients can modulate the solar cycle amplitude and periodicity. Our results suggest the viability of an alternate magnetic flux transport paradigm—mediated via turbulent pumping—for sustaining solar-stellar dynamo action.« less

Numerical study of laminar plasma dynamo in cylindrical and spherical geometries

NASA Astrophysics Data System (ADS)

Khalzov, Ivan; Bayliss, Adam; Ebrahimi, Fatima; Forest, Cary; Schnack, Dalton

2009-05-01

We have performed the numerical investigation of possibility of laminar dynamo in two new experiments, Plasma Couette and Plasma Dynamo, which have been designed at the University of Wisconsin-Madison. The plasma is confined by a strong multipole magnetic field localized at the boundary of cylindrical (Plasma Couette) or spherical (Plasma Dynamo) chamber. Electrodes positioned between the magnet rings can be biased with arbitrary potentials so that Lorenz force ExB drives any given toroidal velocity profile at the surface. Using the extended MHD code, NIMROD, we have modeled several types of plasma flows appropriate for dynamo excitation. It is found that for high magnetic Reynolds numbers the counter-rotating von Karman flow (in cylinder) and Dudley-James flow (in sphere) can lead to self-generation of non-axisymmetric magnetic field. This field saturates at certain amplitude corresponding to a new stable equilibrium. The structure of this equilibrium is considered.

Wave-driven dynamo action in spherical magnetohydrodynamic systems.

PubMed

Reuter, K; Jenko, F; Tilgner, A; Forest, C B

2009-11-01

Hydrodynamic and magnetohydrodynamic numerical studies of a mechanically forced two-vortex flow inside a sphere are reported. The simulations are performed in the intermediate regime between the laminar flow and developed turbulence, where a hydrodynamic instability is found to generate internal waves with a characteristic m=2 zonal wave number. It is shown that this time-periodic flow acts as a dynamo, although snapshots of the flow as well as the mean flow are not dynamos. The magnetic fields' growth rate exhibits resonance effects depending on the wave frequency. Furthermore, a cyclic self-killing and self-recovering dynamo based on the relative alignment of the velocity and magnetic fields is presented. The phenomena are explained in terms of a mixing of nonorthogonal eigenstates of the time-dependent linear operator of the magnetic induction equation. The potential relevance of this mechanism to dynamo experiments is discussed.

Dynamo Catalogue: Geometrical tools and data management for particle picking in subtomogram averaging of cryo-electron tomograms.

PubMed

Castaño-Díez, Daniel; Kudryashev, Mikhail; Stahlberg, Henning

2017-02-01

Cryo electron tomography allows macromolecular complexes within vitrified, intact, thin cells or sections thereof to be visualized, and structural analysis to be performed in situ by averaging over multiple copies of the same molecules. Image processing for subtomogram averaging is specific and cumbersome, due to the large amount of data and its three dimensional nature and anisotropic resolution. Here, we streamline data processing for subtomogram averaging by introducing an archiving system, Dynamo Catalogue. This system manages tomographic data from multiple tomograms and allows visual feedback during all processing steps, including particle picking, extraction, alignment and classification. The file structure of a processing project file structure includes logfiles of performed operations, and can be backed up and shared between users. Command line commands, database queries and a set of GUIs give the user versatile control over the process. Here, we introduce a set of geometric tools that streamline particle picking from simple (filaments, spheres, tubes, vesicles) and complex geometries (arbitrary 2D surfaces, rare instances on proteins with geometric restrictions, and 2D and 3D crystals). Advanced functionality, such as manual alignment and subboxing, is useful when initial templates are generated for alignment and for project customization. Dynamo Catalogue is part of the open source package Dynamo and includes tools to ensure format compatibility with the subtomogram averaging functionalities of other packages, such as Jsubtomo, PyTom, PEET, EMAN2, XMIPP and Relion. Copyright © 2016. Published by Elsevier Inc.

Increasing Helicity to Achieve a Dynamo State on the Three-Meter Model of the Earth's Core

NASA Astrophysics Data System (ADS)

Rojas, R.; Perevalov, A.; Lathrop, D. P.

2017-12-01

Dynamo theory describes the generation of magnetic fields in the flows of conducting fluids, for example, in stars and planetary cores. Spherical Couette flows, which are flows between two concentric and independently rotating spheres, is one of the experimental models for achieving this task in the laboratory. We have performed dynamo state search in our three-meter spherical-Couette model reaching up to Reynolds number near 108 with amplifications of the field between 10-30% but without a self-sustained dynamo magnetic field. A recent numerical work [K. Finke and A. Tilgner. Phys. Rev. E, 86:016310, Jul 2012] suggested that a roughened inner core reduces the threshold for dynamo action. The mean flow would have more poloidal component than the one we are generating with our current smooth sphere setup. With baffles flow would be expelled radially outward on the equatorial plane and returned at the poles, with opposite helicities in the two hemispheres. Baffles welded on our smooth inner sphere are proposed to achieve this task. We are working to perform experiments on a scaled water model of our experimental setup with Reynolds number near 105 to measure the helicity improvements of different baffle designs in support of upcoming Three-Meter modifications. We gratefully acknowledge support from NSF EAR-1417148.

Simulations of Dynamo and Magnetorotational Instability in Madison Plasma Experiments and Astrophysical Disks

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ebrahimi, Fatima

Magnetic fields are observed to exist on all scales in many astrophysical sources such as stars, galaxies, and accretion discs. Understanding the origin of large scale magnetic fields, whereby the field emerges on spatial scales large compared to the fluctuations, has been a particularly long standing challenge. Our physics objective are: 1) what are the minimum ingredients for large-scale dynamo growth? 2) could a large-scale magnetic field grow out of turbulence and sustained despite the presence of dissipation? These questions are fundamental for understanding the large-scale dynamo in both laboratory and astrophysical plasmas. Here, we report major new findings inmore » the area of Large-Scale Dynamo (magnetic field generation).« less

Measurement of the dynamo effect in a plasma

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ji, H.; Prager, S.C.; Almagri, A.F.

1995-11-01

A series of the detailed experiments has been conducted in three laboratory plasma devices to measure the dynamo electric field along the equilibrium field line (the {alpha} effect) arising from the correlation between the fluctuating flow velocity and magnetic field. The fluctuating flow velocity is obtained from probe measurement of the fluctuating E x B drift and electron diamagnetic drift. The three major findings are (1) the {alpha} effect accounts for the dynamo current generation, even in the time dependence through a ``sawtooth`` cycle; (2) at low collisionality the dynamo is explained primarily by the widely studied pressureless Magnetohydrodynamic (MHD)more » model, i.e., the fluctuating velocity is dominated by the E x B drift; (3) at high collisionality, a new ``electron diamagnetic dynamo`` is observed, in which the fluctuating velocity is dominated by the diamagnetic drift. In addition, direct measurements of the helicity flux indicate that the dynamo activity transports magnetic helicity from one part of the plasma to another, but the total helicity is roughly conserved, verifying J.B. Taylor`s conjecture.« less

Residual fields from extinct dynamos

NASA Astrophysics Data System (ADS)

Parker, E. N.

The generation of magnetic fields in convective zones of declining vigor and/or thickness is considered, the goal being to explain the magnetic fields observed in A-stars. The investigation is restricted to kinematical dynamos in order to show some of the many possibilities, which depend on the assumed conditions of decline of the convection. The examples illustrate the quantitative detail required to describe the convection in order to extract any firm conclusions concerning specific stars. The first example treats the basic problem of diffusion from a layer of declining thickness. The second has a buoyant rise added to the field in the layer. The third deals with plane dynamo waves in a region with declining eddy diffusivity, dynamo coefficient, and large-scale shear. It is noted that the dynamo number may increase or decrease with declining convection, with an increase expected if the large-scale shear does not decline as rapidly as the eddy diffusivity. It is shown that one of the components of the field may increase without bound even when the dynamo number declines to zero.

Stochastic generation of MAC waves and implications for convection in Earth's core

NASA Astrophysics Data System (ADS)

Buffett, Bruce; Knezek, Nicholas

2018-03-01

Convection in Earth's core can sustain magnetic-Archemedes-Coriolis (MAC) waves through a variety of mechanisms. Buoyancy and Lorentz forces are viable sources for wave motion, together with the effects of magnetic induction. We develop a quantitative description for zonal MAC waves and assess the source mechanisms using a numerical dynamo model. The largest sources at conditions accessible to the dynamo model are due to buoyancy forces and magnetic induction. However, when these sources are extrapolated to conditions expected in Earth's core, the Lorentz force emerges as the dominant generation mechanism. This source is expected to produce wave velocities of roughly 2 km yr-1 when the internal magnetic field is characterized by a dimensionless Elsasser number of roughly Λ ≈ 10 and the root-mean-square convective velocity defines a magnetic Reynolds number of Rm ≈ 103. Our preferred model has a radially varying stratification and a constant (radial) background magnetic field. It predicts a broad power spectrum for the wave velocity with most power distributed across periods from 30 to 100 yr.

A theoretical model of the variation of the meridional circulation with the solar cycle

NASA Astrophysics Data System (ADS)

Hazra, Gopal; Choudhuri, Arnab Rai

2017-12-01

Observations of the meridional circulation of the Sun, which plays a key role in the operation of the solar dynamo, indicate that its speed varies with the solar cycle, becoming faster during the solar minima and slower during the solar maxima. To explain this variation of the meridional circulation with the solar cycle, we construct a theoretical model by coupling the equation of the meridional circulation (the ϕ component of the vorticity equation within the solar convection zone) with the equations of the flux transport dynamo model. We consider the back reaction due to the Lorentz force of the dynamo-generated magnetic fields and study the perturbations produced in the meridional circulation due to it. This enables us to model the variations of the meridional circulation without developing a full theory of the meridional circulation itself. We obtain results which reproduce the observational data of solar cycle variations of the meridional circulation reasonably well. We get the best results on assuming the turbulent viscosity acting on the velocity field to be comparable to the magnetic diffusivity (i.e. on assuming the magnetic Prandtl number to be close to unity). We have to assume an appropriate bottom boundary condition to ensure that the Lorentz force cannot drive a flow in the subadiabatic layers below the bottom of the tachocline. Our results are sensitive to this bottom boundary condition. We also suggest a hypothesis on how the observed inward flow towards the active regions may be produced.

Dynamo generation of magnetic field in the white dwarf GD 358

NASA Technical Reports Server (NTRS)

Markiel, J. Andrew; Thomas, John H.; Van Horn, H. M.

1994-01-01

On the basis of Whole Earth Telescope observations of the g-mode oscillation spectrum of the white dwarf GD 358, Winget et al. find evidence for significant differential rotation and for a time-varying magnetic field concentrated in the surface layers of this star. Here we argue on theoretical grounds that this magnetic field is produced by an alpha omega dynamo operating in the lower part of a surface convection zone in GD 358. Our argument is based on numerical solutions of the nonlinear, local dynamo equations of Robinson & Durney, with specific parameters based on our detailed models of white-dwarf convective envelopes, and universal constants determined by a calibration with the the Sun's dynamo. The calculations suggest a dynamo cycle period of about 6 years for the fundamental mode, and periods as short as 1 year for the higher-order modes that are expected to dominate in view of the large dynamo number we estimate for GD 358. These dynamo periods are consistent with the changes in the magnetic field of GD 358 over the span of 1 month inferred by Winget et. al. from their observations. Our calculations also suggest a peak dynamo magnetic field strength at the base of the surface convection zone of about 1800 G, which is consistent with the field strength inferred from the observations.

Dynamic Modeling of the Madison Dynamo Experiment

NASA Astrophysics Data System (ADS)

Truitt, J. L.; Forest, C. B.; Wright, J. C.

1999-11-01

This work focuses on a computer simulation of the Magnetohydrodynamic equations applied in the geometry of the Madison Dynamo Experiemnt. An integration code is used to evolve both the magnetic field and the velocity field numerically in spherical coordinates using a pseudo-spectral algorithm. The focus is to realistically model an experiment to be undertaken by the Madison Dynamo Experiment Group. The first flows studied are the well documented ones of Dudley and James. The main goals of the simulation are to observe the dynamo effect with the back-reaction allowed, to observe the equipartition of magnetic and kinetic energy due to theoretically proposed turbulent effects, and to isolate and study the α and β effects.

Magnetic Field Amplification in Supernova Remnants

NASA Astrophysics Data System (ADS)

Xu, Siyao; Lazarian, Alex

2017-12-01

Based on the new findings on the turbulent dynamo in Xu & Lazarian, we examine the magnetic field amplification in the context of supernova remnants. Due to the strong ion-neutral collisional damping in the weakly ionized interstellar medium, the dynamo in the preshock turbulence remains in the damping kinematic regime, which leads to a linear-in-time growth of the magnetic field strength. The resultant magnetic field structure enables effective diffusion upstream and shock acceleration of cosmic rays to energies above the “knee.” Differently, the nonlinear dynamo in the postshock turbulence leads to a linear-in-time growth of the magnetic energy due to the turbulent magnetic diffusion. Given a weak initial field strength in the postshock region, the magnetic field saturates at a significant distance from the shock front as a result of the inefficiency of the nonlinear dynamo. This result is in a good agreement with existing numerical simulations and well explains the X-ray spots detected far behind the shock front.

Self-consistent simulations of a von Kármán type dynamo in a spherical domain with metallic walls.

PubMed

Guervilly, Céline; Brummell, Nicholas H

2012-10-01

We have performed numerical simulations of boundary-driven dynamos using a three-dimensional nonlinear magnetohydrodynamical model in a spherical shell geometry. A conducting fluid of magnetic Prandtl number Pm=0.01 is driven into motion by the counter-rotation of the two hemispheric walls. The resulting flow is of von Kármán type, consisting of a layer of zonal velocity close to the outer wall and a secondary meridional circulation. Above a certain forcing threshold, the mean flow is unstable to non-axisymmetric motions within an equatorial belt. For fixed forcing above this threshold, we have studied the dynamo properties of this flow. The presence of a conducting outer wall is essential to the existence of a dynamo at these parameters. We have therefore studied the effect of changing the material parameters of the wall (magnetic permeability, electrical conductivity, and thickness) on the dynamo. In common with previous studies, we find that dynamos are obtained only when either the conductivity or the permeability is sufficiently large. However, we find that the effect of these two parameters on the dynamo process are different and can even compete to the detriment of the dynamo. Our self-consistent approach allow us to analyze in detail the dynamo feedback loop. The dynamos we obtain are typically dominated by an axisymmetric toroidal magnetic field and an axial dipole component. We show that the ability of the outer shear layer to produce a strong toroidal field depends critically on the presence of a conducting outer wall, which shields the fluid from the vacuum outside. The generation of the axisymmetric poloidal field, on the other hand, occurs in the equatorial belt and does not depend on the wall properties.

Generation of large-scale vorticity in rotating stratified turbulence with inhomogeneous helicity: mean-field theory

NASA Astrophysics Data System (ADS)

Kleeorin, N.

2018-06-01

We discuss a mean-field theory of the generation of large-scale vorticity in a rotating density stratified developed turbulence with inhomogeneous kinetic helicity. We show that the large-scale non-uniform flow is produced due to either a combined action of a density stratified rotating turbulence and uniform kinetic helicity or a combined effect of a rotating incompressible turbulence and inhomogeneous kinetic helicity. These effects result in the formation of a large-scale shear, and in turn its interaction with the small-scale turbulence causes an excitation of the large-scale instability (known as a vorticity dynamo) due to a combined effect of the large-scale shear and Reynolds stress-induced generation of the mean vorticity. The latter is due to the effect of large-scale shear on the Reynolds stress. A fast rotation suppresses this large-scale instability.

MAGNETIC CYCLES IN A DYNAMO SIMULATION OF FULLY CONVECTIVE M-STAR PROXIMA CENTAURI

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yadav, Rakesh K.; Wolk, Scott J.; Christensen, Ulrich R.

2016-12-20

The recent discovery of an Earth-like exoplanet around Proxima Centauri has shined a spot light on slowly rotating fully convective M-stars. When such stars rotate rapidly (period ≲20 days), they are known to generate very high levels of activity that is powered by a magnetic field much stronger than the solar magnetic field. Recent theoretical efforts are beginning to understand the dynamo process that generates such strong magnetic fields. However, the observational and theoretical landscape remains relatively uncharted for fully convective M-stars that rotate slowly. Here, we present an anelastic dynamo simulation designed to mimic some of the physical characteristicsmore » of Proxima Centauri, a representative case for slowly rotating fully convective M-stars. The rotating convection spontaneously generates differential rotation in the convection zone that drives coherent magnetic cycles where the axisymmetric magnetic field repeatedly changes polarity at all latitudes as time progress. The typical length of the “activity” cycle in the simulation is about nine years, in good agreement with the recently proposed activity cycle length of about seven years for Proxima Centauri. Comparing our results with earlier work, we hypothesis that the dynamo mechanism undergoes a fundamental change in nature as fully convective stars spin down with age.« less

Modelling the dynamo in fully convective M-stars

NASA Astrophysics Data System (ADS)

Yadav, Rakesh Kumar; Christensen, Ulrich; Morin, Julien; Wolk, Scott; Poppenhaeger, Katja; Reiners, Ansgar; gastine, Thomas

2017-05-01

M-stars are among the most active and numerous stars in our galaxy. Their activity plays a fundamentally important role in shaping the exoplanetary biosphere since the habitable zones are very close to these stars. Therefore, modeling M-star activity has become a focal point in habitability studies. The fully convective members of the M-star population demand more immediate attention due to the discovery of Earth-like exoplanets around our stellar neighbors Proxima Centauri and TRAPPIST-1 which are both fully convective. The activity of these stars is driven by their convective dynamo, which may be fundamentally different from the solar dynamo due the absence of radiative cores. We model this dynamo mechanism using high-resolution 3D anelastic MHD simulations. To understand the evolution of the dynamo mechanism we simulate two cases, one with a fast enough rotation period to model a star in the `saturated' regime of the rotation-activity realtionship and the other with a slower period to represent cases in the `unsaturated' regime. We find the rotation period fundamentally controls the behavior of the dynamo solution: faster rotation promotes strong magnetic fields (of order kG) on both small and large length scales and the dipolar component of the magnetic field is dominant and stable, however, slower rotation leads to weaker magnetic fields which exhibit cyclic behavior. In this talk, I will present the simulation results and discuss how we can use them to interpret several observed features of the M-star activity.

Evidence from numerical experiments for a feedback dynamo generating Mercury's magnetic field.

PubMed

Heyner, Daniel; Wicht, Johannes; Gómez-Pérez, Natalia; Schmitt, Dieter; Auster, Hans-Ulrich; Glassmeier, Karl-Heinz

2011-12-23

The observed weakness of Mercury's magnetic field poses a long-standing puzzle to dynamo theory. Using numerical dynamo simulations, we show that it could be explained by a negative feedback between the magnetospheric and the internal magnetic fields. Without feedback, a small internal field was amplified by the dynamo process up to Earth-like values. With feedback, the field strength saturated at a much lower level, compatible with the observations at Mercury. The classical saturation mechanism via the Lorentz force was replaced by the external field impact. The resulting surface field was dominated by uneven harmonic components. This will allow the feedback model to be distinguished from other models once a more accurate field model is constructed from MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) and BepiColombo data.

New Mexico Liquid Metal αω -dynamo experiment: Most Recent Progress

NASA Astrophysics Data System (ADS)

Si, Jiahe; Sonnenfeld, Richard; Colgate, Art; Li, Hui

2017-10-01

The goal of the New Mexico Liquid Metal αω -dynamo experiment is to demonstrate a galactic dynamo can be generated through two phases, the ω-phase and α-phase by two semi-coherent flows in laboratory. We have demonstrated an 8-fold poloidal-to-toroidal flux amplification from differential rotation (the ω-effect) by minimizing turbulence in our apparatus. To demonstrate the α-effect, major upgrades are needed. The upgrades include building a helicity injection facility, mounting new 100hp motors and new sensors, designing a new data acquisition system capable of transmitting data from about 80 sensors in a high speed rotating frame with an overall 200kS/sec sampling rate. We hope the upgrade can be utilized to answer the question of whether a self-sustaining αω -dynamo can be implemented with a realistic lab fluid flow field, as well as to obtain more details to understand dynamo action in highly turbulent Couette flow.

Numerical modelling of the Madison Dynamo Experiment.

NASA Astrophysics Data System (ADS)

Bayliss, R. A.; Wright, J. C.; Forest, C. B.; O'Connell, R.; Truitt, J. L.

2000-10-01

Growth, saturation and turbulent evolution of the Madison dynamo experiment is investigated numerically using a newly developed 3-D pseudo-spectral simulation of the MHD equations; results of the simulations will be compared to the experimental results obtained from the experiment. The code, Dynamo, is in Fortran90 and allows for full evolution of the magnetic and velocity fields. The induction equation governing B and the Navier-Stokes equation governing V are solved. The code uses a spectral representation via spherical harmonic basis functions of the vector fields in longitude and latitude, and finite differences in the radial direction. The magnetic field evolution has been benchmarked against the laminar kinematic dynamo predicted by M.L. Dudley and R.W. James (M.L. Dudley and R.W. James, Time-dependant kinematic dynamos with stationary flows, Proc. R. Soc. Lond. A 425, p. 407 (1989)). Initial results on magnetic field saturation, generated by the simultaneous evolution of magnetic and velocity fields be presented using a variety of mechanical forcing terms.

Mean-field dynamo in a turbulence with shear and kinetic helicity fluctuations.

PubMed

Kleeorin, Nathan; Rogachevskii, Igor

2008-03-01

We study the effects of kinetic helicity fluctuations in a turbulence with large-scale shear using two different approaches: the spectral tau approximation and the second-order correlation approximation (or first-order smoothing approximation). These two approaches demonstrate that homogeneous kinetic helicity fluctuations alone with zero mean value in a sheared homogeneous turbulence cannot cause a large-scale dynamo. A mean-field dynamo is possible when the kinetic helicity fluctuations are inhomogeneous, which causes a nonzero mean alpha effect in a sheared turbulence. On the other hand, the shear-current effect can generate a large-scale magnetic field even in a homogeneous nonhelical turbulence with large-scale shear. This effect was investigated previously for large hydrodynamic and magnetic Reynolds numbers. In this study we examine the threshold required for the shear-current dynamo versus Reynolds number. We demonstrate that there is no need for a developed inertial range in order to maintain the shear-current dynamo (e.g., the threshold in the Reynolds number is of the order of 1).

Dynamo: a flexible, user-friendly development tool for subtomogram averaging of cryo-EM data in high-performance computing environments.

PubMed

Castaño-Díez, Daniel; Kudryashev, Mikhail; Arheit, Marcel; Stahlberg, Henning

2012-05-01

Dynamo is a new software package for subtomogram averaging of cryo Electron Tomography (cryo-ET) data with three main goals: first, Dynamo allows user-transparent adaptation to a variety of high-performance computing platforms such as GPUs or CPU clusters. Second, Dynamo implements user-friendliness through GUI interfaces and scripting resources. Third, Dynamo offers user-flexibility through a plugin API. Besides the alignment and averaging procedures, Dynamo includes native tools for visualization and analysis of results and data, as well as support for third party visualization software, such as Chimera UCSF or EMAN2. As a demonstration of these functionalities, we studied bacterial flagellar motors and showed automatically detected classes with absent and present C-rings. Subtomogram averaging is a common task in current cryo-ET pipelines, which requires extensive computational resources and follows a well-established workflow. However, due to the data diversity, many existing packages offer slight variations of the same algorithm to improve results. One of the main purposes behind Dynamo is to provide explicit tools to allow the user the insertion of custom designed procedures - or plugins - to replace or complement the native algorithms in the different steps of the processing pipeline for subtomogram averaging without the burden of handling parallelization. Custom scripts that implement new approaches devised by the user are integrated into the Dynamo data management system, so that they can be controlled by the GUI or the scripting capacities. Dynamo executables do not require licenses for third party commercial software. Sources, executables and documentation are freely distributed on http://www.dynamo-em.org. Copyright © 2012 Elsevier Inc. All rights reserved.

Generation of dynamo waves by spatially separated sources in the Earth and other celestial bodies

NASA Astrophysics Data System (ADS)

Popova, E.

2017-12-01

The amplitude and the spatial configuration of the planetary and stellar magnetic field can changing over the years. Celestial bodies can have cyclic, chaotic or unchanging in time magnetic activity which is connected with a dynamo mechanism. This mechanism is based on the consideration of the joint influence of the alpha-effect and differential rotation. Dynamo sources can be located at different depths (active layers) of the celestial body and can have different intensities. Application of this concept allows us to get different forms of solutions and some of which can include wave propagating inside the celestial body. We analytically showed that in the case of spatially separated sources of magnetic field each source generates a wave whose frequency depends on the physical parameters of its source. We estimated parameters of sources required for the generation nondecaying waves. We discus structure of such sources and matter motion (including meridional circulation) in the liquid outer core of the Earth and active layers of other celestial bodies.

Optimum reduction of the dynamo threshold by a ferromagnetic layer located in the flow.

PubMed

Herault, J; Pétrélis, F

2014-09-01

We consider a fluid dynamo model generated by the flow on both sides of a moving layer. The magnetic permeability of the layer is larger than that of the flow. We show that there exists an optimum value of magnetic permeability for which the critical magnetic Reynolds number for dynamo onset is smaller than for a nonmagnetic material and also smaller than for a layer of infinite magnetic permeability. We present a mechanism that provides an explanation for recent experimental results. A similar effect occurs when the electrical conductivity of the layer is large.

EFFECTS OF LARGE-SCALE NON-AXISYMMETRIC PERTURBATIONS IN THE MEAN-FIELD SOLAR DYNAMO

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pipin, V. V.; Kosovichev, A. G.

2015-11-10

We explore the response of a nonlinear non-axisymmetric mean-field solar dynamo model to shallow non-axisymmetric perturbations. After a relaxation period, the amplitude of the non-axisymmetric field depends on the initial condition, helicity conservation, and the depth of perturbation. It is found that a perturbation that is anchored at 0.9 R{sub ⊙} has a profound effect on the dynamo process, producing a transient magnetic cycle of the axisymmetric magnetic field, if it is initiated at the growing phase of the cycle. The non-symmetric, with respect to the equator, perturbation results in a hemispheric asymmetry of the magnetic activity. The evolution ofmore » the axisymmetric and non-axisymmetric fields depends on the turbulent magnetic Reynolds number R{sub m}. In the range of R{sub m} = 10{sup 4}–10{sup 6} the evolution returns to the normal course in the next cycle, in which the non-axisymmetric field is generated due to a nonlinear α-effect and magnetic buoyancy. In the stationary state, the large-scale magnetic field demonstrates a phenomenon of “active longitudes” with cyclic 180° “flip-flop” changes of the large-scale magnetic field orientation. The flip-flop effect is known from observations of solar and stellar magnetic cycles. However, this effect disappears in the model, which includes the meridional circulation pattern determined by helioseismology. The rotation rate of the non-axisymmetric field components varies during the relaxation period and carries important information about the dynamo process.« less

Building Magnetic Fields in White Dwarfs

NASA Astrophysics Data System (ADS)

Kohler, Susanna

2017-03-01

White dwarfs, the compact remnants left over at the end of low- and medium-mass stars lifetimes, are often found to have magnetic fields with strengths ranging from thousands to billions of times that of Earth. But how do these fields form?MultiplePossibilitiesAround 1020% of white dwarfs have been observed to have measurable magnetic fields with a wide range of strengths. There are several theories as to how these fields might be generated:The fields are fossil.The original weak magnetic fields of the progenitor stars were amplified as the stars cores evolved into white dwarfs.The fields are caused by binary interactions.White dwarfs that formed in the merger of a binary pair might have had a magnetic field amplified as a result of a dynamo that was generated during the merger.The fields were produced by some other internal physical mechanism during the cooling of the white dwarf itself.In a recent publication, a team of authors led by Jordi Isern (Institute of Space Sciences, CSIC, and Institute for Space Studies of Catalonia, Spain) explored this third possibility.Dynamos from CrystallizationThe inner and outer boundaries of the convective mantle of carbon/oxygen white dwarfs of two different masses (top vs. bottom panel) as a function of luminosity. As the white dwarf cools (toward the right), the mantle grows thinner due to the crystallization and settling of material. [Isern et al. 2017]As white dwarfs have no nuclear fusion at their centers, they simply radiate heat and gradually cool over time. The structure of the white dwarf undergoes an interesting change as it cools, however: though the object begins as a fluid composed primarily of an ionized mixture of carbon and oxygen (and a few minor species like nickel and iron), it gradually crystallizes as its temperature drops.The crystallized phase of the white dwarf is oxygen-rich which is denser than the liquid, so the crystallized material sinks to the center of the dwarf as it solidifies. As a result, the white dwarf forms a solid, oxygen-rich core with a liquid, carbon-rich mantle thats Rayleigh-Taylor unstable: as crystallization continues, the solids continue to sink out of the mantle.By analytically modeling this process, Isern and collaborators demonstrate that the Rayleigh-Taylor instabilities in the convective mantle can drive a dynamo large enough to generate the magnetic field strengths weve observed in white dwarfs.Magnetic field density as a function of the dynamo energy density. The plots show Earth and Jupiter (black dots), T Tauri stars (cyan), M dwarf stars (magenta), and two types of white dwarfs (blue and red). Do these lie on the same scaling relation? [Isern et al. 2017]A Universal Process?This setup the solid core with an unstable liquid mantle on top is exactly the structure expected to occur in planets such as Earth and Jupiter. These planets magnetic fields are similarly thought to be generated by convective dynamos powered by the cooling and chemical separation of their interiors and the process can also be scaled up to account for the magnetic fields of fully convective objects like T Tauri stars, as well.If white-dwarf magnetic fields are generated by the same type of dynamo, this may be a universal process for creating magnetic fields in astrophysical objects though other processes may well be at work too.CitationJordi Isern et al 2017 ApJL 836 L28. doi:10.3847/2041-8213/aa5eae

Probing Core Processes in the Earth and Small Bodies Using Paleomagnetism

NASA Astrophysics Data System (ADS)

Fu, R. R.; Weiss, B. P.; Lima, E. A.; Glenn, D. R.; Kehayias, P.; Walsworth, R. L.

2015-12-01

Convective motion in the cores of differentiated metal-silicate bodies may sustain a global dynamo magnetic field. Progressive crystallization in a dynamo-generating core is expected to play a central role in determining the observable properties of the hosted magnetic field. Importantly, the release of light elements and latent heat during core crystallization is a key source of entropy for sustaining core convection. Therefore, the persistence and intensity of a dynamo magnetic field depend directly on the extent and style of core crystallization. We present and discuss paleomagnetic data from the Earth and asteroid-sized bodies to characterize internally generated magnetic fields during the early histories of these objects. In the case of the Earth, recent and ongoing paleomagnetic experiments of zircons from the Jack Hills of Australia can potentially constrain the existence and intensity of the geodynamo before 3.5 Ga. If robust, such measurements hold strong implications for the energy budget of the Earth's early core and the dynamics of the early mantle. We will discuss both recently published and preliminary results and assess carefully the challenges and uncertainties of paleomagnetic experimentation on ancient zircon samples. In the case of small bodies, several classes of meteorites record ancient magnetic fields likely produced by core dynamos on their parent bodies. Data from the CV carbonaceous chondrites and pallasites indicate that dynamos in planetesimal-sized bodies persisted for a broad range of timescales between ~10 My and >100 My. Meanwhile, measurements of the angrite group of achondrites show that their earliest-forming members crystallized in an almost non-magnetic environment, suggesting a delayed onset of the planetesimal dynamo until several My after initial differentiation. We will discuss the possible causes for this observed diversity of small body dynamo properties, including the role of core crystallization and the distribution of short-lived radioisotopes.

A study of the required Rayleigh number to sustain dynamo with various inner core radius

NASA Astrophysics Data System (ADS)

Nishida, Y.; Katoh, Y.; Matsui, H.; Kumamoto, A.

2017-12-01

It is widely accepted that the geomagnetic field is sustained by thermal and compositional driven convections of a liquid iron alloy in the outer core. The generation process of the geomagnetic field has been studied by a number of MHD dynamo simulations. Recent studies of the ratio of the Earth's core evolution suggest that the inner solid core radius ri to the outer liquid core radius ro changed from ri/ro = 0 to 0.35 during the last one billion years. There are some studies of dynamo in the early Earth with smaller inner core than the present. Heimpel et al. (2005) revealed the Rayleigh number Ra of the onset of dynamo process as a function of ri/ro from simulation, while paleomagnetic observation shows that the geomagnetic field has been sustained for 3.5 billion years. While Heimpel and Evans (2013) studied dynamo processes taking into account the thermal history of the Earth's interior, there were few cases corresponding to the early Earth. Driscoll (2016) performed a series of dynamo based on a thermal evolution model. Despite a number of dynamo simulations, dynamo process occurring in the interior of the early Earth has not been fully understood because the magnetic Prandtl numbers in these simulations are much larger than that for the actual outer core.In the present study, we performed thermally driven dynamo simulations with different aspect ratio ri/ro = 0.15, 0.25 and 0.35 to evaluate the critical Ra for the thermal convection and required Ra to maintain the dynamo. For this purpose, we performed simulations with various Ra and fixed the other control parameters such as the Ekman, Prandtl, and magnetic Prandtl numbers. For the initial condition and boundary conditions, we followed the dynamo benchmark case 1 by Christensen et al. (2001). The results show that the critical Ra increases with the smaller aspect ratio ri/ro. It is confirmed that larger amplitude of buoyancy is required in the smaller inner core to maintain dynamo.

Cosmological magnetic fields as string dynamo seeds and axion fields in torsioned spacetime

DOE Office of Scientific and Technical Information (OSTI.GOV)

De Andrade, L.C. Garcia, E-mail: garcia@dft.if.uerj.br

2014-08-01

In this paper two examples of the generation cosmological magnetic fields (CMF) are given. The first is the string dynamo seed cosmological magnetic field estimated as B{sub seed}∼10{sup -24} Gauss from a static spin polarised cylinder in Einstein-Cartan-Maxwell spacetime. The string dynamo seeds from a static spin polarised cylinder is given by B∼σ{sup 2}R{sup 2} where σ is the spin-torsion density while R is the string radius. The B-field value above is able to seed galactic dynamo. In the BBN the magnetic fields around 10{sup 12} Gauss give rise to a string radius as small as 10{sup 17}l{sub P} where l{sub P}more » is the Planck length. The second is the CMF from axionic torsion field which is given by B{sub seed}∼10{sup -27} Gauss which is stronger than the primordial magnetic field B{sub BICEP2}∼10{sup -30} Gauss from the BICEP2 recent experiment on primordial gravitational waves and cosmological inflation to axionic torsion. The interaction Lagrangean between axionic torsion scalar φ and magnetic fields used in this last example is given by f{sup 2}(φ)F{sub μν}F{sup μν}. A similar lagrangean has been used by K. Bamba et al. [JCAP 10 (2012) 058] so generate magnetic fields without dynamo action. Since axionic torsion can be associated with axionic domain walls both examples discussed here could be consider as topological defects examples of the generation of primordial magnetic fields in universes endowed with spacetime torsion.« less

Numerical simulation of laminar plasma dynamos in a cylindrical von Karman flow

DOE Office of Scientific and Technical Information (OSTI.GOV)

Khalzov, I. V.; Brown, B. P.; Schnack, D. D.

2011-03-15

The results of a numerical study of the magnetic dynamo effect in cylindrical von Karman plasma flow are presented with parameters relevant to the Madison Plasma Couette Experiment. This experiment is designed to investigate a broad class of phenomena in flowing plasmas. In a plasma, the magnetic Prandtl number Pm can be of order unity (i.e., the fluid Reynolds number Re is comparable to the magnetic Reynolds number Rm). This is in contrast to liquid metal experiments, where Pm is small (so, Re>>Rm) and the flows are always turbulent. We explore dynamo action through simulations using the extended magnetohydrodynamic NIMRODmore » code for an isothermal and compressible plasma model. We also study two-fluid effects in simulations by including the Hall term in Ohm's law. We find that the counter-rotating von Karman flow results in sustained dynamo action and the self-generation of magnetic field when the magnetic Reynolds number exceeds a critical value. For the plasma parameters of the experiment, this field saturates at an amplitude corresponding to a new stable equilibrium (a laminar dynamo). We show that compressibility in the plasma results in an increase of the critical magnetic Reynolds number, while inclusion of the Hall term in Ohm's law changes the amplitude of the saturated dynamo field but not the critical value for the onset of dynamo action.« less

Impact of DYNAMO observations on NASA GEOS-5 reanalyses and the representation of MJO initiation

NASA Astrophysics Data System (ADS)

Achuthavarier, D.; Wang, H.; Schubert, S. D.; Sienkiewicz, M.

2017-01-01

This study examines the impact of the Dynamics of the Madden-Julian Oscillation (DYNAMO) campaign in situ observations on NASA Goddard Earth Observing System version 5 (GEOS-5) reanalyses and the improvements gained thereby in the representation of the Madden-Julian Oscillation (MJO) initiation processes. To this end, we produced a global, high-resolution (1/4° spatially) reanalysis that assimilates the level-4, quality-controlled DYNAMO upper air soundings from about 87 stations in the equatorial Indian Ocean region along with a companion data-denied control reanalysis. The DYNAMO reanalysis produces a more realistic vertical structure of the temperature and moisture in the central tropical Indian Ocean by correcting the model biases, namely, the cold and dry biases in the lower troposphere and warm bias in the upper troposphere. The reanalysis horizontal winds are substantially improved, in that, the westerly acceleration and vertical shear of the zonal wind are enhanced. The DYNAMO reanalysis shows enhanced low-level diabatic heating, moisture anomalies and vertical velocity during the MJO initiation. Due to the warmer lower troposphere, the deep convection is invigorated, which is evident in convective cloud fraction. The GEOS-5 atmospheric general circulation model (AGCM) employed in the reanalysis is overall successful in assimilating the additional DYNAMO observations, except for an erroneous model response for medium rain rates, between 700 and 600 hPa, reminiscent of a bias in earlier versions of the AGCM. The moist heating profile shows a sharp decrease there due to the excessive convective rain re-evaporation, which is partly offset by the temperature increment produced by the analysis.

Cyclic Evolution of Coronal Fields from a Coupled Dynamo Potential-Field Source-Surface Model.

PubMed

Dikpati, Mausumi; Suresh, Akshaya; Burkepile, Joan

The structure of the Sun's corona varies with the solar-cycle phase, from a near spherical symmetry at solar maximum to an axial dipole at solar minimum. It is widely accepted that the large-scale coronal structure is governed by magnetic fields that are most likely generated by dynamo action in the solar interior. In order to understand the variation in coronal structure, we couple a potential-field source-surface model with a cyclic dynamo model. In this coupled model, the magnetic field inside the convection zone is governed by the dynamo equation; these dynamo-generated fields are extended from the photosphere to the corona using a potential-field source-surface model. Assuming axisymmetry, we take linear combinations of associated Legendre polynomials that match the more complex coronal structures. Choosing images of the global corona from the Mauna Loa Solar Observatory at each Carrington rotation over half a cycle (1986 - 1991), we compute the coefficients of the associated Legendre polynomials up to degree eight and compare with observations. We show that at minimum the dipole term dominates, but it fades as the cycle progresses; higher-order multipolar terms begin to dominate. The amplitudes of these terms are not exactly the same for the two limbs, indicating that there is a longitude dependence. While both the 1986 and the 1996 minimum coronas were dipolar, the minimum in 2008 was unusual, since there was a substantial departure from a dipole. We investigate the physical cause of this departure by including a North-South asymmetry in the surface source of the magnetic fields in our flux-transport dynamo model, and find that this asymmetry could be one of the reasons for departure from the dipole in the 2008 minimum.

The precession dynamo experiment at HZDR

NASA Astrophysics Data System (ADS)

Giesecke, A.; Gundrum, T.; Herault, J.; Stefani, F.; Gerbeth, G.

2015-12-01

In a next generation dynamo experiment currently under development atthe Helmholtz-Zentrum Dresden-Rossendorf (HZDR) a fluid flow of liquidsodium, solely driven by precession, will be considered as a possiblesource for magnetic field generation. The experiment is mainlymotivated by alternative concepts for astrophysical dynamos that arebased on mechanical flow driving. For example, it has long beendiscussed whether precession may be a complementary power source forthe geodynamo (Malkus, Science 1968) or for the ancient lunar dynamodue to the Earth-driven precession of the lunar spin axis (Dwyer, Nature 2011).We will present the current state of development of the dynamoexperiment together with results from non-linear hydrodynamicsimulations with moderate precessional forcing. Our simulations reveala non-axisymmetric forced mode with an amplitude of up to one fourthof the rotation velocity of the cylindrical container confirming thatprecession provides a rather efficient flow driving mechanism even atmoderate precession rates.More relevant for dynamo action might be free Kelvin modes (thenatural flow eigenmodes in a rotating cylinder) with higher azimuthalwave number. These modes may become relevant when constituting atriadic resonance with the fundamental forced mode, i.e., when theheight of the container matches their axial wave lengths. We findtriadic resonances at aspect ratios close to those predicted by thelinear theory except around the primary resonance of the forcedmode. In that regime we still identify free Kelvin modes propagatingin retrograde direction but none of them can be assigned to a triade.Our results will enter into the development of flow models that willbe used in kinematic simulations of the electromagnetic inductionequation in order to determine whether a precession driven flow willbe capable to drive a dynamo at all and to limit the parameter spacewithin which the occurrence of dynamo action is most promising.

Numerical study of dynamo action at low magnetic Prandtl numbers.

PubMed

Ponty, Y; Mininni, P D; Montgomery, D C; Pinton, J-F; Politano, H; Pouquet, A

2005-04-29

We present a three-pronged numerical approach to the dynamo problem at low magnetic Prandtl numbers P(M). The difficulty of resolving a large range of scales is circumvented by combining direct numerical simulations, a Lagrangian-averaged model and large-eddy simulations. The flow is generated by the Taylor-Green forcing; it combines a well defined structure at large scales and turbulent fluctuations at small scales. Our main findings are (i) dynamos are observed from P(M)=1 down to P(M)=10(-2), (ii) the critical magnetic Reynolds number increases sharply with P(M)(-1) as turbulence sets in and then it saturates, and (iii) in the linear growth phase, unstable magnetic modes move to smaller scales as P(M) is decreased. Then the dynamo grows at large scales and modifies the turbulent velocity fluctuations.

Manifestations of dynamo driven large-scale magnetic field in accretion disks of compact objects

NASA Technical Reports Server (NTRS)

Chagelishvili, G. D.; Chanishvili, R. G.; Lominadze, J. G.; Sokhadze, Z. A.

1991-01-01

A turbulent dynamo nonlinear theory of turbulence was developed that shows that in the compact objects of accretion disks, the generated large-scale magnetic field (when the generation takes place) has a practically toroidal configuration. Its energy density can be much higher than turbulent pulsations energy density, and it becomes comparable with the thermal energy density of the medium. On this basis, the manifestations to which the large-scale magnetic field can lead at the accretion onto black holes and gravimagnetic rotators, respectively, are presented.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kitiashvili, I. N.; Mansour, N. N.; Wray, A. A.

Magnetic fields are usually observed in the quiet Sun as small-scale elements that cover the entire solar surface (the “salt-and-pepper” patterns in line-of-sight magnetograms). By using 3D radiative MHD numerical simulations, we find that these fields result from a local dynamo action in the top layers of the convection zone, where extremely weak “seed” magnetic fields (e.g., from a 10{sup −6} G) can locally grow above the mean equipartition field to a stronger than 2000 G field localized in magnetic structures. Our results reveal that the magnetic flux is predominantly generated in regions of small-scale helical downflows. We find thatmore » the local dynamo action takes place mostly in a shallow, about 500 km deep, subsurface layer, from which the generated field is transported into the deeper layers by convective downdrafts. We demonstrate that the observed dominance of vertical magnetic fields at the photosphere and horizontal fields above the photosphere can be explained by small-scale magnetic loops produced by the dynamo. Such small-scale loops play an important role in the structure and dynamics of the solar atmosphere and their detection in observations is critical for understanding the local dynamo action on the Sun.« less

Magnetic flux pumping in 3D nonlinear magnetohydrodynamic simulations

NASA Astrophysics Data System (ADS)

Krebs, I.; Jardin, S. C.; Günter, S.; Lackner, K.; Hoelzl, M.; Strumberger, E.; Ferraro, N.

2017-10-01

A self-regulating magnetic flux pumping mechanism in tokamaks that maintains the core safety factor at q ≈1 , thus preventing sawteeth, is analyzed in nonlinear 3D magnetohydrodynamic simulations using the M3D-C1 code. In these simulations, the most important mechanism responsible for the flux pumping is that a saturated (m =1 ,n =1 ) quasi-interchange instability generates an effective negative loop voltage in the plasma center via a dynamo effect. It is shown that sawtoothing is prevented in the simulations if β is sufficiently high to provide the necessary drive for the (m =1 ,n =1 ) instability that generates the dynamo loop voltage. The necessary amount of dynamo loop voltage is determined by the tendency of the current density profile to centrally peak which, in our simulations, is controlled by the peakedness of the applied heat source profile.

Generation and maintenance of bisymmetric spiral magnetic fields in disk galaxies in differential rotation

NASA Astrophysics Data System (ADS)

Sawa, Takeyasu; Fujimoto, M.

1993-05-01

The approximate dynamo equation, which yields asymptotic solutions for the large scale bisymmetric spiral (BSS) magnetic fields rotating rigidly over a large area of the galactic disk, is derived. The vertical thickness and the dynamo strength of the gaseous disk which are necessary to generate and sustain the BSS magnetic fields is determined. The globally BSS magnetic fields which propagate over the disk as a wave without being twisted more tightly are reproduced. A poloidal field configuration is theoretically predicted in the halo around the disk, and is observed in the edge-on galaxy NGC4631. Mathematical methods for the galactic dynamo are shown to be equivalent. Those methods give different growth rates between the BSS and the axisymmetric spiral (ASS) magnetic fields in the disk. Magnetohydrodynamical excitation is discussed between the BSS magnetic fields and the two armed spiral density waves.

Magnetic flux concentrations from dynamo-generated fields

NASA Astrophysics Data System (ADS)

Jabbari, S.; Brandenburg, A.; Losada, I. R.; Kleeorin, N.; Rogachevskii, I.

2014-08-01

Context. The mean-field theory of magnetized stellar convection gives rise to two distinct instabilities: the large-scale dynamo instability, operating in the bulk of the convection zone and a negative effective magnetic pressure instability (NEMPI) operating in the strongly stratified surface layers. The latter might be important in connection with magnetic spot formation. However, as follows from theoretical analysis, the growth rate of NEMPI is suppressed with increasing rotation rates. On the other hand, recent direct numerical simulations (DNS) have shown a subsequent increase in the growth rate. Aims: We examine quantitatively whether this increase in the growth rate of NEMPI can be explained by an α2 mean-field dynamo, and whether both NEMPI and the dynamo instability can operate at the same time. Methods: We use both DNS and mean-field simulations (MFS) to solve the underlying equations numerically either with or without an imposed horizontal field. We use the test-field method to compute relevant dynamo coefficients. Results: DNS show that magnetic flux concentrations are still possible up to rotation rates above which the large-scale dynamo effect produces mean magnetic fields. The resulting DNS growth rates are quantitatively reproduced with MFS. As expected for weak or vanishing rotation, the growth rate of NEMPI increases with increasing gravity, but there is a correction term for strong gravity and large turbulent magnetic diffusivity. Conclusions: Magnetic flux concentrations are still possible for rotation rates above which dynamo action takes over. For the solar rotation rate, the corresponding turbulent turnover time is about 5 h, with dynamo action commencing in the layers beneath.

Effects of anisotropies in turbulent magnetic diffusion in mean-field solar dynamo models

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pipin, V. V.; Kosovichev, A. G.

2014-04-10

We study how anisotropies of turbulent diffusion affect the evolution of large-scale magnetic fields and the dynamo process on the Sun. The effect of anisotropy is calculated in a mean-field magnetohydrodynamics framework assuming that triple correlations provide relaxation to the turbulent electromotive force (so-called the 'minimal τ-approximation'). We examine two types of mean-field dynamo models: the well-known benchmark flux-transport model and a distributed-dynamo model with a subsurface rotational shear layer. For both models, we investigate effects of the double- and triple-cell meridional circulation, recently suggested by helioseismology and numerical simulations. To characterize the anisotropy effects, we introduce a parameter ofmore » anisotropy as a ratio of the radial and horizontal intensities of turbulent mixing. It is found that the anisotropy affects the distribution of magnetic fields inside the convection zone. The concentration of the magnetic flux near the bottom and top boundaries of the convection zone is greater when the anisotropy is stronger. It is shown that the critical dynamo number and the dynamo period approach to constant values for large values of the anisotropy parameter. The anisotropy reduces the overlap of toroidal magnetic fields generated in subsequent dynamo cycles, in the time-latitude 'butterfly' diagram. If we assume that sunspots are formed in the vicinity of the subsurface shear layer, then the distributed dynamo model with the anisotropic diffusivity satisfies the observational constraints from helioseismology and is consistent with the value of effective turbulent diffusion estimated from the dynamics of surface magnetic fields.« less

Laminar and Turbulent Dynamos in Chiral Magnetohydrodynamics. II. Simulations

NASA Astrophysics Data System (ADS)

Schober, Jennifer; Rogachevskii, Igor; Brandenburg, Axel; Boyarsky, Alexey; Fröhlich, Jürg; Ruchayskiy, Oleg; Kleeorin, Nathan

2018-05-01

Using direct numerical simulations (DNS), we study laminar and turbulent dynamos in chiral magnetohydrodynamics with an extended set of equations that accounts for an additional contribution to the electric current due to the chiral magnetic effect (CME). This quantum phenomenon originates from an asymmetry between left- and right-handed relativistic fermions in the presence of a magnetic field and gives rise to a chiral dynamo. We show that the magnetic field evolution proceeds in three stages: (1) a small-scale chiral dynamo instability, (2) production of chiral magnetically driven turbulence and excitation of a large-scale dynamo instability due to a new chiral effect (α μ effect), and (3) saturation of magnetic helicity and magnetic field growth controlled by a conservation law for the total chirality. The α μ effect becomes dominant at large fluid and magnetic Reynolds numbers and is not related to kinetic helicity. The growth rate of the large-scale magnetic field and its characteristic scale measured in the numerical simulations agree well with theoretical predictions based on mean-field theory. The previously discussed two-stage chiral magnetic scenario did not include stage (2), during which the characteristic scale of magnetic field variations can increase by many orders of magnitude. Based on the findings from numerical simulations, the relevance of the CME and the chiral effects revealed in the relativistic plasma of the early universe and of proto-neutron stars are discussed.

Magnetic field variation caused by rotational speed change in a magnetohydrodynamic dynamo.

PubMed

Miyagoshi, Takehiro; Hamano, Yozo

2013-09-20

We have performed numerical magnetohydrodynamic dynamo simulations in a spherical shell with rotational speed or length-of-day (LOD) variation, which is motivated by correlations between geomagnetic field and climatic variations with ice and non-ice ages. The results show that LOD variation leads to magnetic field variation whose amplitude is considerably larger than that of LOD variation. The heat flux at the outer sphere and the zonal flow also change. The mechanism of the magnetic field variation due to LOD variation is also found. The keys are changes of dynamo activity and Joule heating.

Precession Driven Instabilities and Dynamos in the Early Moon

NASA Astrophysics Data System (ADS)

Cebron, D.; Laguerre, R.; Noir, J.; Vidal, J.; Schaeffer, N.

2017-12-01

The Early Moon magnetic fields are probably due to a strong temporary dynamo, which may be due to lunar precession [1]. However, precession driven dynamos remain badly known, with only few studied cases [2,3,4]. Given the uncertainties of the early Moon precession, wider ranges of parameters need to be explored in order to assess if such lunar dynamos are possible. Using the efficient dynamo code XSHELLS, we have thus performed many simulations of precessing spherical shells, varying the parameters in a systematic way. This allows us to characterize the various excited instabilities, and to propose scaling laws. We also obtain that precession driven dynamos seem scarce and weak in our simulations, which makes difficult and uncertain the extrapolation of these dynamos to the Moon. However, our dynamo simulations, as every other in the literature, neglect the topographic torque effect on instabilities in order to use fast spectral codes [5]. By contrast, the topographic torque is dominant for the lunar core. Before exploring this effect numerically, which is a real challenge, we choose to study it theoretically. To do so, we have developed a novel global linear stability analysis of mechanically-driven flows in triaxial ellipsoids, with leading order viscous effects. Internal dissipation is obtained for the first time by extending the Greenspan's theory (1968) of geostrophic and inertial modes. By contrast with pioneering theories [6], we propose a new linear viscous model valid in arbitrary ellipsoid and for any precessing forcing. Then we perform the linear stability analysis by considering ellipsoidal perturbations of unprecedented spatial complexity with a self-consistent model of viscous damping. We show that forced precession-driven basic flows are bistable in triaxial ellipsoids. Then, we present the first stability analysis of precessing-flows in triaxial ellipsoids. [1] Dwyer et al. (2011), Nature, 479, 212-214.[2] Tilgner (2005), Phy. Fluids, 17, 034104.[3] Tilgner (2007), Geo. Astro. Fluid Dyn., 101 (1), 1-9.[4] Lin et al. (2016), Phys. Fluids, 28, 066601.[5] Tian et al., EPSL, in revision.[6] Busse (1968), J. Fluid. Mech, 33 (04), 739-751.

The spectrum of random magnetic fields in the mean field dynamo theory of the Galactic magnetic field

NASA Technical Reports Server (NTRS)

Kulsrud, Russell M.; Anderson, Stephen W.

1992-01-01

The fluctuation spectrum that must arise in a mean field dynamo generation of galactic fields if the initial field is weak is considered. A kinetic equation for its evolution is derived and solved. The spectrum evolves by transfer of energy from one magnetic mode to another by interaction with turbulent velocity modes. This kinetic equation is valid in the limit that the rate of evolution of the magnetic modes is slower than the reciprocal decorrelation time of the turbulent modes. This turns out to be the case by a factor greater than 3. Most of the fluctuation energy concentrates on small scales, shorter than the hydrodynamic turbulent scales. The fluctuation energy builds up to equipartition with the turbulent energy in times that are short compared to the e-folding time of the mean field. The turbulence becomes strongly modified before the dynamo amplification starts. Thus, the kinematic assumption of the mean dynamo theory is invalid. Thus, the galactic field must have a primordial origin, although it may subsequently be modified by dynamo action.

Evolution of Our Understanding of the Solar Dynamo During Solar Cycle 24

NASA Astrophysics Data System (ADS)

Munoz-Jaramillo, A.

2017-12-01

Solar cycle 24 has been an exciting cycle for our understanding of the solar dynamo: 1. It was the first cycle for which dynamo based predictions were ever used teaching us valuable lessons. 2. It has given us the opportunity to observe a deep minimum and a weak cycle with a high level of of observational detail . 3. It is full of breaktrhoughs in anelastic MHD dynamo simulations (regular cycles, buoyant flux-tubes, mounder-like events). 4. It has seen the creation of bridges between the kinematic flux-transport and anelastic MHD approaches. 5. It has ushered a new generation of realistic surface flux-transport simulations 6. We have achieved significant observational progress in our understanding of solar cycle propagation. The objective of this talk is to highlight some of the most important results, giving special emphasis on what they have taught us about solar cycle predictability.

Scaling and intermittency in incoherent α-shear dynamo

NASA Astrophysics Data System (ADS)

Mitra, Dhrubaditya; Brandenburg, Axel

2012-03-01

We consider mean-field dynamo models with fluctuating α effect, both with and without large-scale shear. The α effect is chosen to be Gaussian white noise with zero mean and a given covariance. In the presence of shear, we show analytically that (in infinitely large domains) the mean-squared magnetic field shows exponential growth. The growth rate of the fastest growing mode is proportional to the shear rate. This result agrees with earlier numerical results of Yousef et al. and the recent analytical treatment by Heinemann, McWilliams & Schekochihin who use a method different from ours. In the absence of shear, an incoherent α2 dynamo may also be possible. We further show by explicit calculation of the growth rate of third- and fourth-order moments of the magnetic field that the probability density function of the mean magnetic field generated by this dynamo is non-Gaussian.

Marshall N. Rosenbluth Outstanding Doctoral Thesis Award: Magnetorotational turbulence and dynamo

NASA Astrophysics Data System (ADS)

Squire, Jonathan

2017-10-01

Accretion disks are ubiquitous in astrophysics and power some of the most luminous sources in the universe. In many disks, the transport of angular momentum, and thus the mass accretion itself, is thought to be caused by the magnetorotational instability (MRI). As the MRI saturates into strong turbulence, it also generates ordered magnetic fields, acting as a magnetic dynamo powered by the background shear flow. However, despite its importance for astrophysical accretion processes, basic aspects of MRI turbulence-including its saturation amplitude-remain poorly understood. In this talk, I will outline progress towards improving this situation, focusing in particular on the nonlinear shear dynamo and how this controls the turbulence. I will discuss how novel statistical simulation methods can be used to better understand this shear dynamo, in particular the distinct mechanisms that may play a role in MRI turbulence and how these depend on important physical parameters.

Local magnetohydrodynamic instabilities and the wave-driven dynamo in accretion disks

NASA Technical Reports Server (NTRS)

Vishniac, Ethan T.; Diamond, Patrick

1992-01-01

We consider the consequences of magnetic buoyancy and the magnetic shearing instability (MSI) on the strength and organization of the magnetic field in a thin accretion disk. We discuss a model in which the wave-driven dynamo growth rate is balanced by the dissipative effects of the MSI. As in earlier work, the net helicity is due to small advective motions driven by nonlinear interactions between internal waves. Assuming a simple model of the internal wave spectrum generated from the primary m = 1 internal waves, we find that the magnetic energy density saturates at about (H/r) exp 4/3 times the local pressure (where H is the disk thickness and r is its radius). On very small scales the shearing instability will produce an isotropic fluctuating field. For a stationary disk this is equivalent to a dimensionless 'viscosity' of about (H/r) exp 4/3. The vertical and radial diffusion coefficients will be comparable to each other. Magnetic buoyancy will be largely suppressed by the turbulence due to the MSI. We present a rough estimate of its effects and find that it removes magnetic flux from the disk at a rate comparable to that caused by turbulent diffusion.

Turbulent transport coefficients in spherical wedge dynamo simulations of solar-like stars

NASA Astrophysics Data System (ADS)

Warnecke, J.; Rheinhardt, M.; Tuomisto, S.; Käpylä, P. J.; Käpylä, M. J.; Brandenburg, A.

2018-01-01

Aims: We investigate dynamo action in global compressible solar-like convective dynamos in the framework of mean-field theory. Methods: We simulate a solar-type star in a wedge-shaped spherical shell, where the interplay between convection and rotation self-consistently drives a large-scale dynamo. To analyze the dynamo mechanism we apply the test-field method for azimuthally (φ) averaged fields to determine the 27 turbulent transport coefficients of the electromotive force, of which six are related to the α tensor. This method has previously been used either in simulations in Cartesian coordinates or in the geodynamo context and is applied here for the first time to fully compressible simulations of solar-like dynamos. Results: We find that the φφ-component of the α tensor does not follow the profile expected from that of kinetic helicity. The turbulent pumping velocities significantly alter the effective mean flows acting on the magnetic field and therefore challenge the flux transport dynamo concept. All coefficients are significantly affected by dynamically important magnetic fields. Quenching as well as enhancement are being observed. This leads to a modulation of the coefficients with the activity cycle. The temporal variations are found to be comparable to the time-averaged values and seem to be responsible for a nonlinear feedback on the magnetic field generation. Furthermore, we quantify the validity of the Parker-Yoshimura rule for the equatorward propagation of the mean magnetic field in the present case.

Effect of metallic walls on dynamos generated by laminar boundary-driven flow in a spherical domain.

PubMed

Guervilly, Céline; Wood, Toby S; Brummell, Nicholas H

2013-11-01

We present a numerical study of dynamo action in a conducting fluid encased in a metallic spherical shell. Motions in the fluid are driven by differential rotation of the outer metallic shell, which we refer to as "the wall." The two hemispheres of the wall are held in counter-rotation, producing a steady, axisymmetric interior flow consisting of differential rotation and a two-cell meridional circulation with radial inflow in the equatorial plane. From previous studies, this type of flow is known to maintain a stationary equatorial dipole by dynamo action if the magnetic Reynolds number is larger than about 300 and if the outer boundary is electrically insulating. We vary independently the thickness, electrical conductivity, and magnetic permeability of the wall to determine their effect on the dynamo action. The main results are the following: (a) Increasing the conductivity of the wall hinders the dynamo by allowing eddy currents within the wall, which are induced by the relative motion of the equatorial dipole field and the wall. This processes can be viewed as a skin effect or, equivalently, as the tearing apart of the dipole by the differential rotation of the wall, to which the field lines are anchored by high conductivity. (b) Increasing the magnetic permeability of the wall favors dynamo action by constraining the magnetic field lines in the fluid to be normal to the wall, thereby decoupling the fluid from any induction in the wall. (c) Decreasing the wall thickness limits the amplitude of the eddy currents, and is therefore favorable for dynamo action, provided that the wall is thinner than the skin depth. We explicitly demonstrate these effects of the wall properties on the dynamo field by deriving an effective boundary condition in the limit of vanishing wall thickness.

The solar dynamo and prediction of sunspot cycles

NASA Astrophysics Data System (ADS)

Dikpati, Mausumi

2012-07-01

Much progress has been made in understanding the solar dynamo since Parker first developed the concepts of dynamo waves and magnetic buoyancy around 1955, and the German school first formulated the solar dynamo using the mean-field formalism. The essential ingredients of these mean-field dynamos are turbulent magnetic diffusivity, a source of lifting of flux, or 'alpha-effect', and differential rotation. With the advent of helioseismic and other observations at the Sun's photosphere and interior, as well as theoretical understanding of solar interior dynamics, solar dynamo models have evolved both in the realm of mean-field and beyond mean-field models. After briefly discussing the status of these models, I will focus on a class of mean-field model, called flux-transport dynamos, which include meridional circulation as an essential additional ingredient. Flux-transport dynamos have been successful in simulating many global solar cycle features, and have reached the stage that they can be used for making solar cycle predictions. Meridional circulation works in these models like a conveyor-belt, carrying a memory of the magnetic fields from 5 to 20 years back in past. The lower is the magnetic diffusivity, the longer is the model's memory. In the terrestrial system, the great-ocean conveyor-belt in oceanic models and Hadley, polar and Ferrel circulation cells in the troposphere, carry signatures from the past climatological events and influence the determination of future events. Analogously, the memory provided by the Sun's meridional circulation creates the potential for flux-transport dynamos to predict future solar cycle properties. Various groups in the world have built flux-transport dynamo-based predictive tools, which nudge the Sun's surface magnetic data and integrated forward in time to forecast the amplitude of the currently ascending cycle 24. Due to different initial conditions and different choices of unknown model-ingredients, predictions can vary; so it is for their cycle 24 forecasts. We all await the peak of cycle 24. I will close by discussing the prospects of improving dynamo-based predictive tools using more sophisticated data-assimilation techniques, such as the Ensemble Kalman Filter method and variational approaches.

Why does substorm-associated auroral surge travel westward?

NASA Astrophysics Data System (ADS)

Ebihara, Y.; Tanaka, T.

2018-01-01

A substorm is a long-standing unsolved issue in solar-terrestrial physics. One of the big challenges is to explain reasonably the evolution of the morphological structure of the aurora associated with the substorm. The sudden appearance of a bright aurora and an auroral surge traveling westward (westward traveling surge, WTS) are noticeable features of the aurora during the substorm expansion phase. By using a global magnetohydrodynamics (MHD) simulation, we obtained the following results regarding the WTS. When the interplanetary magnetic field turns southward, a persistent dynamo appears in the cusp/mantle region, driving the two-cell magnetospheric convection. Then, the substorm growth phase begins. When magnetic reconnection takes place in the magnetotail, plasma is accelerated earthward in the plasma sheet, and accelerated toward the equatorial plane in the lobe. The second dynamo appears in the near-Earth region, which is closely associated with the generation of the field-aligned current (FAC) on the nightside. When the FAC reaches the ionosphere, the aurora becomes bright, and the onset of the expansion phase begins. In the ionosphere, the conductivity is intensified in the bright aurora due to the precipitation of accelerated electrons. The conductivity gradient gives rise to the overflow of the Hall current, which acts as the third dynamo. The overflow results in the accumulation of space charge, which causes a divergent electric field. The divergent electric field generates a thin, structured upward FAC adjacent to the bright aurora. The opposite process takes place on the opposite side of the bright aurora. In short, the upward FAC increases (appearance of aurora) at the leading edge of the surge, and decreases (disappearance of aurora) at the trailing edge of the surge. By repeating these processes, the surge seems to travel westward.

SMALL-SCALE AND GLOBAL DYNAMOS AND THE AREA AND FLUX DISTRIBUTIONS OF ACTIVE REGIONS, SUNSPOT GROUPS, AND SUNSPOTS: A MULTI-DATABASE STUDY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Muñoz-Jaramillo, Andrés; Windmueller, John C.; Amouzou, Ernest C.

2015-02-10

In this work, we take advantage of 11 different sunspot group, sunspot, and active region databases to characterize the area and flux distributions of photospheric magnetic structures. We find that, when taken separately, different databases are better fitted by different distributions (as has been reported previously in the literature). However, we find that all our databases can be reconciled by the simple application of a proportionality constant, and that, in reality, different databases are sampling different parts of a composite distribution. This composite distribution is made up by linear combination of Weibull and log-normal distributions—where a pure Weibull (log-normal) characterizesmore » the distribution of structures with fluxes below (above) 10{sup 21}Mx (10{sup 22}Mx). Additionally, we demonstrate that the Weibull distribution shows the expected linear behavior of a power-law distribution (when extended to smaller fluxes), making our results compatible with the results of Parnell et al. We propose that this is evidence of two separate mechanisms giving rise to visible structures on the photosphere: one directly connected to the global component of the dynamo (and the generation of bipolar active regions), and the other with the small-scale component of the dynamo (and the fragmentation of magnetic structures due to their interaction with turbulent convection)« less

An ancient core dynamo in asteroid Vesta.

PubMed

Fu, Roger R; Weiss, Benjamin P; Shuster, David L; Gattacceca, Jérôme; Grove, Timothy L; Suavet, Clément; Lima, Eduardo A; Li, Luyao; Kuan, Aaron T

2012-10-12

The asteroid Vesta is the smallest known planetary body that has experienced large-scale igneous differentiation. However, it has been previously uncertain whether Vesta and similarly sized planetesimals formed advecting metallic cores and dynamo magnetic fields. Here we show that remanent magnetization in the eucrite meteorite Allan Hills A81001 formed during cooling on Vesta 3.69 billion years ago in a surface magnetic field of at least 2 microteslas. This field most likely originated from crustal remanence produced by an earlier dynamo, suggesting that Vesta formed an advecting liquid metallic core. Furthermore, the inferred present-day crustal fields can account for the lack of solar wind ion-generated space weathering effects on Vesta.

A unified large/small-scale dynamo in helical turbulence

NASA Astrophysics Data System (ADS)

Bhat, Pallavi; Subramanian, Kandaswamy; Brandenburg, Axel

2016-09-01

We use high resolution direct numerical simulations (DNS) to show that helical turbulence can generate significant large-scale fields even in the presence of strong small-scale dynamo action. During the kinematic stage, the unified large/small-scale dynamo grows fields with a shape-invariant eigenfunction, with most power peaked at small scales or large k, as in Subramanian & Brandenburg. Nevertheless, the large-scale field can be clearly detected as an excess power at small k in the negatively polarized component of the energy spectrum for a forcing with positively polarized waves. Its strength overline{B}, relative to the total rms field Brms, decreases with increasing magnetic Reynolds number, ReM. However, as the Lorentz force becomes important, the field generated by the unified dynamo orders itself by saturating on successively larger scales. The magnetic integral scale for the positively polarized waves, characterizing the small-scale field, increases significantly from the kinematic stage to saturation. This implies that the small-scale field becomes as coherent as possible for a given forcing scale, which averts the ReM-dependent quenching of overline{B}/B_rms. These results are obtained for 10243 DNS with magnetic Prandtl numbers of PrM = 0.1 and 10. For PrM = 0.1, overline{B}/B_rms grows from about 0.04 to about 0.4 at saturation, aided in the final stages by helicity dissipation. For PrM = 10, overline{B}/B_rms grows from much less than 0.01 to values of the order the 0.2. Our results confirm that there is a unified large/small-scale dynamo in helical turbulence.

Magnetorotational dynamo action in the shearing box

NASA Astrophysics Data System (ADS)

Walker, Justin; Boldyrev, Stanislav

2017-09-01

Magnetic dynamo action caused by the magnetorotational instability is studied in the shearing-box approximation with no imposed net magnetic flux. Consistent with recent studies, the dynamo action is found to be sensitive to the aspect ratio of the box: it is much easier to obtain in tall boxes (stretched in the direction normal to the disc plane) than in long boxes (stretched in the radial direction). Our direct numerical simulations indicate that the dynamo is possible in both cases, given a large enough magnetic Reynolds number. To explain the relatively larger effort required to obtain the dynamo action in a long box, we propose that the turbulent eddies caused by the instability most efficiently fold and mix the magnetic field lines in the radial direction. As a result, in the long box the scale of the generated strong azimuthal (stream-wise directed) magnetic field is always comparable to the scale of the turbulent eddies. In contrast, in the tall box the azimuthal magnetic flux spreads in the vertical direction over a distance exceeding the scale of the turbulent eddies. As a result, different vertical sections of the tall box are permeated by large-scale non-zero azimuthal magnetic fluxes, facilitating the instability. In agreement with this picture, the cases when the dynamo is efficient are characterized by a strong intermittency of the local azimuthal magnetic fluxes.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yadav, Rakesh K.; Poppenhaeger, Katja; Wolk, Scott J.

Despite the lack of a shear-rich tachocline region, low-mass fully convective (FC) stars are capable of generating strong magnetic fields, indicating that a dynamo mechanism fundamentally different from the solar dynamo is at work in these objects. We present a self-consistent three-dimensional model of magnetic field generation in low-mass FC stars. The model utilizes the anelastic magnetohydrodynamic equations to simulate compressible convection in a rotating sphere. A distributed dynamo working in the model spontaneously produces a dipole-dominated surface magnetic field of the observed strength. The interaction of this field with the turbulent convection in outer layers shreds it, producing small-scalemore » fields that carry most of the magnetic flux. The Zeeman–Doppler-Imaging technique applied to synthetic spectropolarimetric data based on our model recovers most of the large-scale field. Our model simultaneously reproduces the morphology and magnitude of the large-scale field as well as the magnitude of the small-scale field observed on low-mass FC stars.« less

Estimating the Magnetic Field Strength in Hot Jupiters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yadav, Rakesh K.; Thorngren, Daniel P., E-mail: rakesh_yadav@fas.harvard.edu

A large fraction of known Jupiter-like exoplanets are inflated as compared to Jupiter. These “hot” Jupiters orbit close to their parent star and are bombarded with intense starlight. Many theories have been proposed to explain their radius inflation and several suggest that a small fraction of the incident starlight is injected into the planetary interior, which helps to puff up the planet. How will such energy injection affect the planetary dynamo? In this Letter, we estimate the surface magnetic field strength of hot Jupiters using scaling arguments that relate energy available in planetary interiors to the dynamo-generated magnetic fields. Wemore » find that if we take into account the energy injected in the planetary interior that is sufficient to inflate hot Jupiters to observed radii, then the resulting dynamo should be able generate magnetic fields that are more than an order of magnitude stronger than the Jovian values. Our analysis highlights the potential fundamental role of the stellar light in setting the field strength in hot Jupiters.« less

Strong-field dynamo action in rapidly rotating convection with no inertia.

PubMed

Hughes, David W; Cattaneo, Fausto

2016-06-01

The earth's magnetic field is generated by dynamo action driven by convection in the outer core. For numerical reasons, inertial and viscous forces play an important role in geodynamo models; however, the primary dynamical balance in the earth's core is believed to be between buoyancy, Coriolis, and magnetic forces. The hope has been that by setting the Ekman number to be as small as computationally feasible, an asymptotic regime would be reached in which the correct force balance is achieved. However, recent analyses of geodynamo models suggest that the desired balance has still not yet been attained. Here we adopt a complementary approach consisting of a model of rapidly rotating convection in which inertial forces are neglected from the outset. Within this framework we are able to construct a branch of solutions in which the dynamo generates a strong magnetic field that satisfies the expected force balance. The resulting strongly magnetized convection is dramatically different from the corresponding solutions in which the field is weak.

The magnetic shear-current effect: Generation of large-scale magnetic fields by the small-scale dynamo

DOE PAGES

Squire, J.; Bhattacharjee, A.

2016-03-14

A novel large-scale dynamo mechanism, the magnetic shear-current effect, is discussed and explored. Here, the effect relies on the interaction of magnetic fluctuations with a mean shear flow, meaning the saturated state of the small-scale dynamo can drive a large-scale dynamo – in some sense the inverse of dynamo quenching. The dynamo is non-helical, with the mean fieldmore » $${\\it\\alpha}$$coefficient zero, and is caused by the interaction between an off-diagonal component of the turbulent resistivity and the stretching of the large-scale field by shear flow. Following up on previous numerical and analytic work, this paper presents further details of the numerical evidence for the effect, as well as an heuristic description of how magnetic fluctuations can interact with shear flow to produce the required electromotive force. The pressure response of the fluid is fundamental to this mechanism, which helps explain why the magnetic effect is stronger than its kinematic cousin, and the basic idea is related to the well-known lack of turbulent resistivity quenching by magnetic fluctuations. As well as being interesting for its applications to general high Reynolds number astrophysical turbulence, where strong small-scale magnetic fluctuations are expected to be prevalent, the magnetic shear-current effect is a likely candidate for large-scale dynamo in the unstratified regions of ionized accretion disks. Evidence for this is discussed, as well as future research directions and the challenges involved with understanding details of the effect in astrophysically relevant regimes.« less

Disorder in the Disk: The Influence of Accretion Disk Thickness on the Large-scale Magnetic Dynamo.

NASA Astrophysics Data System (ADS)

Hogg, J. Drew; Reynolds, Christopher S.

2018-01-01

The evolution of the magnetic field from the enigmatic large-scale dynamo is often considered a central feature of the accretion disk around a black hole. The resulting low-frequency oscillations introduced from the growth and decay of the field strength, along with the change in field orientation, are thought to be intimately tied to variability from the disk. Several factors are at play, but the dynamo can either be directly tied to observable signatures through modulation of the heating rate, or indirectly as the source of quasiperiodic oscillations, the driver of nonlinear structure from propagating fluctuations in mass accretion rate, or even the trigger of state transitions. We present a selection of results from a recent study of this process using a suite of four global, high-resolution, MHD accretion disk simulations. We systematically vary the scale height ratio and find the large-scale dynamo fails to develop above a scale height ratio of h/r ≥ 0.2. Using “butterfly” diagrams of the azimuthal magnetic field, we show the large-scale dynamo exists in the thinner accretion disk models, but fails to excite when the scale height ratio is increased, a feature which is also reflected in 2D Fourier transforms. Additionally, we calculate the dynamo α-parameter through correlations in the averaged magnetic field and turbulent electromotive force, and also generate synthetic light curves from the disk cooling. Using our emission proxy, we find the disks have markedly different characters as photometric fluctuations are larger and less ordered when the disk is thicker and the dynamo is absent.

Identification of vortexes obstructing the dynamo mechanism in laboratory experiments

NASA Astrophysics Data System (ADS)

Limone, A.; Hatch, D. R.; Forest, C. B.; Jenko, F.

2013-06-01

The magnetohydrodynamic dynamo effect explains the generation of self-sustained magnetic fields in electrically conducting flows, especially in geo- and astrophysical environments. Yet the details of this mechanism are still unknown, e.g., how and to which extent the geometry, the fluid topology, the forcing mechanism, and the turbulence can have a negative effect on this process. We report on numerical simulations carried out in spherical geometry, analyzing the predicted velocity flow with the so-called singular value decomposition, a powerful technique that allows us to precisely identify vortexes in the flow which would be difficult to characterize with conventional spectral methods. We then quantify the contribution of these vortexes to the growth rate of the magnetic energy in the system. We identify an axisymmetric vortex, whose rotational direction changes periodically in time, and whose dynamics are decoupled from those of the large scale background flow, that is detrimental for the dynamo effect. A comparison with experiments is carried out, showing that similar dynamics were observed in cylindrical geometry. These previously unexpected eddies, which impede the dynamo effect, offer an explanation for the experimental difficulties in attaining a dynamo in spherical geometry.

Mericitabine and Either Boceprevir or Telaprevir in Combination with Peginterferon Alfa-2a plus Ribavirin for Patients with Chronic Hepatitis C Genotype 1 Infection and Prior Null Response: The Randomized DYNAMO 1 and DYNAMO 2 Studies.

PubMed

Wedemeyer, Heiner; Forns, Xavier; Hézode, Christophe; Lee, Samuel S; Scalori, Astrid; Voulgari, Athina; Le Pogam, Sophie; Nájera, Isabel; Thommes, James A

2016-01-01

Most patients with chronic hepatitis C virus (HCV) genotype 1 infection who have had a previous null response (<2-log10 reduction in HCV RNA by treatment week 12) to peginterferon/ribavirin (PegIFN/RBV) do not achieve a sustained virological response (SVR) when re-treated with a first-generation HCV protease inhibitor (PI) administered in combination with PegIFN/RBV. We studied the incremental benefits associated with adding mericitabine (nucleoside analog inhibitor of HCV polymerase) to PI plus PegIFN alfa-2a/RBV-based therapy in two double-blind randomized multicenter phase 2 trials (with boceprevir in DYNAMO 1, and with telaprevir in DYNAMO 2). The primary endpoint in both trials was SVR, defined as HCV RNA <25 IU/mL 12 weeks after the end of treatment (SVR12). Overall, the addition of mericitabine to PI plus PegIFN alfa-2a/RBV therapy resulted in SVR12 rates of 60-70% in DYNAMO 1 and of 71-96% in DYNAMO 2. SVR12 rates were similar in patients infected with HCV genotype 1a and 1b in both trials. The placebo control arms in both studies were stopped because of high rates of virological failure. Numerically lower relapse rates were associated with longer treatment with mericitabine (24 versus 12 weeks), telaprevir-containing regimens, and regimens that included 48 weeks of PegIFN alfa-2a/RBV therapy. No mericitabine resistance mutations were identified in any patient in either trial. The addition of mericitabine did not add to the safety burden associated with either telaprevir or boceprevir-based regimens. These studies demonstrate increased SVR rates and reduced relapse rates in difficult-to-treat patients when a nucleoside polymerase inhibitor with intermediate antiviral potency is added to regimens containing a first-generation PI. ClinicalTrials.gov NCT01482403 and ClinicalTrials.gov NCT01482390.

Magnetic Flux Concentrations in Stratified Turbulent Plasma Due to Negative Effective Magnetic Pressure Instability

NASA Astrophysics Data System (ADS)

Jabbari, Sarah

2015-08-01

We study a system of a highly stratified turbulent plasma. In such a system, when the magnetic Reynolds number is large enough and there is a background field of suitable strength, a new effect will play role in con- centrating magnetic fields such that it leads to the formation of magnetic spots and bipolar regions. This effect is due to the fact that the turbu- lent pressure is suppressed by the large-scale magnetic field, which adds a negative term to the total mean-field (effective) pressure. This leads to an instability, which is known as the negative effective magnetic pressure instability (NEMPI). Direct numerical simulations (DNS) of isothermally forced turbulence have shown that NEMPI leads to the formation of spots in the presence of an imposed field. Our main aim now is to use NEMPI to explain the formation of active regions and sunspots. To achieve this goal, we need to move progressively to more realistic models. Here we extend our model by allowing the magnetic field to be generated by a dy- namo. A dynamo plays an important role in solar activity. Therefore, it is of interest to investigate NEMPI in the presence of dynamo-generated magnetic fields. Mean-field simulations (MFS) of such systems in spheri- cal geometry have shown how these two instabilities work in concert. In fact NEMPI will be activated as long as the strength of the magnetic field generated by the dynamo is in a proper range (for more detail see Jab- bari et al. 2013). In our new study, we use DNS to investigate a similar system. The turbulence is forced in the entire spherical shell, but the forc- ing is made helical in the lower 30% of the shell, similar to the model of Mitra et al. (2014). We perform simulations using the Pencil Code for different density contrasts and other input parameters. We applied ver- tical field boundary conditions in the r direction. The results show that, when the stratification is high enough, intense bipolar regions form and as time passes, they expand, merge and create giant structures. At the same time, the new structures appear at different latitudes. By extending in φ direction, the size of the bipolar regions decreases. When the helical zone is thinner, the structures appear at a later time.

The nonlinear differential equations governing a hierarchy of self-exciting coupled Faraday-disk homopolar dynamos

NASA Astrophysics Data System (ADS)

Hide, Raymond

1997-02-01

This paper discusses the derivation of the autonomous sets of dimensionless nonlinear ordinary differential equations (ODE's) that govern the behaviour of a hierarchy of related electro-mechanical self-exciting Faraday-disk homopolar dynamo systems driven by steady mechanical couples. Each system comprises N interacting units which could be arranged in a ring or lattice. Within each unit and connected in parallel or in series with the coil are electric motors driven into motion by the dynamo, all having linear characteristics, so that nonlinearity arises entirely through the coupling between components. By introducing simple extra terms into the equations it is possible to represent biasing effects arising from impressed electromotive forces due to thermoelectric or chemical processes and from the presence of ambient magnetic fields. Dissipation in the system is due not only to ohmic heating but also to mechanical friction in the disk and the motors, with the latter agency, no matter how weak, playing an unexpectedly crucial rôle in the production of régimes of chaotic behaviour. This has already been demonstrated in recent work on a case of a single unit incorporating just one series motor, which is governed by a novel autonomous set of nonlinear ODE's with three time-dependent variables and four control parameters. It will be of mathematical as well as geophysical and astrophysical interest to investigate systematically phase and amplitude locking and other types of behaviour in the more complicated cases that arise when N > 1, which can typically involve up to 6 N dependent variables and 19 N-5 control parameters. Even the simplest members of the hierarchy, with N as low as 1, 2 or 3, could prove useful as physically-realistic low-dimensional models in theoretical studies of fluctuating stellar and planetary magnetic fields. Geomagnetic polarity reversals could be affected by the presence of the Earth's solid metallic inner core, driven like an electric motor by currents generated by self-exciting magnetohydrodynamic (MHD) dynamo action involving motional induction associated with buoyancy-driven flow in the liquid metallic outer core. The study of biased disk dynamos could bear on the theory of the magnetic fields of natural systems where a significant background field is present (e.g., Galilean satellites of Jupiter) or when the action of motional induction is modified by electromotive forces produced by other mechanisms, such as thermoelectric processes, as in certain stars.

Short-term variability in the ionosphere due to the nonlinear interaction between the 6 day wave and migrating tides

NASA Astrophysics Data System (ADS)

Gan, Quan; Oberheide, Jens; Yue, Jia; Wang, Wenbin

2017-08-01

Using the thermosphere-ionosphere-mesosphere electrodynamics general circulation model simulations, we investigate the short-term ionospheric variability due to the child waves and altered tides produced by the nonlinear interaction between the 6 day wave and migrating tides. Via the Fourier spectral diagnostics and least squares fittings, the [21 h, W2] and [13 h, W1] child waves, generated by the interaction of the 6 day wave with the DW1 and SW2, respectively, are found to play the leading roles on the subdiurnal variability (e.g., ±10 m/s in the ion drift and 50% in the NmF2) in the F region vertical ion drift changes through the dynamo modulation induced by the low-latitude zonal wind and the meridional wind at higher latitudes. The relatively minor contribution of the [11 h, W3] child wave is explicit as well. Although the [29 h, W0] child wave has the largest magnitude in the E region, its effect is totally absent in the vertical ion drift due to the zonally uniform structure. But the [29 h, W0] child wave shows up in the NmF2. It is found that the NmF2 short-term variability is attributed to the wave modulations on both E region dynamo and in situ F region composition. Also, the altered migrating tides due to the interaction will not contribute to the ionospheric changes significantly.

Understanding lunar magnetic field through magnetization and dynamo mechanism

NASA Astrophysics Data System (ADS)

Singh, K. H.; Kuang, W.

2016-12-01

It has been known that the Moon does not have an active global magnetic field. But past missions to the Moon (e.g. Apollo missions, Lunar Prospector) have detected magnetic anomalies in many areas on the lunar surface. They carry rich information about geophysical processes on and within the Moon, thus central for understanding the structure and dynamics in the interior, e.g. the core and the suggested magma ocean. One unsettling problem for understanding the lunar magnetic anomaly is its origin. There have been several mechanisms suggested in the past, either on the anomalies in specific regions, or only at the conceptual stage. The latter include the paleo dynamo. The lunar dynamo mechanism is conceptually very simple: lunar crustal magnetization was acquired in an internal magnetic field that was generated and maintained by dynamo action in the lunar core. Could this simple mechanism suffice to explain most of the observed lunar magnetic anomalies? We present our theoretical calculations of possible paleo-lunar magnetic field strengths based on paleomagnetic measurements of Apollo samples.

Modeling the Solar Convective Dynamo and Emerging Flux

NASA Astrophysics Data System (ADS)

Fan, Y.

2017-12-01

Significant advances have been made in recent years in global-scale fully dynamic three-dimensional convective dynamo simulations of the solar/stellar convective envelopes to reproduce some of the basic features of the Sun's large-scale cyclic magnetic field. It is found that the presence of the dynamo-generated magnetic fields plays an important role for the maintenance of the solar differential rotation, without which the differential rotation tends to become anti-solar (with a faster rotating pole instead of the observed faster rotation at the equator). Convective dynamo simulations are also found to produce emergence of coherent super-equipartition toroidal flux bundles with a statistically significant mean tilt angle that is consistent with the mean tilt of solar active regions. The emerging flux bundles are sheared by the giant cell convection into a forward leaning loop shape with its leading side (in the direction of rotation) pushed closer to the strong downflow lanes. Such asymmetric emerging flux pattern may lead to the observed asymmetric properties of solar active regions.

Effect of soft-iron impellers on the von Kármán-sodium dynamo.

PubMed

Xu, Mingtian

2014-01-01

The explanation for the observed axisymmetric magnetic field in the von Kármán-sodium (VKS) dynamo experiment is still an unresolved question. In this paper, the integral equation approach is extended to investigate the VKS dynamo action by taking into account the discontinuity of the magnetic permeability and electrical conductivity in the conducting region. When the relative magnetic permeability of the soft-iron impellers is set to 65, a steady toroidal field that is apparently axisymmetric is excited at the critical magnetic Reynolds number, Rmc≈27.23, which is close to the experimental result, Rmc≈30. Our results show that the critical magnetic Reynolds number declines as the relative magnetic permeability of the impellers increases. Furthermore, when the relative magnetic permeability is not greater than 37, an equatorial magnetic field with an azimuthal wave number m=1 is the dominant mode, otherwise a steady toroidal field with an azimuthal wave number m=0 predominates the magnetic field generated by the VKS dynamo action.

Faraday rotation signatures of fluctuation dynamos in young galaxies

NASA Astrophysics Data System (ADS)

Sur, Sharanya; Bhat, Pallavi; Subramanian, Kandaswamy

2018-03-01

Observations of Faraday rotation through high-redshift galaxies have revealed that they host coherent magnetic fields that are of comparable strengths to those observed in nearby galaxies. These fields could be generated by fluctuation dynamos. We use idealized numerical simulations of such dynamos in forced compressible turbulence up to rms Mach number of 2.4 to probe the resulting rotation measure (RM) and the degree of coherence of the magnetic field. We obtain rms values of RM at dynamo saturation of the order of 45-55 per cent of the value expected in a model where fields are assumed to be coherent on the forcing scale of turbulence. We show that the dominant contribution to the RM in subsonic and transonic cases comes from the general sea of volume filling fields, rather than from the rarer structures. However, in the supersonic case, strong field regions as well as moderately overdense regions contribute significantly. Our results can account for the observed RMs in young galaxies.

ON THE CAUSE OF SOLAR-LIKE EQUATORWARD MIGRATION IN GLOBAL CONVECTIVE DYNAMO SIMULATIONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Warnecke, Jörn; Käpylä, Petri J.; Käpylä, Maarit J.

2014-11-20

We present results from four convectively driven stellar dynamo simulations in spherical wedge geometry. All of these simulations produce cyclic and migrating mean magnetic fields. Through detailed comparisons, we show that the migration direction can be explained by an αΩ dynamo wave following the Parker-Yoshimura rule. We conclude that the equatorward migration in this and previous work is due to a positive (negative) α effect in the northern (southern) hemisphere and a negative radial gradient of Ω outside the inner tangent cylinder of these models. This idea is supported by a strong correlation between negative radial shear and toroidal fieldmore » strength in the region of equatorward propagation.« less

Dynamo Enhancement and Mode Selection Triggered by High Magnetic Permeability.

PubMed

Kreuzahler, S; Ponty, Y; Plihon, N; Homann, H; Grauer, R

2017-12-08

We present results from consistent dynamo simulations, where the electrically conducting and incompressible flow inside a cylinder vessel is forced by moving impellers numerically implemented by a penalization method. The numerical scheme models jumps of magnetic permeability for the solid impellers, resembling various configurations tested experimentally in the von Kármán sodium experiment. The most striking experimental observations are reproduced in our set of simulations. In particular, we report on the existence of a time-averaged axisymmetric dynamo mode, self-consistently generated when the magnetic permeability of the impellers exceeds a threshold. We describe a possible scenario involving both the turbulent flow in the vicinity of the impellers and the high magnetic permeability of the impellers.

Intermittent behavior of galactic dynamo activities

NASA Technical Reports Server (NTRS)

Ko, C. M.; Parker, E. N.

1989-01-01

Recent observations by Beck and Golla of far-infrared and radio continuum emission from nearby spiral galaxies suggest that the galactic magnetic field strength is connected to the current star formation rate. The role of star formation on the generation of large-scale galactic magnetic field is studied in this paper. Using a simple galactic model, it is shown how the galactic dynamo depends strongly on the turbulent velocity of the interstellar medium. When the star formation efficiency is high, the ISM is churned which in turn amplifies the galactic magnetic field. Between active star formation epochs, the magnetic field is in dormant state and decays at a negligible rate. If density waves trigger star formation, then they also turn on the otherwise dormant dynamo.

The Invariant Twist of Magnetic Fields in the Relativistic Jets of Active Galactic Nuclei

NASA Technical Reports Server (NTRS)

Contopoulos, Ioannis; Christodoulou, Dimitris M.; Kazanas, Demosthenes; Gabuzda, Denise C.

2009-01-01

The origin of cosmic magnetic (B) fields remains an open question. It is generally believed that very weak primordial B fields are amplified by dynamo processes, but it appears unlikely that the amplification proceeds fast enough to account for the fields presently observed in galaxies and galaxy clusters. In an alternative scenario, cosmic B fields are generated near the inner edges of accretion disks in Active Galactic Nuclei (AGNs) by azimuthal electric currents due to the difference between the plasma electron and ion velocities that arises when the electrons are retarded by interactions with photons. While dynamo processes show no preference for the polarity of the (presumably random) seed field that they amplify, this alternative mechanism uniquely relates the polarity of the poloidal B field to the angular velocity of the accretion disk, resulting in a unique direction for the toroidal B field induced by disk rotation. Observations of the toroidal fields of 29 AGN jets revealed by parsec-scale Faraday rotation measurements show a clear asymmetry that is consistent with this model, with the probability that this asymmetry came about by chance being less than 1 %. This lends support to the hypothesis that the Universe is seeded by B fields that are generated in AGN via this mechanism

Health Impacts of Increased Physical Activity from Changes in Transportation Infrastructure: Quantitative Estimates for Three Communities

PubMed Central

2015-01-01

Recently, two quantitative tools have emerged for predicting the health impacts of projects that change population physical activity: the Health Economic Assessment Tool (HEAT) and Dynamic Modeling for Health Impact Assessment (DYNAMO-HIA). HEAT has been used to support health impact assessments of transportation infrastructure projects, but DYNAMO-HIA has not been previously employed for this purpose nor have the two tools been compared. To demonstrate the use of DYNAMO-HIA for supporting health impact assessments of transportation infrastructure projects, we employed the model in three communities (urban, suburban, and rural) in North Carolina. We also compared DYNAMO-HIA and HEAT predictions in the urban community. Using DYNAMO-HIA, we estimated benefit-cost ratios of 20.2 (95% C.I.: 8.7–30.6), 0.6 (0.3–0.9), and 4.7 (2.1–7.1) for the urban, suburban, and rural projects, respectively. For a 40-year time period, the HEAT predictions of deaths avoided by the urban infrastructure project were three times as high as DYNAMO-HIA's predictions due to HEAT's inability to account for changing population health characteristics over time. Quantitative health impact assessment coupled with economic valuation is a powerful tool for integrating health considerations into transportation decision-making. However, to avoid overestimating benefits, such quantitative HIAs should use dynamic, rather than static, approaches. PMID:26504832

Health Impacts of Increased Physical Activity from Changes in Transportation Infrastructure: Quantitative Estimates for Three Communities.

PubMed

Mansfield, Theodore J; MacDonald Gibson, Jacqueline

2015-01-01

Recently, two quantitative tools have emerged for predicting the health impacts of projects that change population physical activity: the Health Economic Assessment Tool (HEAT) and Dynamic Modeling for Health Impact Assessment (DYNAMO-HIA). HEAT has been used to support health impact assessments of transportation infrastructure projects, but DYNAMO-HIA has not been previously employed for this purpose nor have the two tools been compared. To demonstrate the use of DYNAMO-HIA for supporting health impact assessments of transportation infrastructure projects, we employed the model in three communities (urban, suburban, and rural) in North Carolina. We also compared DYNAMO-HIA and HEAT predictions in the urban community. Using DYNAMO-HIA, we estimated benefit-cost ratios of 20.2 (95% C.I.: 8.7-30.6), 0.6 (0.3-0.9), and 4.7 (2.1-7.1) for the urban, suburban, and rural projects, respectively. For a 40-year time period, the HEAT predictions of deaths avoided by the urban infrastructure project were three times as high as DYNAMO-HIA's predictions due to HEAT's inability to account for changing population health characteristics over time. Quantitative health impact assessment coupled with economic valuation is a powerful tool for integrating health considerations into transportation decision-making. However, to avoid overestimating benefits, such quantitative HIAs should use dynamic, rather than static, approaches.

STELLAR DYNAMOS AND CYCLES FROM NUMERICAL SIMULATIONS OF CONVECTION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dubé, Caroline; Charbonneau, Paul, E-mail: dube@astro.umontreal.ca, E-mail: paulchar@astro.umontreal.ca

We present a series of kinematic axisymmetric mean-field αΩ dynamo models applicable to solar-type stars, for 20 distinct combinations of rotation rates and luminosities. The internal differential rotation and kinetic helicity profiles required to calculate source terms in these dynamo models are extracted from a corresponding series of global three-dimensional hydrodynamical simulations of solar/stellar convection, so that the resulting dynamo models end up involving only one free parameter, namely, the turbulent magnetic diffusivity in the convecting layers. Even though the αΩ dynamo solutions exhibit a broad range of morphologies, and sometimes even double cycles, these models manage to reproduce relativelymore » well the observationally inferred relationship between cycle period and rotation rate. On the other hand, they fail in capturing the observed increase of magnetic activity levels with rotation rate. This failure is due to our use of a simple algebraic α-quenching formula as the sole amplitude-limiting nonlinearity. This suggests that α-quenching is not the primary mechanism setting the amplitude of stellar magnetic cycles, with magnetic reaction on large-scale flows emerging as the more likely candidate. This inference is coherent with analyses of various recent global magnetohydrodynamical simulations of solar/stellar convection.« less

Stochastic flux freezing and magnetic dynamo.

PubMed

Eyink, Gregory L

2011-05-01

Magnetic flux conservation in turbulent plasmas at high magnetic Reynolds numbers is argued neither to hold in the conventional sense nor to be entirely broken, but instead to be valid in a statistical sense associated to the "spontaneous stochasticity" of Lagrangian particle trajectories. The latter phenomenon is due to the explosive separation of particles undergoing turbulent Richardson diffusion, which leads to a breakdown of Laplacian determinism for classical dynamics. Empirical evidence is presented for spontaneous stochasticity, including numerical results. A Lagrangian path-integral approach is then exploited to establish stochastic flux freezing for resistive hydromagnetic equations and to argue, based on the properties of Richardson diffusion, that flux conservation must remain stochastic at infinite magnetic Reynolds number. An important application of these results is the kinematic, fluctuation dynamo in nonhelical, incompressible turbulence at magnetic Prandtl number (Pr(m)) equal to unity. Numerical results on the Lagrangian dynamo mechanisms by a stochastic particle method demonstrate a strong similarity between the Pr(m)=1 and 0 dynamos. Stochasticity of field-line motion is an essential ingredient of both. Finally, some consequences for nonlinear magnetohydrodynamic turbulence, dynamo, and reconnection are briefly considered. © 2011 American Physical Society

Simulations of Magnetic Flux Emergence in Cool, Low-Mass Stars: Toward Linking Dynamo Action with Starspots

NASA Astrophysics Data System (ADS)

Weber, Maria Ann; Browning, Matthew; Nelson, Nicholas

2018-01-01

Starspots are windows into a star’s internal dynamo mechanism. However, the manner by which the dynamo-generated magnetic field traverses the stellar interior to emerge at the surface is not especially well understood. Establishing the details of magnetic flux emergence plays a key role in deciphering stellar dynamos and observed starspot properties. In the solar context, insight into this process has been obtained by assuming the magnetism giving rise to sunspots consists partly of idealized thin flux tubes (TFTs). Here, we present three sets of TFT simulations in rotating spherical shells of convection: one representative of the Sun, the second of a solar-like rapid rotator, and the third of a fully convective M dwarf. Our solar simulations reproduce sunspot observables such as low-latitude emergence, tilting action toward the equator following the Joy’s Law trend, and a phenomenon akin to active longitudes. Further, we compare the evolution of rising flux tubes in our (computationally inexpensive) TFT simulations to buoyant magnetic structures that arise naturally in a unique global simulation of a rapidly rotating Sun. We comment on the role of rapid rotation, the Coriolis force, and external torques imparted by the surrounding convection in establishing the trajectories of the flux tubes across the convection zone. In our fully convective M dwarf simulations, the expected starspot latitudes deviate from the solar trend, favoring significantly poleward latitudes unless the differential rotation is sufficiently prograde or the magnetic field is strongly super-equipartition. Together our work provides a link between dynamo-generated magnetic fields, turbulent convection, and observations of starspots along the lower main sequence.

26. Port side of engine room looking forward from aft ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

26. Port side of engine room looking forward from aft bulkhead. This area contains mostly electrical equipment. Two single-cylinder steam-driven dynamos are located near the engine bed, one at right foreground, the other in background. At left in image are a motor-generator set installed to convert DC current (from dynamos) to AC current. Edge-on view of control panel appears near center of image. - Ferry TICONDEROGA, Route 7, Shelburne, Chittenden County, VT

The Madison Dynamo Experiment

NASA Astrophysics Data System (ADS)

Bastian, N.; O'Connell, R.; Kendrick, R.; Goldwin, J.; Forest, C. B.

1998-11-01

A liquid metal magneto-hydrodynamic (MHD) experiment at the University of Wisconsin is being constructed in order to validate 3 key elements of MHD dynamo theory: magnetic instabilities driven by flow shear, the effects of turbulence on current generation (primarily the α and β effects) and the nature of saturation for these on these processes. The experiment consists of two main stages, the first of which uses water to test impeller designs that are used to generate flows capable of supporting a dynamo. Since water has nearly the same viscosity and mass density as sodium, it is the ideal substance with which to test our impeller designs. The second stage of the experiment uses a one meter diameter sphere filled with ≈ 200 gallons of liquid sodium to directly test MHD theory. Impellers will be used to impose flows on the liquid sodium that are predicted by MHD theory to lead to a growing magnetic field. In addition, large scale flows will lead to small-scale turbulence which can produce a dynamo effect and a current. This is known as the turbulent α-effect which we will attempt to observe. The MHD theory also predicts an anomalously high resistivity or magnetic diffusivity (the β-effect). Once a growing magnetic field is present it should be possible to measure the effect that the growing magnetic field has on the flow that created it.

Constraining Substellar Magnetic Dynamos using Brown Dwarf Radio Aurorae

NASA Astrophysics Data System (ADS)

Kao, Melodie Minyu

Brown dwarfs share characteristics with both low-mass stars and gas giant planets, making them useful laboratories for studying physics occurring in objects throughout this low mass and temperature range. Of particular interest in this dissertation is the nature of the engine driving their magnetic fields. Fully convective magnetic dynamos can operate in low mass stars, brown dwarfs, gas giant planets, and even fluid metal cores in small rocky planets. Objects in this wide mass range are capable of hosting strong magnetic fields, which shape much of the evolution of planets and stars: strong fields can protect planetary atmospheres from evaporating, generate optical and infrared emission that masquerade as clouds in the atmospheres of other worlds, and affect planet formation mechanisms. Thus, implications from understanding convective dynamo mechanisms also extend to exoplanet habitability. How the convective dynamos driving these fields operate remains an important open problem. While we have extensive data to inform models of magnetic dynamo mechanisms in higher mass stars like our Sun, the coolest and lowest-mass objects that probe the substellar-planetary boundary do not possess the internal structures necessary to drive solar-type dynamos. A number of models examining fully convective dynamo mechanisms have been proposed but they remain unconstrained by magnetic field measurements in the lowest end of the substellar mass and temperature space. Detections of highly circularly polarized pulsed radio emission provide our only window into magnetic field measurements for objects in the ultracool brown dwarf regime, but these detections are very rare; until this dissertation, only one attempt out of 60 had been successful. The work presented in this dissertation seeks to address this problem and examines radio emission from late L, T, and Y spectral type brown dwarfs spanning 1-6 times the surface temperature of Earth and explores implications for fully convective magnetic dynamo models.

Turbulent dynamo in a collisionless plasma

NASA Astrophysics Data System (ADS)

Rincon, François; Califano, Francesco; Schekochihin, Alexander A.; Valentini, Francesco

2016-04-01

Magnetic fields pervade the entire universe and affect the formation and evolution of astrophysical systems from cosmological to planetary scales. The generation and dynamical amplification of extragalactic magnetic fields through cosmic times (up to microgauss levels reported in nearby galaxy clusters, near equipartition with kinetic energy of plasma motions, and on scales of at least tens of kiloparsecs) are major puzzles largely unconstrained by observations. A dynamo effect converting kinetic flow energy into magnetic energy is often invoked in that context; however, extragalactic plasmas are weakly collisional (as opposed to magnetohydrodynamic fluids), and whether magnetic field growth and sustainment through an efficient turbulent dynamo instability are possible in such plasmas is not established. Fully kinetic numerical simulations of the Vlasov equation in a 6D-phase space necessary to answer this question have, until recently, remained beyond computational capabilities. Here, we show by means of such simulations that magnetic field amplification by dynamo instability does occur in a stochastically driven, nonrelativistic subsonic flow of initially unmagnetized collisionless plasma. We also find that the dynamo self-accelerates and becomes entangled with kinetic instabilities as magnetization increases. The results suggest that such a plasma dynamo may be realizable in laboratory experiments, support the idea that intracluster medium turbulence may have significantly contributed to the amplification of cluster magnetic fields up to near-equipartition levels on a timescale shorter than the Hubble time, and emphasize the crucial role of multiscale kinetic physics in high-energy astrophysical plasmas.

Turbulent dynamo in a collisionless plasma

PubMed Central

Rincon, François; Califano, Francesco; Schekochihin, Alexander A.; Valentini, Francesco

2016-01-01

Magnetic fields pervade the entire universe and affect the formation and evolution of astrophysical systems from cosmological to planetary scales. The generation and dynamical amplification of extragalactic magnetic fields through cosmic times (up to microgauss levels reported in nearby galaxy clusters, near equipartition with kinetic energy of plasma motions, and on scales of at least tens of kiloparsecs) are major puzzles largely unconstrained by observations. A dynamo effect converting kinetic flow energy into magnetic energy is often invoked in that context; however, extragalactic plasmas are weakly collisional (as opposed to magnetohydrodynamic fluids), and whether magnetic field growth and sustainment through an efficient turbulent dynamo instability are possible in such plasmas is not established. Fully kinetic numerical simulations of the Vlasov equation in a 6D-phase space necessary to answer this question have, until recently, remained beyond computational capabilities. Here, we show by means of such simulations that magnetic field amplification by dynamo instability does occur in a stochastically driven, nonrelativistic subsonic flow of initially unmagnetized collisionless plasma. We also find that the dynamo self-accelerates and becomes entangled with kinetic instabilities as magnetization increases. The results suggest that such a plasma dynamo may be realizable in laboratory experiments, support the idea that intracluster medium turbulence may have significantly contributed to the amplification of cluster magnetic fields up to near-equipartition levels on a timescale shorter than the Hubble time, and emphasize the crucial role of multiscale kinetic physics in high-energy astrophysical plasmas. PMID:27035981

Turbulent dynamo in a collisionless plasma.

PubMed

Rincon, François; Califano, Francesco; Schekochihin, Alexander A; Valentini, Francesco

2016-04-12

Magnetic fields pervade the entire universe and affect the formation and evolution of astrophysical systems from cosmological to planetary scales. The generation and dynamical amplification of extragalactic magnetic fields through cosmic times (up to microgauss levels reported in nearby galaxy clusters, near equipartition with kinetic energy of plasma motions, and on scales of at least tens of kiloparsecs) are major puzzles largely unconstrained by observations. A dynamo effect converting kinetic flow energy into magnetic energy is often invoked in that context; however, extragalactic plasmas are weakly collisional (as opposed to magnetohydrodynamic fluids), and whether magnetic field growth and sustainment through an efficient turbulent dynamo instability are possible in such plasmas is not established. Fully kinetic numerical simulations of the Vlasov equation in a 6D-phase space necessary to answer this question have, until recently, remained beyond computational capabilities. Here, we show by means of such simulations that magnetic field amplification by dynamo instability does occur in a stochastically driven, nonrelativistic subsonic flow of initially unmagnetized collisionless plasma. We also find that the dynamo self-accelerates and becomes entangled with kinetic instabilities as magnetization increases. The results suggest that such a plasma dynamo may be realizable in laboratory experiments, support the idea that intracluster medium turbulence may have significantly contributed to the amplification of cluster magnetic fields up to near-equipartition levels on a timescale shorter than the Hubble time, and emphasize the crucial role of multiscale kinetic physics in high-energy astrophysical plasmas.

21. Generator Room (basement beneath Blacksmith Shop): view looking north ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

21. Generator Room (basement beneath Blacksmith Shop): view looking north showing remains of 1923 six-cylinder Studebaker engine and a dynamo - Ben Thresher's Mill, State Aid No. 1, Barnet, Caledonia County, VT

Growth rate degeneracies in kinematic dynamos

NASA Astrophysics Data System (ADS)

Favier, B.; Proctor, M. R. E.

2013-09-01

We consider the classical problem of kinematic dynamo action in simple steady flows. Due to the adjointness of the induction operator, we show that the growth rate of the dynamo will be exactly the same for two types of magnetic boundary conditions: the magnetic field can be normal (infinite magnetic permeability, also called pseudovacuum) or tangent (perfect electrical conductor) to the boundaries of the domain. These boundary conditions correspond to well-defined physical limits often used in numerical models and relevant to laboratory experiments. The only constraint is for the velocity field u to be reversible, meaning there exists a transformation changing u into -u. We illustrate this surprising property using S2T2 type of flows in spherical geometry inspired by [Dudley and James, Proc. R. Soc. London A1364-502110.1098/rspa.1989.0112 425, 407 (1989)]. Using both types of boundary conditions, it is shown that the growth rates of the dynamos are identical, although the corresponding magnetic eigenmodes are drastically different.

Magnetic field generation by pointwise zero-helicity three-dimensional steady flow of an incompressible electrically conducting fluid

NASA Astrophysics Data System (ADS)

Rasskazov, Andrey; Chertovskih, Roman; Zheligovsky, Vladislav

2018-04-01

We introduce six families of three-dimensional space-periodic steady solenoidal flows, whose kinetic helicity density is zero at any point. Four families are analytically defined. Flows in four families have zero helicity spectrum. Sample flows from five families are used to demonstrate numerically that neither zero kinetic helicity density nor zero helicity spectrum prohibit generation of large-scale magnetic field by the two most prominent dynamo mechanisms: the magnetic α -effect and negative eddy diffusivity. Our computations also attest that such flows often generate small-scale field for sufficiently small magnetic molecular diffusivity. These findings indicate that kinetic helicity and helicity spectrum are not the quantities controlling the dynamo properties of a flow regardless of whether scale separation is present or not.

Magnetorotational Dynamo Action in the Shearing Box

NASA Astrophysics Data System (ADS)

Walker, Justin; Boldyrev, Stanislav

2017-10-01

Magnetic dynamo action caused by the magnetorotational instability is studied in the shearing-box approximation with no imposed net magnetic flux. Consistent with recent studies, the dynamo action is found to be sensitive to the aspect ratio of the box: it is much easier to obtain in tall boxes (stretched in the direction normal to the disk plane) than in long boxes (stretched in the radial direction). Our direct numerical simulations indicate that the dynamo is possible in both cases, given a large enough magnetic Reynolds number. To explain the relatively larger effort required to obtain the dynamo action in a long box, we propose that the turbulent eddies caused by the instability most efficiently fold and mix the magnetic field lines in the radial direction. As a result, in the long box the scale of the generated strong azimuthal (stream-wise directed) magnetic field is always comparable to the scale of the turbulent eddies. In contrast, in the tall box the azimuthal magnetic flux spreads in the vertical direction over a distance exceeding the scale of the turbulent eddies. As a result, different vertical sections of the tall box are permeated by large-scale nonzero azimuthal magnetic fluxes, facilitating the instability. NSF AGS-1261659, Vilas Associates Award, NSF-Teragrid Project TG-PHY110016.

Using Data Assimilation Methods of Prediction of Solar Activity

NASA Technical Reports Server (NTRS)

Kitiashvili, Irina N.; Collins, Nancy S.

2017-01-01

The variable solar magnetic activity known as the 11-year solar cycle has the longest history of solar observations. These cycles dramatically affect conditions in the heliosphere and the Earth's space environment. Our current understanding of the physical processes that make up global solar dynamics and the dynamo that generates the magnetic fields is sketchy, resulting in unrealistic descriptions in theoretical and numerical models of the solar cycles. The absence of long-term observations of solar interior dynamics and photospheric magnetic fields hinders development of accurate dynamo models and their calibration. In such situations, mathematical data assimilation methods provide an optimal approach for combining the available observational data and their uncertainties with theoretical models in order to estimate the state of the solar dynamo and predict future cycles. In this presentation, we will discuss the implementation and performance of an Ensemble Kalman Filter data assimilation method based on the Parker migratory dynamo model, complemented by the equation of magnetic helicity conservation and long-term sunspot data series. This approach has allowed us to reproduce the general properties of solar cycles and has already demonstrated a good predictive capability for the current cycle, 24. We will discuss further development of this approach, which includes a more sophisticated dynamo model, synoptic magnetogram data, and employs the DART Data Assimilation Research Testbed.

Early Estimation of Solar Activity Cycle: Potential Capability and Limits

NASA Technical Reports Server (NTRS)

Kitiashvili, Irina N.; Collins, Nancy S.

2017-01-01

The variable solar magnetic activity known as the 11-year solar cycle has the longest history of solar observations. These cycles dramatically affect conditions in the heliosphere and the Earth's space environment. Our current understanding of the physical processes that make up global solar dynamics and the dynamo that generates the magnetic fields is sketchy, resulting in unrealistic descriptions in theoretical and numerical models of the solar cycles. The absence of long-term observations of solar interior dynamics and photospheric magnetic fields hinders development of accurate dynamo models and their calibration. In such situations, mathematical data assimilation methods provide an optimal approach for combining the available observational data and their uncertainties with theoretical models in order to estimate the state of the solar dynamo and predict future cycles. In this presentation, we will discuss the implementation and performance of an Ensemble Kalman Filter data assimilation method based on the Parker migratory dynamo model, complemented by the equation of magnetic helicity conservation and longterm sunspot data series. This approach has allowed us to reproduce the general properties of solar cycles and has already demonstrated a good predictive capability for the current cycle, 24. We will discuss further development of this approach, which includes a more sophisticated dynamo model, synoptic magnetogram data, and employs the DART Data Assimilation Research Testbed.

Dynamo magnetic field-induced angular momentum transport in protostellar nebulae - The 'minimum mass' protosolar nebula

NASA Technical Reports Server (NTRS)

Stepinski, T. F.; Levy, E. H.

1990-01-01

Magnetic torques can produce angular momentum redistribution in protostellar nebulas. Dynamo magnetic fields can be generated in differentially rotating and turbulent nebulas and can be the source of magnetic torques that transfer angular momentum from a protostar to a disk, as well as redistribute angular momentum within a disk. A magnetic field strength of 100-1000 G is needed to transport the major part of a protostar's angular momentum into a surrounding disk in a time characteristic of star formation, thus allowing formation of a solar-system size protoplanetary nebula in the usual 'minimum-mass' model of the protosolar nebula. This paper examines the possibility that a dynamo magnetic field could have induced the needed angular momentum transport from the proto-Sun to the protoplanetary nebula.

Aurora on Uranus - A Faraday disc dynamo mechanism

NASA Technical Reports Server (NTRS)

Hill, T. W.; Rassbach, M. E.; Dessler, A. J.

1983-01-01

A mechanism is proposed whereby the solar wind flowing past the magnetosphere of Uranus causes a Faraday disk dynamo topology to be established and power to be extracted from the kinetic energy of rotation of Uranus. An immediate consequence of this dynamo is the generation of Birkeland currents that flow in and out of the sunlit polar cap with the accompanying production of polar aurora. The power extracted from planetary rotation is calculated as a function of planetary dipole magnetic moment and the ionospheric conductivity of Uranus. For plausible values of ionospheric conductivity, the observed auroral power requires a magnetic moment corresponding to a surface equatorial field of the order of 4 Gauss, slightly larger than the value 1.8 Gauss given by the empirical 'magnetic Bodes law'.

Dynamo room (compartment A21) with view of port side, art ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

Dynamo room (compartment A-21) with view of port side, art electrical generator in background. Note cables, speaking tubes and steering crank arm at top center of photograph. These rise through an armored tube to the Conning Tower. The electrical distribution board at left is a reproduction of the board as it may have looked in the 1920's. Reproduction was done in the 1970's and the 1980's. (015) - USS Olympia, Penn's Landing, 211 South Columbus Boulevard, Philadelphia, Philadelphia County, PA

Generation of electric fields and currents by neutral flows in weakly ionized plasmas through collisional dynamos

NASA Astrophysics Data System (ADS)

Dimant, Y. S.; Oppenheim, M. M.; Fletcher, A. C.

2016-08-01

In weakly ionized plasmas neutral flows drag plasma across magnetic field lines generating intense electric fields and currents. An example occurs in the Earth's ionosphere near the geomagnetic equator. Similar processes take place in the Solar chromosphere and magnetohydrodynamic generators. This paper argues that not all convective neutral flows generate electric fields and currents and it introduces the corresponding universal criterion for their formation, ∇×(U ×B )≠∂B /∂t , where U is the neutral flow velocity, B is the magnetic field, and t is time. This criterion does not depend on the conductivity tensor, σ ̂ . For many systems, the displacement current, ∂B /∂t , is negligible making the criterion even simpler. This theory also shows that the neutral-dynamo driver that generates E-fields and currents plays the same role as the DC electric current plays for the generation of the magnetic field in the Biot-Savart law.

Solar magnetic fields

NASA Astrophysics Data System (ADS)

Hood, Alan W.; Hughes, David W.

2011-08-01

This review provides an introduction to the generation and evolution of the Sun's magnetic field, summarising both observational evidence and theoretical models. The eleven year solar cycle, which is well known from a variety of observed quantities, strongly supports the idea of a large-scale solar dynamo. Current theoretical ideas on the location and mechanism of this dynamo are presented. The solar cycle influences the behaviour of the global coronal magnetic field and it is the eruptions of this field that can impact on the Earth's environment. These global coronal variations can be modelled to a surprising degree of accuracy. Recent high resolution observations of the Sun's magnetic field in quiet regions, away from sunspots, show that there is a continual evolution of a small-scale magnetic field, presumably produced by small-scale dynamo action in the solar interior. Sunspots, a natural consequence of the large-scale dynamo, emerge, evolve and disperse over a period of several days. Numerical simulations can help to determine the physical processes governing the emergence of sunspots. We discuss the interaction of these emerging fields with the pre-existing coronal field, resulting in a variety of dynamic phenomena.

CONSISTENT SCALING LAWS IN ANELASTIC SPHERICAL SHELL DYNAMOS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yadav, Rakesh K.; Gastine, Thomas; Christensen, Ulrich R.

2013-09-01

Numerical dynamo models always employ parameter values that differ by orders of magnitude from the values expected in natural objects. However, such models have been successful in qualitatively reproducing properties of planetary and stellar dynamos. This qualitative agreement fuels the idea that both numerical models and astrophysical objects may operate in the same asymptotic regime of dynamics. This can be tested by exploring the scaling behavior of the models. For convection-driven incompressible spherical shell dynamos with constant material properties, scaling laws had been established previously that relate flow velocity and magnetic field strength to the available power. Here we analyzemore » 273 direct numerical simulations using the anelastic approximation, involving also cases with radius-dependent magnetic, thermal, and viscous diffusivities. These better represent conditions in gas giant planets and low-mass stars compared to Boussinesq models. Our study provides strong support for the hypothesis that both mean velocity and mean magnetic field strength scale as a function of the power generated by buoyancy forces in the same way for a wide range of conditions.« less

An impact-driven dynamo for the early Moon.

PubMed

Le Bars, M; Wieczorek, M A; Karatekin, O; Cébron, D; Laneuville, M

2011-11-09

The origin of lunar magnetic anomalies remains unresolved after their discovery more than four decades ago. A commonly invoked hypothesis is that the Moon might once have possessed a thermally driven core dynamo, but this theory is problematical given the small size of the core and the required surface magnetic field strengths. An alternative hypothesis is that impact events might have amplified ambient fields near the antipodes of the largest basins, but many magnetic anomalies exist that are not associated with basin antipodes. Here we propose a new model for magnetic field generation, in which dynamo action comes from impact-induced changes in the Moon's rotation rate. Basin-forming impact events are energetic enough to have unlocked the Moon from synchronous rotation, and we demonstrate that the subsequent large-scale fluid flows in the core, excited by the tidal distortion of the core-mantle boundary, could have powered a lunar dynamo. Predicted surface magnetic field strengths are on the order of several microteslas, consistent with palaeomagnetic measurements, and the duration of these fields is sufficient to explain the central magnetic anomalies associated with several large impact basins.

Convection and magnetic field generation in the interior of planets (August Love Medal Lecture)

NASA Astrophysics Data System (ADS)

Christensen, U. R.

2009-04-01

Thermal convection driven by internal energy plays a role of paramount importance in planetary bodies. Its numerical modeling has been an essential tool for understanding how the internal engine of a planet works. Solid state convection in the silicate or icy mantles is the cause of endogenic tectonic activity, volcanism and, in the case of Earth, of plate motion. It also regulates the energy budget of the entire planet, including that of its core, and controls the presence or absence of a dynamo. The complex rheology of solid minerals, effects of phase transitions, and chemical heterogeneity are important issues in mantle convection. Examples discussed here are the convection pattern in Mars and the complex morphology of subducted slabs that are observed by seismic tomography in the Earth's mantle. Internally driven convection in the deep gas envelopes of the giant planets is possibly the cause for the strong jet streams at the surfaces that give rise to their banded appearance. Modeling of the magnetohydrodynamic flow in the conducting liquid core of the Earth has been remarkably successful in reproducing the primary properties of the geomagnetic field. As an examplefor attempts to explain also secondary properties, I will discuss dynamo models that account for the thermal coupling to the mantle. The understanding of the somewhat enigmatic magnetic fields of some other planets is less advanced. Here I will show that dynamos that operate below a stable conducting layer in the upper part of the planetary core can explain the unusual magnetic field properties of Mercury and Saturn. The question what determines the strength of a dynamo-generated magnetic field has been a matter of debate. From a large set of numerical dynamo simulations that cover a fair range of control parameters, we find a rule that relates magnetic field strength to the part of the energy flux that is thermodynamically available to be transformed into other forms of energy. This rules predicts correctly not only the magnetic field strength of planets with sufficiently simple dynamos (Earth and Jupiter), but also that of rapidly rotating stars.

Mericitabine and Either Boceprevir or Telaprevir in Combination with Peginterferon Alfa-2a plus Ribavirin for Patients with Chronic Hepatitis C Genotype 1 Infection and Prior Null Response: The Randomized DYNAMO 1 and DYNAMO 2 Studies

PubMed Central

Wedemeyer, Heiner; Forns, Xavier; Hézode, Christophe; Lee, Samuel S.; Scalori, Astrid; Voulgari, Athina; Le Pogam, Sophie; Nájera, Isabel; Thommes, James A.

2016-01-01

Most patients with chronic hepatitis C virus (HCV) genotype 1 infection who have had a previous null response (<2-log10 reduction in HCV RNA by treatment week 12) to peginterferon/ribavirin (PegIFN/RBV) do not achieve a sustained virological response (SVR) when re-treated with a first-generation HCV protease inhibitor (PI) administered in combination with PegIFN/RBV. We studied the incremental benefits associated with adding mericitabine (nucleoside analog inhibitor of HCV polymerase) to PI plus PegIFN alfa-2a/RBV-based therapy in two double-blind randomized multicenter phase 2 trials (with boceprevir in DYNAMO 1, and with telaprevir in DYNAMO 2). The primary endpoint in both trials was SVR, defined as HCV RNA <25 IU/mL 12 weeks after the end of treatment (SVR12). Overall, the addition of mericitabine to PI plus PegIFN alfa-2a/RBV therapy resulted in SVR12 rates of 60–70% in DYNAMO 1 and of 71–96% in DYNAMO 2. SVR12 rates were similar in patients infected with HCV genotype 1a and 1b in both trials. The placebo control arms in both studies were stopped because of high rates of virological failure. Numerically lower relapse rates were associated with longer treatment with mericitabine (24 versus 12 weeks), telaprevir-containing regimens, and regimens that included 48 weeks of PegIFN alfa-2a/RBV therapy. No mericitabine resistance mutations were identified in any patient in either trial. The addition of mericitabine did not add to the safety burden associated with either telaprevir or boceprevir-based regimens. These studies demonstrate increased SVR rates and reduced relapse rates in difficult-to-treat patients when a nucleoside polymerase inhibitor with intermediate antiviral potency is added to regimens containing a first-generation PI. Trial Registration: ClinicalTrials.gov NCT01482403 and ClinicalTrials.gov NCT01482390 PMID:26752189

COMPARISON OF CHAOTIC AND FRACTAL PROPERTIES OF POLAR FACULAE WITH SUNSPOT ACTIVITY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Deng, L. H.; Xiang, Y. Y.; Dun, G. T.

The solar magnetic activity is governed by a complex dynamo mechanism and exhibits a nonlinear dissipation behavior in nature. The chaotic and fractal properties of solar time series are of great importance to understanding the solar dynamo actions, especially with regard to the nonlinear dynamo theories. In the present work, several nonlinear analysis approaches are proposed to investigate the nonlinear dynamical behavior of the polar faculae and sunspot activity for the time interval from 1951 August to 1998 December. The following prominent results are found: (1) both the high- and the low-latitude solar activity are governed by a three-dimensional chaoticmore » attractor, and the chaotic behavior of polar faculae is the most complex, followed by that of the sunspot areas, and then the sunspot numbers; (2) both the high- and low-latitude solar activity exhibit a high degree of persistent behavior, and their fractal nature is due to such long-range correlation; (3) the solar magnetic activity cycle is predictable in nature, but the high-accuracy prediction should only be done for short- to mid-term due to its intrinsically dynamical complexity. With the help of the Babcock–Leighton dynamo model, we suggest that the nonlinear coupling of the polar magnetic fields with strong active-region fields exhibits a complex manner, causing the statistical similarities and differences between the polar faculae and the sunspot-related indicators.« less

Fluctuation dynamo based on magnetic reconnections

NASA Astrophysics Data System (ADS)

Baggaley, A. W.; Shukurov, A.; Barenghi, C. F.; Subramanian, K.

2010-01-01

We develop a new model of the fluctuation dynamo in which the magnetic field is confined to thin flux ropes advected by a multi-scale flow which models turbulence. Magnetic dissipation occurs only via reconnections of flux ropes. The model is particularly suitable for rarefied plasma, such as the solar corona or galactic halos. We investigate the kinetic energy release into heat, mediated by dynamo action, both in our model and by solving the induction equation with the same flow. We find that the flux rope dynamo is more than an order of magnitude more efficient at converting mechanical energy into heat. The probability density of the magnetic energy released during reconnections has a power-law form with the slope -3, consistent with the solar corona heating by nanoflares. We also present a nonlinear extension of the model. This shows that a plausible saturation mechanism of the fluctuation dynamo is the suppression of turbulent magnetic diffusivity, due to suppression of random stretching at the location of the flux ropes. We confirm that the probability distribution function of the magnetic line curvature has a power-law form suggested by \\citet{Sheck:2002b}. We argue, however, using our results that this does not imply a persistent folded structure of magnetic field, at least in the nonlinear stage.

Generation of Currents in Weakly Ionized Plasmas through a Collisional Dynamo

NASA Astrophysics Data System (ADS)

Dimant, Yakov; Oppenheim, Meers; Fletcher, Alex

2016-10-01

Intense electric currents called electrojets occur in weakly ionized magnetized plasmas. An example occurs in the Earth's ionosphere near the magnetic equator where neutral winds drive the plasma across the geomagnetic field. Similar processes take place in the Solar chromosphere and MHD generators. We argue that not all convective neutral flows generate electrojets and it introduces the corresponding universal criterion for the current formation, ∇ × (U-> × B->) ≠ ∂ B-> / ∂ t , where U-> is the neutral flow velocity, B-> is the magnetic field, and t is time. This criterion does not depend on the conductivity tensor, σ̂ . For many systems, the displacement current, ∂ B-> / ∂ t , is negligible, making the criterion even simpler. This theory also shows that the neutral-dynamo driver that generates electrojets plays the same role as the DC electric current plays for the generation of the magnetic field in the Biot-Savart law. Work supported by NSF/DOE Grant PHY-1500439.

Modeling the Conducting Stably-Stratified Layer of the Earth's Core

NASA Astrophysics Data System (ADS)

Petitdemange, L.; Philidet, J.; Gissinger, C.

2017-12-01

Observations of the Earth magnetic field as well as recent theoretical works tend to show that the Earth's outer liquid core is mostly comprised of a convective zone in which the Earth's magnetic field is generated - likely by dynamo action -, but also features a thin, stably stratified layer at the top of the core.We carry out direct numerical simulations by modeling this thin layer as an axisymmetric spherical Couette flow for a stably stratified fluid embedded in a dipolar magnetic field. The dynamo region is modeled by a conducting inner core rotating slightly faster than the insulating mantle due to magnetic torques acting on it, such that a weak differential rotation (low Rossby limit) can develop in the stably stratified layer.In the case of a non-stratified fluid, the combined action of the differential rotation and the magnetic field leads to the well known regime of `super-rotation', in which the fluid rotates faster than the inner core. Whereas in the classical case, this super-rotation is known to vanish in the magnetostrophic limit, we show here that the fluid stratification significantly extends the magnitude of the super-rotation, keeping this phenomenon relevant for the Earth core. Finally, we study how the shear layers generated by this new state might give birth to magnetohydrodynamic instabilities or waves impacting the secular variations or jerks of the Earth's magnetic field.

Magnetic helicity generation in the frame of Kazantsev model

NASA Astrophysics Data System (ADS)

Yushkov, Egor V.; Lukin, Alexander S.

2017-11-01

Using a magnetic dynamo model, suggested by Kazantsev (J. Exp. Theor. Phys. 1968, vol. 26, p. 1031), we study the small-scale helicity generation in a turbulent electrically conducting fluid. We obtain the asymptotic dependencies of dynamo growth rate and magnetic correlation functions on magnetic Reynolds numbers. Special attention is devoted to the comparison of a longitudinal correlation function and a function of magnetic helicity for various conditions of asymmetric turbulent flows. We compare the analytical solutions on small scales with numerical results, calculated by an iterative algorithm on non-uniform grids. We show that the exponential growth of current helicity is simultaneous with the magnetic energy for Reynolds numbers larger than some critical value and estimate this value for various types of asymmetry.

Modeling of the Coupled Magnetospheric and Neutral Wind Dynamos

NASA Technical Reports Server (NTRS)

Thayer, Jeffrey P.

1997-01-01

Over the past four years of funding, SRI, in collaboration with the University of Texas at Dallas, has been involved in assessing the influence of thermospheric neutral winds on the electric field and current systems at high latitudes. The initial direction of the project was to perform a set of numerical experiments concerning the contribution of the magnetospheric and neutral wind dynamo processes, under specific boundary conditions, to the polarization electric field and/or the field-aligned current distribution at high latitudes. To facilitate these numerical experiments we developed a numerical scheme that relied on using output from the NCAR Thermosphere-Ionosphere General Circulation Model (NCAR-TIGCM), expanding them in the form of spherical harmonics and solving the dynamo equations spectrally. Once initial calculations were completed, it was recognized that the neutral wind contribution could be significant but its actual contribution to the electric field or currents depended strongly on the generator properties of the magnetosphere. Solutions to this problem are not unique because of the unknown characteristics of the magnetospheric generator, therefore the focus was on two limiting cases. One limiting case was to consider the magnetosphere as a voltage generator delivering a fixed voltage to the high-latitude ionosphere and allowing for the neutral wind dynamo to contribute only to the current system. The second limiting case was to consider the magnetosphere as a current generator and allowing for the neutral wind dynamo to contribute only to the generation of polarization electric fields. This work was completed and presented at the l994 Fall AGU meeting. The direction of the project then shifted to applying the Poynting flux concept to the high-latitude ionosphere. This concept was more attractive as it evaluated the influence of neutral winds on the high-latitude electrodynamics without actually having to determine the generator characteristics of the magnetosphere. The influence of the neutral wind was then determined not by estimating how much electric potential or current density it provides, but by determining the contribution of the neutral wind to the net electromagnetic energy transferred between the ionosphere and magnetosphere. The estimate of the net electromagnetic energy transfer and the role of the neutral winds proves to be a more fundamental quantity in studies of magnetosphere- ionosphere coupling also showed that by using electric and magnetic field measurements from the HILAT satellite, the Poynting flux could be a measurable quantity from polar-orbiting, low- altitude spacecraft. Through collaboration with Dr. Heelis and others at UTD and their expertise of the electric field measurements on the DE-B satellite, an extensive analysis was planned to determine the Poynting flux from the DE-B measurements in combination with a modeling effort to help interpret the observations taking into account the coupled magnetosphere-ionosphere.

Tidal Excitation of the Core Dynamo of Mars

NASA Astrophysics Data System (ADS)

Seyed-Mahmoud, B.; Arkani-Hamed, J.; Aldridge, K.

2007-05-01

The lack of magnetic anomalies inside the giant impact basins Hellas, Isidis, Utopia and Argyre, inside the northern low lands, over the Tharsis bulge, and over the Tharsis and Olympus mounts suggests that the core field of Mars ceased to exist by about 4 Gyr ago, almost when the giant basins were formed. On the other hand, the giant basins are located on a great circle, implying that the basins were likely produced by fragments of a large asteroid that broke apart as it entered the Roche limit of Mars. This scenario offers a causative relationship for the apparent coincidence of the formation of the giant basins and the cessation of the core dynamo. We suggest that the core dynamo was excited by tidally driven elliptical instability in the Martian core. The breaking of the asteroid and its final impact on Mars eliminated the excitation and thus killed the dynamo. We show that a retrograde asteroid captured in a Keplerian orbit around Mars at a distance of about 50,000-100,000 km could orbit Mars for several hundreds of millions of years before impacting the planet due to the tidal coupling of the asteroid and Mars. Because of relatively very short growth time of the elliptical instability, less than 50,000 years, the asteroid was capable of retaining the elliptical instability and energizing the core dynamo for a geologically long period prior to 4 Ga. Our laboratory observations of a parametric instability of a rotating incompressible fluid, contained in a flexible-walled spherical cavity, confirm the possibility that an early Martian dynamo could have been powered by tidal straining.

Recent Lunar Magnetism

NASA Astrophysics Data System (ADS)

Buz, J.; Weiss, B. P.; Garrick-Bethell, I.

2010-12-01

Although the Moon today does not have a core dynamo magnetic field [1], paleomagnetic analyses of Apollo samples and spacecraft magnetometry measurements of the lunar crust show magnetization and suggest there were magnetic fields on the Moon > 3 billion years ago [2]. It is unclear whether this magnetization is the product of an ancient core dynamo or that of impact-generated plasmas [3,4,5]. A key way to distinguish between these two hypotheses is to conduct paleomagnetic analyses of lunar impact glasses that formed after any putative core dynamo. Here we present a paleomagnetic study of Apollo 12 basalt 12017. This sample consists of a 3.2 billion year old basalt covered by ~9000 year old impact glass [6,7,8]. We have found that both the rock and glass are magnetized, but in widely divergent directions. The intensity of the fields which magnetized the rock and glass were 40 μT and 1 μT, respectively. Given the near certain absence of a lunar dynamo 9000 years ago, we have two hypotheses to explain the magnetization of the glass: magnetization by an impact-generated field and magnetization by magnetic fields generated by the rock underneath. The long cooling time of the glass (~10 s) relative to that expected for impact-generated field (milliseconds) suggests that impact-generated magnetization is highly improbable. We are currently modeling the magnetic fields of the underlying rock in order to determine whether it had sufficient strength and appropriate orientation to explain the magnetization of the glass. Initial calculations suggest that this is possible. [1] Russell et al., JGR, 79, 1105-1109, 1974 [2] Garrick-Bethell et al., Science,323, 356-359, 2009 [3] Wieczorek et al., Reviews in Mineralogy and Geochemistry, 60, 221-364, 2006 [4] Crawford and Schultz, International Journal of Impact Engineering, 23, 169-180, 1999 [5] Hood and Artemieva, Icarus, 193, 485-502, 2007 [6] Horn et al., Meteoritical Society, 417-418, 1975 [7] Morrisson et al., Proceedings of the 3rd Lunar Science Conference, 3, 2767-2791, 1973 [8] Fleischer et al., Proceedings of the 2nd Lunar Science Conference, 2, 2559-2568, 1971

Understanding the Interiors of Saturn and Mercury through Magnetic Field Observation and Dynamo Modeling

NASA Astrophysics Data System (ADS)

Cao, Hao

Understanding the interior structure and dynamics of a planet is a key step towards understanding the formation and evolution of a planet. In this thesis, I combine field observation and dynamo modeling to understand planetary interiors. Focus has been put on planets Saturn and Mercury. The Cassini spacecraft has been taking continuous measurements in the Saturnian system since the Saturn orbital insertion in June 2004. Since the Mercury orbital insertion in March 2011, the MESSENGER spacecraft has been examining planet Mercury. After analyzing the close-in portion of the in-situ Cassini magnetometer measurements around Saturn, I find that Saturn's magnetic field features several surprising characteristics. First, Saturn's magnetic field is extremely axisymmetric. We cannot find any consistent departure from axisymmetry, and have put an extremely tight upper bound on the dipole tilt of Saturn: the dipole tilt of Saturn has to be smaller than 0.06 degrees. Second, we find that Saturn's magnetic field is extremely stable with time. Third, we estimated the magnetic moments of Saturn up to degree 5. This is the first magnetic field model for Saturn which goes beyond degree 3. We find that not only Saturn's intrinsic magnetic field is dominated by the axial moments; among these axial moments the odd degree ones dominate. In addition, the first three odd degree axial moments all take the same sign. This sign pattern of Saturn's magnetic moments is in contrast to that of the Earth's magnetic moments which takes alternative signs for the past century. The contrast between the geometries of Saturn's magnetic field and the Earth's magnetic field lead us to propose a dynamo hypothesis which speculates that such differences are caused by structural and dynamical differences inside these two planets. Our dynamo hypothesis for Saturn has two essential ingredients. The first concerns about the existence and size of a central core inside Saturn and its influence on Saturn's dynamo action. The second concerns about the possible heterogeneous heat transfer efficiency in the outer envelope of Saturn and its influence on Saturn's dynamo action. We then carried out numerical convective dynamo simulations using the community dynamo code MagIC version 3.44 to test our dynamo hypothesis. In our numerical dynamo experiments, the central core sizes and the outer boundary heat flow heterogeneities are both varied. We find that the central core size is an important factor that can strongly influence the geometry of the dynamo generated magnetic field. Such influence is rendered through the tangent cylinder, which is an imaginary cylinder with its axis parallel to the spin axis of the planet and is tangent to the central core at the equator. We find that both the convective motion and the magnetic field generation efficiency, represented by kinetic helicity, are weaker inside the tangent cylinder than those outside the tangent cylinder. As a result, the magnetic fields inside the tangent cylinder are consistently weaker than those outside the tangent cylinder. Thus the lack of a polar field minimum region at Saturn could be indicative of the absence or a small central core inside Saturn. MESSENGER observations revealed that Mercury's magnetic field is more unusual than previously thought. In particular, Mercury's magnetic field is strongly north-south asymmetric: the magnetic field strength in the northern hemisphere is three times as strong as that in the southern hemisphere. Yet, there is no evidence for any such north-south asymmetry in the basic properties of Mercury that could possibly influence the present-day dynamo action. Here we propose a mechanism to break the equatorial symmetry of Mercury's magnetic field within the framework of convective dynamos. The essence of our mechanism is the mutual excitation of two fundamental modes of columnar convection in rapidly rotating spherical shells. Such mutual excitation results in equatorially asymmetric kinetic helicity, which then leads to equatorially asymmetric magnetic field. With numerical dynamo experiments, we find two necessary conditions to reproduce the equatorial symmetry breaking of Mercury's magnetic field with equatorially symmetric core-mantle boundary (CMB) heat flows. The first is that buoyancy sources need to be distributed within an extended volume of the outer core rather than being concentrated near the inner boundary. The second is an equatorially peaked CMB heat flow. From this study, we conclude that 1) Mercury's core dynamo is likely powered by distributed buoyancy sources and thus is different from the present-day geodynamo which is predominantly powered by bottom-up inner core growth; 2) Mercury's mantle structure and dynamics could be favoring higher heat flow from the equatorial region of Mercury's core. (Abstract shortened by UMI.)

Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence

DOE PAGES

Squire, J.; Bhattacharjee, A.

2015-11-02

Here, this article presents a calculation of the mean electromotive force arising from general small-scale magnetohydrodynamical turbulence, within the framework of the second-order correlation approximation. With the goal of improving understanding of the accretion disk dynamo, effects arising through small-scale magnetic fluctuations, velocity gradients, density and turbulence stratification, and rotation, are included. The primary result, which supplements numerical findings, is that an off-diagonal turbulent resistivity due to magnetic fluctuations can produce large-scale dynamo action-the magnetic analog of the "shear-current" effect. In addition, consideration of alpha effects in the stratified regions of disks gives the puzzling result that there is nomore » strong prediction for a sign of alpha, since the effects due to kinetic and magnetic fluctuations, as well as those due to shear and rotation, are each of opposing signs and tend to cancel each other.« less

Simulation of Dynamo Action Generated by a Precession Driven Flow.

NASA Astrophysics Data System (ADS)

Giesecke, A.; Vogt, T.; Gundrum, T.; Stefani, F.

2017-12-01

Since many years precession is regarded as an alternative flow drivingmechanism that may account, e.g., for remarkable features of theancient lunar magnetic field [Dwyer 2011; Noir 2013; Weiss 2014] or asa complementary power source for the geodynamo [Malkus 1968; Vanyo1991]. Precessional forcing is also of great interest from theexperimental point of view because it represents a natural forcingmechanism that allows an efficient driving of conducting fluid flowson the laboratory scale without making use of propellers orpumps. Within the project DRESDYN (DREsden Sodium facility for DYNamoand thermohydraulic studies) a dynamo experiment is under developmentat Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in which a precessiondriven flow of liquid sodium with a magnetic Reynolds number of up toRm=700 will be used to drive dynamo action.Our present study addresses preparative numerical simulations and flowmeasurements at a small model experiment running with water. Theresulting flow pattern and amplitude provide the essential ingredientsfor kinematic dynamo models that are used to estimate whether theparticular flow is able to drive a dynamo. In the strongly non-linearregime the flow essentially consists of standing inertial waves (see Figure). Most remarkable feature is the occurrence of a resonant-like axisymmetricmode which emerges around a precession ratio of Ωp/Ωc = 0.1on top of the directly forced re-circulation flow. The combination ofthis axisymmetric mode and the forced m=1 Kelvin mode is indeedcapable of driving a dynamo at a critical magnetic Reynolds number ofRmc=430 which is well within the range achievable in theexperiment. However, the occurrence of the axisymmetric mode slightlydepends on the absolute rotation rate of the cylinder and futureexperiments are required to indicate whether it persists at theextremely large Re that will be obtained in the large scale sodiumexperiment.

Transitions in rapidly rotating convection dynamos

NASA Astrophysics Data System (ADS)

Tilgner, A.

2013-12-01

It is commonly assumed that buoyancy in the fluid core powers the geodynamo. We study here the minimal model of a convection driven dynamo, which is a horizontal plane layer in a gravity field, filled with electrically conducting fluid, heated from below and cooled from above, and rotating about a vertical axis. Such a plane layer may be viewed as a local approximation to the geophysically more relevant spherical geometry. The numerical simulations have been run on graphics processing units with at least 960 cores. If the convection is driven stronger and stronger at fixed rotation rate, the flow behaves at some point as if it was not rotating. This transition shows in the scaling of the heat transport which can be used to distinguish slow from rapid rotation. One expects dynamos to behave differently in these two flow regimes. But even within the convection flows which are rapidly rotating according to this criterion, it will be shown that different types of dynamos exist. In one state, the magnetic field strength obeys a scaling indicative of a magnetostrophic balance, in which the Lorentz force is in equilibrium with the Coriolis force. The flow in this case is helical. A different state exists at higher magnetic Reynolds numbers, in which the magnetic energy obeys a different scaling law and the helicity of the flow is much reduced. As one increases the Rayleigh number, all other parameters kept constant, one may find both types of dynamos separated by an interval of Rayleigh numbers in which there are no dynamos at all. The effect of these transitions on energy dissipation and mean field generation have also been studied.

Statistical dynamo theory: Mode excitation.

PubMed

Hoyng, P

2009-04-01

We compute statistical properties of the lowest-order multipole coefficients of the magnetic field generated by a dynamo of arbitrary shape. To this end we expand the field in a complete biorthogonal set of base functions, viz. B= summation operator_{k}a;{k}(t)b;{k}(r) . The properties of these biorthogonal function sets are treated in detail. We consider a linear problem and the statistical properties of the fluid flow are supposed to be given. The turbulent convection may have an arbitrary distribution of spatial scales. The time evolution of the expansion coefficients a;{k} is governed by a stochastic differential equation from which we infer their averages a;{k} , autocorrelation functions a;{k}(t)a;{k *}(t+tau) , and an equation for the cross correlations a;{k}a;{l *} . The eigenfunctions of the dynamo equation (with eigenvalues lambda_{k} ) turn out to be a preferred set in terms of which our results assume their simplest form. The magnetic field of the dynamo is shown to consist of transiently excited eigenmodes whose frequency and coherence time is given by Ilambda_{k} and -1/Rlambda_{k} , respectively. The relative rms excitation level of the eigenmodes, and hence the distribution of magnetic energy over spatial scales, is determined by linear theory. An expression is derived for |a;{k}|;{2}/|a;{0}|;{2} in case the fundamental mode b;{0} has a dominant amplitude, and we outline how this expression may be evaluated. It is estimated that |a;{k}|;{2}/|a;{0}|;{2} approximately 1/N , where N is the number of convective cells in the dynamo. We show that the old problem of a short correlation time (or first-order smoothing approximation) has been partially eliminated. Finally we prove that for a simple statistically steady dynamo with finite resistivity all eigenvalues obey Rlambda_{k}<0 .

A prevalence of dynamo-generated magnetic fields in the cores of intermediate-mass stars.

PubMed

Stello, Dennis; Cantiello, Matteo; Fuller, Jim; Huber, Daniel; García, Rafael A; Bedding, Timothy R; Bildsten, Lars; Aguirre, Victor Silva

2016-01-21

Magnetic fields play a part in almost all stages of stellar evolution. Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes. Intermediate-mass stars do not have deep convective envelopes, although 10 per cent exhibit strong surface fields that are presumed to be residuals from the star formation process. These stars do have convective cores that might produce internal magnetic fields, and these fields might survive into later stages of stellar evolution, but information has been limited by our inability to measure the fields below the stellar surface. Here we report the strength of dipolar oscillation modes for a sample of 3,600 red giant stars. About 20 per cent of our sample show mode suppression, by strong magnetic fields in the cores, but this fraction is a strong function of mass. Strong core fields occur only in red giants heavier than 1.1 solar masses, and the occurrence rate is at least 50 per cent for intermediate-mass stars (1.6-2.0 solar masses), indicating that powerful dynamos were very common in the previously convective cores of these stars.

Production of sunspots and their effects on the corona and solar wind: Insights from a new 3D flux-transport dynamo model

NASA Astrophysics Data System (ADS)

Kumar, Rohit; Jouve, Laurène; Pinto, Rui F.; Rouillard, Alexis P.

2018-01-01

We present a three-dimensional numerical model for the generation and evolution of the magnetic field in the solar convection zone, in which sunspots are produced and contribute to the cyclic reversal of the large-scale magnetic field. We then assess the impact of this dynamo-generated field on the structure of the solar corona and solar wind. This model solves the induction equation in which the velocity field is prescribed. This velocity field is a combination of a solar-like differential rotation and meridional circulation. We develop an algorithm that enables the magnetic flux produced in the interior to be buoyantly transported towards the surface to produce bipolar spots. We find that those tilted bipolar magnetic regions contain a sufficient amount of flux to periodically reverse the polar magnetic field and sustain dynamo action. We then track the evolution of these magnetic features at the surface during a few consecutive magnetic cycles and analyze their effects on the topology of the corona and on properties of the solar wind (distribution of streamers and coronal holes, and of slow and fast wind streams) in connection with current observations of the Sun.

MEAN-FIELD SOLAR DYNAMO MODELS WITH A STRONG MERIDIONAL FLOW AT THE BOTTOM OF THE CONVECTION ZONE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pipin, V. V.; Kosovichev, A. G.

2011-09-01

This paper presents a study of kinematic axisymmetric mean-field dynamo models for the case of meridional circulation with a deep-seated stagnation point and a strong return flow at the bottom of the convection zone. This kind of circulation follows from mean-field models of the angular momentum balance in the solar convection zone. The dynamo models include turbulent sources of the large-scale poloidal magnetic field production due to kinetic helicity and a combined effect due to the Coriolis force and large-scale electric current. In these models the toroidal magnetic field, which is responsible for sunspot production, is concentrated at the bottommore » of the convection zone and is transported to low-latitude regions by a meridional flow. The meridional component of the poloidal field is also concentrated at the bottom of the convection zone, while the radial component is concentrated in near-polar regions. We show that it is possible for this type of meridional circulation to construct kinematic dynamo models that resemble in some aspects the sunspot magnetic activity cycle. However, in the near-equatorial regions the phase relation between the toroidal and poloidal components disagrees with observations. We also show that the period of the magnetic cycle may not always monotonically decrease with the increase of the meridional flow speed. Thus, for further progress it is important to determine the structure of the meridional circulation, which is one of the critical properties, from helioseismology observations.« less

Torsion Bounds from CP Violation α2-DYNAMO in Axion-Photon Cosmic Plasma

NASA Astrophysics Data System (ADS)

Garcia de Andrade, L. C.

Years ago Mohanty and Sarkar [Phys. Lett. B 433, 424 (1998)] have placed bounds on torsion mass from K meson physics. In this paper, associating torsion to axions a la Campanelli et al. [Phys. Rev. D 72, 123001 (2005)], it is shown that it is possible to place limits on spacetime torsion by considering an efficient α2-dynamo CP violation term. Therefore instead of Kostelecky et al. [Phys. Rev. Lett. 100, 111102 (2008)] torsion bounds from Lorentz violation, here torsion bounds are obtained from CP violation through dynamo magnetic field amplification. It is also shown that oscillating photon-axion frequency peak is reduced to 10-7 Hz due to torsion mass (or Planck mass when torsion does not propagate) contribution to the photon-axion-torsion action. Though torsion does not couple to electromagnetic fields at classical level, it does at the quantum level. Recently, Garcia de Andrade [Phys. Lett. B 468, 28 (2011)] has shown that the photon sector of Lorentz violation (LV) Lagrangian leads to linear nonstandard Maxwell equations where the magnetic field decays slower giving rise to a seed for galactic dynamos. Torsion constraints of the order of K0≈10-42 GeV can be obtained which are more stringent than the value obtained by Kostelecky et al. A lower bound for the existence of galactic dynamos is obtained for torsion as K0≈10-37 GeV.

Magnetostrophic balance as the optimal state for turbulent magnetoconvection

PubMed Central

King, Eric M.; Aurnou, Jonathan M.

2015-01-01

The magnetic fields of Earth and other planets are generated by turbulent convection in the vast oceans of liquid metal within them. Although direct observation is not possible, this liquid metal circulation is thought to be dominated by the controlling influences of planetary rotation and magnetic fields through the Coriolis and Lorentz forces. Theory famously predicts that planetary dynamo systems naturally settle into the so-called magnetostrophic state, where the Coriolis and Lorentz forces partially cancel, and convection is optimally efficient. Although this magnetostrophic theory correctly predicts the strength of Earth’s magnetic field, no laboratory experiments have reached the magnetostrophic regime in turbulent liquid metal convection. Furthermore, computational dynamo simulations have as yet failed to produce a magnetostrophic dynamo, which has led some to question the existence of the magnetostrophic state. Here, we present results from the first, to our knowledge, turbulent, magnetostrophic convection experiments using the liquid metal gallium. We find that turbulent convection in the magnetostrophic regime is, in fact, maximally efficient. The experimental results clarify these previously disparate results, suggesting that the dynamically optimal magnetostrophic state is the natural expression of turbulent planetary dynamo systems. PMID:25583512

A Coupled 2 × 2D Babcock-Leighton Solar Dynamo Model. II. Reference Dynamo Solutions

NASA Astrophysics Data System (ADS)

Lemerle, Alexandre; Charbonneau, Paul

2017-01-01

In this paper we complete the presentation of a new hybrid 2 × 2D flux transport dynamo (FTD) model of the solar cycle based on the Babcock-Leighton mechanism of poloidal magnetic field regeneration via the surface decay of bipolar magnetic regions (BMRs). This hybrid model is constructed by allowing the surface flux transport (SFT) simulation described in Lemerle et al. to provide the poloidal source term to an axisymmetric FTD simulation defined in a meridional plane, which in turn generates the BMRs required by the SFT. A key aspect of this coupling is the definition of an emergence function describing the probability of BMR emergence as a function of the spatial distribution of the internal axisymmetric magnetic field. We use a genetic algorithm to calibrate this function, together with other model parameters, against observed cycle 21 emergence data. We present a reference dynamo solution reproducing many solar cycle characteristics, including good hemispheric coupling, phase relationship between the surface dipole and the BMR-generating internal field, and correlation between dipole strength at cycle maximum and peak amplitude of the next cycle. The saturation of the cycle amplitude takes place through the quenching of the BMR tilt as a function of the internal field. The observed statistical scatter about the mean BMR tilt, built into the model, acts as a source of stochasticity which dominates amplitude fluctuations. The model thus can produce Dalton-like epochs of strongly suppressed cycle amplitude lasting a few cycles and can even shut off entirely following an unfavorable sequence of emergence events.

Emergence of Magnetic Flux Generated in a Solar Convective Dynamo. I. The Formation of Sunspots and Active Regions, and The Origin of Their Asymmetries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Feng; Rempel, Matthias; Fan, Yuhong, E-mail: chenfeng@ucar.edu

We present a realistic numerical model of sunspot and active region formation based on the emergence of flux bundles generated in a solar convective dynamo. To this end, we use the magnetic and velocity fields in a horizontal layer near the top boundary of the solar convective dynamo simulation to drive realistic radiative-magnetohydrodynamic simulations of the uppermost layers of the convection zone. The main results are as follows. (1) The emerging flux bundles rise with the mean speed of convective upflows and fragment into small-scale magnetic elements that further rise to the photosphere, where bipolar sunspot pairs are formed throughmore » the coalescence of the small-scale magnetic elements. (2) Filamentary penumbral structures form when the sunspot is still growing through ongoing flux emergence. In contrast to the classical Evershed effect, the inflow seems to prevail over the outflow in a large part of the penumbra. (3) A well-formed sunspot is a mostly monolithic magnetic structure that is anchored in a persistent deep-seated downdraft lane. The flow field outside the spot shows a giant vortex ring that comprises an inflow below 15 Mm depth and an outflow above 15 Mm depth. (4) The sunspots successfully reproduce the fundamental properties of the observed solar active regions, including the more coherent leading spots with a stronger field strength, and the correct tilts of bipolar sunspot pairs. These asymmetries can be linked to the intrinsic asymmetries in the magnetic and flow fields adapted from the convective dynamo simulation.« less

The Dynomak: An advanced spheromak reactor system with imposed-dynamo current drive and next-generation nuclear power technologies

NASA Astrophysics Data System (ADS)

Sutherland, D. A.; Jarboe, T. R.; Marklin, G.; Morgan, K. D.; Nelson, B. A.

2013-10-01

A high-beta spheromak reactor system has been designed with an overnight capital cost that is competitive with conventional power sources. This reactor system utilizes recently discovered imposed-dynamo current drive (IDCD) and a molten salt blanket system for first wall cooling, neutron moderation and tritium breeding. Currently available materials and ITER developed cryogenic pumping systems were implemented in this design on the basis of technological feasibility. A tritium breeding ratio of greater than 1.1 has been calculated using a Monte Carlo N-Particle (MCNP5) neutron transport simulation. High-temperature superconducting tapes (YBCO) were used for the equilibrium coil set, substantially reducing the recirculating power fraction when compared to previous spheromak reactor studies. Using zirconium hydride for neutron shielding, a limiting equilibrium coil lifetime of at least thirty full-power years has been achieved. The primary FLiBe loop was coupled to a supercritical carbon dioxide Brayton cycle due to attractive economics and high thermal efficiencies. With these advancements, an electrical output of 1000 MW from a thermal output of 2486 MW was achieved, yielding an overall plant efficiency of approximately 40%. A paper concerning the Dynomak reactor design is currently being reviewed for publication.

DYNAMO EFFECTS NEAR THE TRANSITION FROM SOLAR TO ANTI-SOLAR DIFFERENTIAL ROTATION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Simitev, Radostin D.; Kosovichev, Alexander G.; Busse, Friedrich H.

2015-09-01

Numerical MHD simulations play an increasingly important role for understanding the mechanisms of stellar magnetism. We present simulations of convection and dynamos in density-stratified rotating spherical fluid shells. We employ a new 3D simulation code for obtaining the solution of a physically consistent anelastic model of the process with a minimum number of parameters. The reported dynamo simulations extend into a “buoyancy-dominated” regime where the buoyancy forcing is dominant while the Coriolis force is no longer balanced by pressure gradients, and strong anti-solar differential rotation develops as a result. We find that the self-generated magnetic fields, despite being relatively weak,more » are able to reverse the direction of differential rotation from anti-solar to solar-like. We also find that convection flows in this regime are significantly stronger in the polar regions than in the equatorial region, leading to non-oscillatory dipole-dominated dynamo solutions, and to a concentration of magnetic field in the polar regions. We observe that convection has a different morphology in the inner and the outer part of the convection zone simultaneously such that organized geostrophic convection columns are hidden below a near-surface layer of well-mixed highly chaotic convection. While we focus our attention on the buoyancy-dominated regime, we also demonstrate that conical differential rotation profiles and persistent regular dynamo oscillations can be obtained in the parameter space of the rotation-dominated regime even within this minimal model.« less

Shear dynamo, turbulence, and the magnetorotational instability

DOE Office of Scientific and Technical Information (OSTI.GOV)

Squire, Jonathan

The formation, evolution, and detailed structure of accretion disks remain poorly understood, with wide implications across a variety of astrophysical disciplines. While the most pressing question – what causes the high angular momentum fluxes that are necessary to explain observations? – is nicely answered by the idea that the disk is turbulent, a more complete grasp of the fundamental processes is necessary to capture the wide variety of behaviors observed in the night sky. This thesis studies the turbulence in ionized accretion disks from a theoretical standpoint, in particular focusing on the generation of magnetic fields in these processes, knownmore » as dynamo. Such fields are expected to be enormously important, both by enabling the magnetorotational instability (which evolves into virulent turbulence), and through large-scale structure formation, which may transport angular momentum in different ways and be fundamental for the formation of jets. The central result of this thesis is the suggestion of a new large-scale dynamo mechanism in shear flows – the “magnetic shear-current effect” – which relies on a positive feedback from smallscale magnetic fields. As well as being a very promising candidate for driving field generation in the central regions of accretion disks, this effect is interesting because small-scale magnetic fields have historically been considered to have a negative effect on the large-scale dynamo, damping growth and leading to dire predictions for final saturation amplitudes. Given that small-scale fields are ubiquitous in plasma turbulence above moderate Reynolds numbers, the finding that they could instead have a positive effect in some situations is interesting from a theoretical and practical standpoint. The effect is studied using direct numerical simulation, analytic techniques, and novel statistical simulation methods. In addition to the dynamo, much attention is given to the linear physics of disks and its relevance to turbulence. This is studied using nonmodal stability theory, which both provides a highly intuitive connection between global domains and the commonly studied shearing box, and suggests that transient linear growth can often be more important than spectral instability. These realizations motivate the use of the quasi-linear models that are applied extensively throughout the turbulence and dynamo studies later in the thesis.« less

On Magnetic Dynamos in Thin Accretion Disks around Compact and Young Stars

NASA Technical Reports Server (NTRS)

Stepinski, T. F.

1993-01-01

A variety of geometrically thin accretion disks commonly associated with such astronomical objects as X-ray binaries, cataclysmic variables, and protostars are likely to be seats of MHD dynamo actions. Thin disk geometry and the particular physical environment make accretion disk dynamos different from stellar, planetary, or even galactic dynamos. We discuss those particular features of disk dynamos with emphasis on the difference between protoplanetary disk dynamos and those associated with compact stars. We then describe normal mode solutions for thin disk dynamos and discuss implications for the dynamical behavior of dynamo-magnetized accretion disks.

Solar and Planetary Dynamos

NASA Astrophysics Data System (ADS)

Proctor, M. R. E.; Matthews, P. C.; Rucklidge, A. M.

2008-02-01

Preface; 1. Magnetic noise and the galactic dynamo; 2. On the oscillation in model Z; 3. Nonlinear dynamos in a spherical shell; 4. The onset of dynamo action in alpha-lambda dynamos; 5. Multifractality, near-singularities and the role of stretching in turbulence; 6. Note on perfect fast dynamo action in a large-amplitude SFS map; 7. A thermally driven disc dynamo; 8. Magnetic instabilities in rapidly rotating systems; 9. Modes of a flux ring lying in the equator of a star; 10. A nonaxisymmetric dynamo in toroidal geometry; 11. Simulating the interaction of convection with magnetic fields in the sun; 12. Experimental aspects of a laboratory scale liquid sodium dynamo model; 13. Influence of the period of an ABC flow on its dynamo action; 14. Numerical calculations of dynamos for ABC and related flows; 15. Incompressible Euler equations; 16. On the quasimagnetostrophic asymptotic approximation related to solar activity; 17. Simple dynamical fast dynamos; 18. A numerical study of dynamos in spherical shells with conducting boundaries; 19. Non-axisymmetric shear layers in a rotating spherical shell; 20. Testing for dynamo action; 21. Alpha-quenching in cylindrical magnetoconvection; 22. On the stretching of line elements in fluids: an approach from different geometry; 23. Instabilities of tidally and precessionally induced flows; 24. Probability distribution of passive scalars with nonlinear mean gradient; 25. Magnetic fluctuations in fast dynamos; 26. A statistical description of MHD turbulence in laboratory plasma; 27. Compressible magnetoconvection in three dimensions; 28. The excitation of nonaxisymmetric magnetic fields in galaxies; 29. Localized magnetic fields in a perfectly conducting fluid; 30. Turbulent dynamo and the geomagnetic secular variation; 31. On-off intermittency: general description and feedback model; 32. Dynamo action in a nearly integrable chaotic flow; 33. The dynamo mechanism in the deep convection zone of the sun; 34. Shearing instabilities in magnetoconvection; 35. On the role of rotation of the internal core relative to the mantle; 36. Evolution of magnetic fields in a swirling jet; 37. Analytic fast dynamo solution for a two-dimensional pulsed flow; 38. On magnetic dynamos in thin accretion disks around compact and young stars; 39. The strong field branch of the Childress-Soward dynamo; 40. Evidence for the suppression of the alpha-effect by weak magnetic fields; 41. Turbulent magnetic transport effects and their relation to magnetic field intermittency; 42. Proving the existence of negative variation of electrical conductivity; 43. Spherical inertial oscillation and convection; 44. Hydrodynamics stability of the ABC flow; 45. Dynamos with ambipolar diffusion; Subject index.

Initial 60Fe Abundance in the Solar Nebula Constrained by Delayed Onset of a Planetesimal Dynamo

NASA Astrophysics Data System (ADS)

Wang, H.; Weiss, B. P.; Crowley, J.

2017-12-01

The paleomagnetism of meteorites provides evidence for advecting metallic core dynamos and large-scale differentiation on their parent planetesimals. Their small sizes relative to planets enable new opportunities to understand the physics of dynamo generation. Wang et al. [2017] studied the paleomagnetism of three volcanic angrites (D'Orbigny, 4563.37±0.12 Ma; Sahara 99555, 4563.54±0.14 Ma; Asuka 881371, 4562.4±1.6 Ma) and one plutonic angrite (Angra dos Reis, 4556.51±0.11 Ma). Their results show that the older volcanic angrites recorded no detectable paleomagnetic field, while the younger plutonic angrite recorded a paleomagnetic field of 17 µT interpreted as evidence of a core dynamo on the angrite parent body (APB). This indicates that the initiation of the APB dynamo was delayed until sometime between 4 and 11 My after the formation of calcium aluminum-rich inclusions (CAIs) at 4567.30 ± 0.16 Ma. This late timing is consistent with recent planetesimal thermal evolution models invoking shallow magma oceans [Neumann et al. 2014], which predict that planetesimal dynamos would not initiate until the core began to crystallize. It is also consistent with thermal evolution models invoking large-scale magma oceans that considered thermal blanketing of the core by 26Al decay in the mantle [Roberts et al. 2013, Sterenborg and Crowley 2013], which would delay thermal convection dynamos until several My after accretion (occurred <0.25 My after CAIs for the APB [Schiller et al. 2015]) and differentiation. Because the presence of even a small amount of 60Fe in the core could effectively remove the thermal blanketing effect of mantle 26Al, we can use the delay in timing of the dynamo to constrain the abundance of 60Fe on the APB. Our planetesimal thermal evolution models show that if the initial solar nebula 60Fe/56Fe ratio was greater than 5×10-9, the APB core dynamo would have to start earlier than 4 My after CAIs, in contradiction to the paleomagnetic constraints. Thus, we argue that 5×10-9 is an upper limit of the initial 60Fe/56Fe ratio in the solar nebula. This upper limit is consistent with independent isotopic measurements of the Sahara 99555 angrite, which found 60Fe/56Fe ratio of (6.96 ± 1.60)×10-9 [Tang and Dauphas, 2015].

Can basal magma oceans generate magnetic fields?

NASA Astrophysics Data System (ADS)

Stegman, D. R.; Ziegler, L. B.; Davies, C.

2015-12-01

Earth's magnetic field is very old, with recent data now showing the field possibly extended back to 4.1 billion years ago (Tarduno et al., Science, 2015). Yet, based upon our current knowledge there are difficulties in sustained a core dynamo over most of Earth's history. Moreover, recent estimates of thermal and electrical conductivity of liquid iron at core conditions from mineral physics experiments indicate that adiabatic heat flux is approximately 15 TW, nearly 3 times larger than previously thought, exacerbating difficulties for driving a core dynamo by convective core cooling alone throughout Earth history. A long-lived basal magma ocean in the lowermost mantle has been proposed to exist in the early Earth, surviving perhaps into the Archean. While the modern, solid lower mantle is an electromagnetic insulator, electrical conductivities of silicate melts are known to be higher, though as yet they are unconstrained for lowermost mantle conditions. Here we explore the geomagnetic consequences of a basal magma ocean layer for a range of possible electrical conductivities. For the highest electrical conductivities considered, we find a basal magma ocean could be a primary dynamo source region. This would suggest the proposed three magnetic eras observed in paleomagnetic data originate from distinct sources for dynamo generation: from 4.5-2.45 Ga within a basal magma ocean, from 2.25-0.4 Ga within a superadiabatically cooled liquid core, and from 0.4-present within a quasi-adiabatic core that includes a solidifying inner core. We have extended this work by developing a new code, Dynamantle, which is a model with an entropy-based approach, similar to those commonly used in core dynamics models. We present new results using this code to assess the conditions under which basal magma oceans can generate positive ohmic dissipation. This is more generally useful than just considering the early Earth, but also for many silicate exoplanets in which basal magma oceans are even more likely to exist.

Derivation of dynamo current drive in a closed-current volume and stable current sustainment in the HIT-SI experiment

DOE PAGES

Hossack, A. C.; Sutherland, D. A.; Jarboe, T. R.

2017-02-01

A derivation is given showing that the current inside a closed-current volume can be sustained against resistive dissipation by appropriately phased magnetic perturbations. Imposed-dynamo current drive (IDCD) theory is used to predict the toroidal current evolution in the HIT-SI experiment as a function of magnetic fluctuations at the edge. Analysis of magnetic fields from a HIT-SI discharge shows that the injector-imposed fluctuations are sufficient to sustain the measured toroidal current without instabilities whereas the small, plasma-generated magnetic fluctuations are not sufficiently large to sustain the current.

Derivation of dynamo current drive in a closed-current volume and stable current sustainment in the HIT-SI experiment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hossack, A. C.; Sutherland, D. A.; Jarboe, T. R.

A derivation is given showing that the current inside a closed-current volume can be sustained against resistive dissipation by appropriately phased magnetic perturbations. Imposed-dynamo current drive (IDCD) theory is used to predict the toroidal current evolution in the HIT-SI experiment as a function of magnetic fluctuations at the edge. Analysis of magnetic fields from a HIT-SI discharge shows that the injector-imposed fluctuations are sufficient to sustain the measured toroidal current without instabilities whereas the small, plasma-generated magnetic fluctuations are not sufficiently large to sustain the current.

New Model for Ionospheric Irregularities at Mars

NASA Astrophysics Data System (ADS)

Keskinen, M. J.

2018-03-01

A new model for ionospheric irregularities at Mars is presented. It is shown that wind-driven currents in the dynamo region of the Martian ionosphere can be unstable to the electromagnetic gradient drift instability. This plasma instability can generate ionospheric density and magnetic field irregularities with scale sizes of approximately 15-20 km down to a few kilometers. We show that the instability-driven magnetic field fluctuation amplitudes relative to background are correlated with the ionospheric density fluctuation amplitudes relative to background. Our results can explain recent observations made by the Mars Atmosphere and Volatile EvolutioN spacecraft in the Martian ionosphere dynamo region.

Using Jupiter's gravitational field to probe the Jovian convective dynamo.

PubMed

Kong, Dali; Zhang, Keke; Schubert, Gerald

2016-03-23

Convective motion in the deep metallic hydrogen region of Jupiter is believed to generate its magnetic field, the strongest in the solar system. The amplitude, structure and depth of the convective motion are unknown. A promising way of probing the Jovian convective dynamo is to measure its effect on the external gravitational field, a task to be soon undertaken by the Juno spacecraft. We calculate the gravitational signature of non-axisymmetric convective motion in the Jovian metallic hydrogen region and show that with sufficiently accurate measurements it can reveal the nature of the deep convection.

Using Jupiter’s gravitational field to probe the Jovian convective dynamo

PubMed Central

Kong, Dali; Zhang, Keke; Schubert, Gerald

2016-01-01

Convective motion in the deep metallic hydrogen region of Jupiter is believed to generate its magnetic field, the strongest in the solar system. The amplitude, structure and depth of the convective motion are unknown. A promising way of probing the Jovian convective dynamo is to measure its effect on the external gravitational field, a task to be soon undertaken by the Juno spacecraft. We calculate the gravitational signature of non-axisymmetric convective motion in the Jovian metallic hydrogen region and show that with sufficiently accurate measurements it can reveal the nature of the deep convection. PMID:27005472

Simulation of an Ice Giant-style Dynamo

NASA Astrophysics Data System (ADS)

Soderlund, K. M.; Aurnou, J. M.

2010-12-01

The Ice Giants, Uranus and Neptune, are unique in the solar system. These planets are the only known bodies to have multipolar magnetic fields where the quadrupole and octopole components have strengths comparable to or greater than that of the dipole. Cloud layer observations show that the planets also have zonal (east-west) flows that are fundamentally different from the banded winds of Jupiter and Saturn. The surface winds are characterized by strong retrograde equatorial jets that are flanked on either side by prograde jets at high latitudes. Thermal emission measurements of Neptune show that the surface energy flux pattern peaks in the equatorial and polar regions with minima at mid-latitudes. (The measurements for Uranus cannot adequately resolve the emission pattern.) The winds and magnetic fields are thought to be the result of convection in the planetary interior, which will also affect the heat flux pattern. Typically, it is implicitly assumed that the zonal winds are generated in a shallow layer, separate from the dynamo generation region. However, if the magnetic fields are driven near the surface, a single region can simultaneously generate both the zonal flows and the magnetic fields. Here, we present a novel numerical model of an Ice Giant-style dynamo to investigate this possibility. An order unity convective Rossby number (ratio of buoyancy to Coriolis forces) has been chosen because retrograde equatorial jets tend to occur in spherical shells when the effects of rotation are relatively weak. Our modeling results qualitatively reproduce all of the structural features of the global dynamical observations. Thus, a self-consistent model can generate magnetic field, zonal flow, and thermal emission patterns that agree with those of Uranus and Neptune. This model, then, leads us to hypothesize that the Ice Giants' zonal flows and magnetic fields are generated via dynamically coupled deep convection processes.

Paleomagnetic record of mare basalt 10017: A lunar core dynamo at 3.6 Ga?

NASA Astrophysics Data System (ADS)

Suavet, C.; Weiss, B. P.; Fuller, M.; Gattacceca, J.; Grove, T. L.; Shuster, D. L.

2011-12-01

Following the Apollo missions, twenty years of paleomagnetic studies of returned samples have failed to demonstrate unambiguously the existence of an ancient lunar core dynamo. As a result of new technologies, more robust analytical methods, and a better understanding of rock magnetism, it is now possible to revisit lunar paleomagnetism. A set of criteria that must be met in order to demonstrate that a sample has recorded a core dynamo field has been defined: the samples must not show petrologic evidence of shock, the magnetization must be a stable thermoremanent magnetization (TRM), mutually oriented subsamples should agree in direction and intensity, and the thermal history should be well constrained, with a cooling timescale longer than the lifetime of impact generated fields (>1h). A critical review of the literature has allowed us to identify Apollo samples that are most likely to provide good records of ancient lunar magnetic fields. The first samples to be studied within this framework were troctolite 76535 (Garrick-Bethell et al., 2009) and mare basalt 10020 (Shea et al., 2010), which have recorded a core dynamo field at 4.2 and 3.7 Ga, respectively. Mare basalt 10017 is a fine grained, vesicular, high-K ilmenite basalt with a crystallization age of 3.6 Ga. It was studied by different groups (Fuller and Meshkov, 1979; Hoffman et al., 1979; Runcorn et al., 1970; Stephenson et al., 1977), all of whom noted the stability of its magnetization. We have measured 7 subsamples of chip 10017,378. Their magnetizations agree in direction, with a low coercivity overprint removed by 10 mT AF demagnetization, and a stable high coercivity component consistent with a TRM. Paleointensity estimations give a conservative minimum of 12 μT for the paleofield. This sample is ~100 Myr younger than the end of the late heavy bombardment, which rules out basin-forming impacts as a possible candidate to explain its magnetization. It extends the lifetime of the putative ancient lunar core dynamo well after thermal convection of the liquid core could have sustained an Earth-like lunar dynamo, thereby supporting non-standard dynamo scenarios such as a mechanical-stirring driven dynamo that could have been triggered by lunar librations from Earth tides or by impacts.

First Numerical Simulations of Turbulent Dynamos Driven by Libration, Precession and Tides in Triaxial Ellipsoids - An Alternative Route for Planetary Magnetism

NASA Astrophysics Data System (ADS)

Le Bars, M.; Kanuganti, S. R.; Favier, B.

2017-12-01

Most of the time, planetary dynamos are - tacitly or not - associated with thermo-solutal convection. The convective dynamo model has indeed proven successful to explain the current Earth's magnetic field. However, its results are sometimes difficult to reconcile with observational data and its validity can be questioned for several celestial bodies. For instance, the small size of the Moon and Ganymede makes it difficult to maintain a sufficient temperature gradient to sustain convection and to explain their past and present magnetic fields, respectively. The same caveat applies to the growing number of planetesimals shown to have generated magnetic fields in their early history. Finally, the energy budget of the early Earth is difficult to reconcile with a convective dynamo before the onset of inner core growth. Significant effort has thus been put into finding new routes for planetary dynamo. In particular, the rotational dynamics of planets, moons and small bodies, where their average spinning motion is periodically perturbed by the small mechanical forcings of libration, precession and/or tides, is now widely accepted as an efficient source of core turbulence. The underlying mechanism relies on a parametric instability where the inertial waves of the rotating fluid core are resonantly excited by the small forcing, leading to exponential growth and bulk filling intense motions, pumping their energy from the orbital dynamics. Dynamos driven by mechanical forcing have been suggested for the Moon, Mars, Io, the early Earth, etc. However, the real dynamo capacity of the corresponding flows has up-to-now been studied only in very limited cases, with simplified spherical/spheroidal geometries and/or overly viscous fluids. We will present here the first numerical simulations of dynamos driven by libration, precession and tides, in the triaxial ellipsoidal geometry and in the turbulent regime relevant for planetary cores. We will describe the numerical techniques required to tackle this challenge and present the first results describing the associated magnetic field in terms of amplitude, energy budget, and spatiotemporal signature. We hope to motivate others to participate in the exploration of the wide parameter space, a necessary work for addressing the variety of observed past and present magnetic fields.

Antipodal Magnetic Anomalies on the Moon, Contributions from Impact Induced Currents Due to Positive Holes and Flexoelectric Phenomina and Dynamo

NASA Technical Reports Server (NTRS)

Kletetschka, G.; Freund, F.; Wasilewski, P. J.; Mikula, V.; Kohout, Tomas

2005-01-01

Large impacts on the Moon generate large pressure pulses that penetrate the whole body. Several of these large impacts may have generated antipodal structure with anomalous magnetic intensity.These regions can be more than a thousand km across, with fields of the order of tens to hundreds of nT. This is the case of Orientale, Imbrium, Serenitatis, Crisium, and Nectaris impact basins. The production of large-scale magnetic fields and associated crustal magnetization due to lunar basin-forming impacts was hypothesized to have an origin in fields external to the impact plasma cloud that are produced by the magnetohydrodynamic interaction of the cloud with ambient magnetic fields and plasmas. During the period of compressed antipodal field amplification, seismic compressional waves from the impact converge at the antipode resulting in transient shock pressures that reach 2 GPa (20 kbar). This can produce conditions for shock magnetic acquisition of the crust antipodal to impact basins.

Covariant and 3 + 1 Equations for Dynamo-Chiral General Relativistic Magnetohydrodynamics

NASA Astrophysics Data System (ADS)

Del Zanna, L.; Bucciantini, N.

2018-06-01

The exponential amplification of initial seed magnetic fields in relativistic plasmas is a very important topic in astrophysics, from the conditions in the early Universe to the interior of neutron stars. While dynamo action in a turbulent plasma is often invoked, in the last years a novel mechanism of quantum origin has gained increasingly more attention, namely the Chiral Magnetic Effect (CME). This has been recognized in semi-metals and it is most likely at work in the quark-gluon plasma formed in heavy-ion collision experiments, where the highest magnetic fields in nature, up to B ˜ 1018 G, are produced. This effect is expected to survive even at large hydrodynamical/MHD scales and it is based on the chiral anomaly due to an imbalance between left- and right-handed relativistic fermions in the constituent plasma. Such imbalance leads to an electric current parallel to an external magnetic field, which is precisely the same mechanism of an α-dynamo action in classical MHD. Here we extend the close parallelism between the chiral and the dynamo effects to relativistic plasmas and we propose a unified, fully covariant formulation of the generalized Ohm's law. Moreover, we derive for the first time the 3 + 1 general relativistic MHD equations for a chiral plasma both in flat and curved spacetimes, in view of numerical investigation of the CME in compact objects, especially magnetars, or of the interplay among the non-ideal magnetic effects of dynamo, the CME and reconnection.

Nanopaleomagnetism of Meteoritic Fe-Ni: the Potential for Time-Resolved Remanence Records within the Cloudy Zone

NASA Astrophysics Data System (ADS)

Harrison, R. J.; Bryson, J. F.; Kasama, T.; Church, N. S.; Herrero Albillos, J.; Kronast, F.; Ghidini, M.; Redfern, S. A.; van der Laan, G.; Tyliszczak, T.

2013-12-01

Paleomagnetic signals recorded by meteorites provide compelling evidence that the liquid cores of differentiated asteroids generated magnetic dynamo fields. Here we argue that magnetic nanostructures unique to meteoritic Fe-Ni metal are capable of carrying a time-resolved record of asteroid dynamo activity, a prospect that could revolutionise our understanding of the thermochemical conditions of differentiated bodies in the early solar system. Using a combination of high-resolution magnetic imaging techniques (including electron holography, magnetic force microscopy, X-ray photoemission electron microscopy and scanning transmission X-ray microscopy) we reveal the origins of the dramatic changes in magnetic properties that are associated with the transition from kamacite - tetrataenite rim - cloudy zone - plessite, typical of Fe-Ni intergrowths. The cloudy zone is comprised of nanoscale islands of tetrataenite (FeNi) coherently intergrown with a hitherto unobserved soft magnetic phase (Fe3Ni). The tetrataenite island diameter decreases with increasing lateral distance from the tetrataenite rim. Exchange coupling between the hard tetrataenite islands and the soft matrix phase leads to an exchange spring effect that lowers the tetrataenite switching field and causes a systematic variation in microcoercivity throughout the cloudy zone. The cloudy zone displays a complex interlocking magnetic domain pattern caused by uniaxial single domain tetrataenite islands with easy axes distributed along all three of the possible <100> crystallographic orientations. The coarse and intermediate cloudy zones contain a random distribution of all three easy axes. The fine cloudy zone, on the other hand, contains one dominant easy axis direction. This easy axis distribution suggests that strong interaction fields (either magnetic or stress) were present in this region at the time of tetrataenite formation, which likely originated from the neighbouring plessite. The easy axis distribution in the coarse and intermediate cloudy zone indicates a lack of interaction fields present at the time of formation, implying that deviations from randomness could be used to detect the presence of an external (e.g. dynamo) field. Zoned metallic grains within chondritic meteorites originating from the top ~5-10% of a differentiated asteroid may have formed their cloudy zones while the core was generating a dynamo field. In this case, as the cloudy zone formed continuously over a period of 10-100 Ma it had the potential to encode sequential information regarding the dynamo field as the spinodal microstructure developed laterally. Thus the local magnetic structure as a function of position throughout the cloudy zone could relate to the time dependence of an asteroid dynamo field. The experimental and analysis methods presented in this study could, in principle, be used to measure the relative strength (proportion of dominant easy axis) and direction (direction of dominant easy axis) of an asteroid dynamo field over ~100 Ma.

A COUPLED 2 × 2D BABCOCK–LEIGHTON SOLAR DYNAMO MODEL. II. REFERENCE DYNAMO SOLUTIONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lemerle, Alexandre; Charbonneau, Paul, E-mail: lemerle@astro.umontreal.ca, E-mail: paulchar@astro.umontreal.ca

In this paper we complete the presentation of a new hybrid 2 × 2D flux transport dynamo (FTD) model of the solar cycle based on the Babcock–Leighton mechanism of poloidal magnetic field regeneration via the surface decay of bipolar magnetic regions (BMRs). This hybrid model is constructed by allowing the surface flux transport (SFT) simulation described in Lemerle et al. to provide the poloidal source term to an axisymmetric FTD simulation defined in a meridional plane, which in turn generates the BMRs required by the SFT. A key aspect of this coupling is the definition of an emergence function describing the probabilitymore » of BMR emergence as a function of the spatial distribution of the internal axisymmetric magnetic field. We use a genetic algorithm to calibrate this function, together with other model parameters, against observed cycle 21 emergence data. We present a reference dynamo solution reproducing many solar cycle characteristics, including good hemispheric coupling, phase relationship between the surface dipole and the BMR-generating internal field, and correlation between dipole strength at cycle maximum and peak amplitude of the next cycle. The saturation of the cycle amplitude takes place through the quenching of the BMR tilt as a function of the internal field. The observed statistical scatter about the mean BMR tilt, built into the model, acts as a source of stochasticity which dominates amplitude fluctuations. The model thus can produce Dalton-like epochs of strongly suppressed cycle amplitude lasting a few cycles and can even shut off entirely following an unfavorable sequence of emergence events.« less

Magnetic Eigenmode Analysis of the Madison Dynamo Experiment

NASA Astrophysics Data System (ADS)

Nornberg, M. D.; Forest, C. B.; Kendrick, Roch; O'Connell, R.; Spence, E. J.

2004-11-01

The magnetic field generated by a spherical homogeneous liquid-sodium dynamo is explored in terms of the magnetic eigenmodes predicted by Dudley and James. The flow geometry chosen corresponds to the T2S2 flow and is created by two counter-rotating propellers driven by 100HP motors with flow velocities up to 15 m/s. A perturbative magnetic field is generated by pulsing a set axial field coils. The largest growing eigenmode is predicted by linear analysis to be a strong equatorial-dipole field. The field is measured using an array of Hall probes both on the surface of the sphere and within the sphere. From the measured field the growth or decay rates of the magnetic eigenmodes are determined. Turbulence in the flow is expected to give rise to modifications of the growth rates and the structure of the eigenmodes.

An efficient spectral method for the simulation of dynamos in Cartesian geometry and its implementation on massively parallel computers

NASA Astrophysics Data System (ADS)

Stellmach, Stephan; Hansen, Ulrich

2008-05-01

Numerical simulations of the process of convection and magnetic field generation in planetary cores still fail to reach geophysically realistic control parameter values. Future progress in this field depends crucially on efficient numerical algorithms which are able to take advantage of the newest generation of parallel computers. Desirable features of simulation algorithms include (1) spectral accuracy, (2) an operation count per time step that is small and roughly proportional to the number of grid points, (3) memory requirements that scale linear with resolution, (4) an implicit treatment of all linear terms including the Coriolis force, (5) the ability to treat all kinds of common boundary conditions, and (6) reasonable efficiency on massively parallel machines with tens of thousands of processors. So far, algorithms for fully self-consistent dynamo simulations in spherical shells do not achieve all these criteria simultaneously, resulting in strong restrictions on the possible resolutions. In this paper, we demonstrate that local dynamo models in which the process of convection and magnetic field generation is only simulated for a small part of a planetary core in Cartesian geometry can achieve the above goal. We propose an algorithm that fulfills the first five of the above criteria and demonstrate that a model implementation of our method on an IBM Blue Gene/L system scales impressively well for up to O(104) processors. This allows for numerical simulations at rather extreme parameter values.

Dynamo Induced by Time-periodic Force

NASA Astrophysics Data System (ADS)

Wei, Xing

2018-03-01

To understand the dynamo driven by time-dependent flow, e.g., turbulence, we investigate numerically the dynamo induced by time-periodic force in rotating magnetohydrodynamic flow and focus on the effect of force frequency on the dynamo action. It is found that the dynamo action depends on the force frequency. When the force frequency is near resonance the force can drive dynamo, but when it is far away from resonance dynamo fails. In the frequency range near resonance to support dynamo, the force frequency at resonance induces a weak magnetic field and magnetic energy increases as the force frequency deviates from the resonant frequency. This is opposite to the intuition that a strong flow at resonance will induce a strong field. It is because magnetic field nonlinearly couples with fluid flow in the self-sustained dynamo and changes the resonance of driving force and inertial wave.

Large-scale dynamo action precedes turbulence in shearing box simulations of the magnetorotational instability

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bhat, Pallavi; Ebrahimi, Fatima; Blackman, Eric G.

Here, we study the dynamo generation (exponential growth) of large-scale (planar averaged) fields in unstratified shearing box simulations of the magnetorotational instability (MRI). In contrast to previous studies restricted to horizontal (x–y) averaging, we also demonstrate the presence of large-scale fields when vertical (y–z) averaging is employed instead. By computing space–time planar averaged fields and power spectra, we find large-scale dynamo action in the early MRI growth phase – a previously unidentified feature. Non-axisymmetric linear MRI modes with low horizontal wavenumbers and vertical wavenumbers near that of expected maximal growth, amplify the large-scale fields exponentially before turbulence and high wavenumbermore » fluctuations arise. Thus the large-scale dynamo requires only linear fluctuations but not non-linear turbulence (as defined by mode–mode coupling). Vertical averaging also allows for monitoring the evolution of the large-scale vertical field and we find that a feedback from horizontal low wavenumber MRI modes provides a clue as to why the large-scale vertical field sustains against turbulent diffusion in the non-linear saturation regime. We compute the terms in the mean field equations to identify the individual contributions to large-scale field growth for both types of averaging. The large-scale fields obtained from vertical averaging are found to compare well with global simulations and quasi-linear analytical analysis from a previous study by Ebrahimi & Blackman. We discuss the potential implications of these new results for understanding the large-scale MRI dynamo saturation and turbulence.« less

Large-scale dynamo action precedes turbulence in shearing box simulations of the magnetorotational instability

DOE PAGES

Bhat, Pallavi; Ebrahimi, Fatima; Blackman, Eric G.

2016-07-06

Here, we study the dynamo generation (exponential growth) of large-scale (planar averaged) fields in unstratified shearing box simulations of the magnetorotational instability (MRI). In contrast to previous studies restricted to horizontal (x–y) averaging, we also demonstrate the presence of large-scale fields when vertical (y–z) averaging is employed instead. By computing space–time planar averaged fields and power spectra, we find large-scale dynamo action in the early MRI growth phase – a previously unidentified feature. Non-axisymmetric linear MRI modes with low horizontal wavenumbers and vertical wavenumbers near that of expected maximal growth, amplify the large-scale fields exponentially before turbulence and high wavenumbermore » fluctuations arise. Thus the large-scale dynamo requires only linear fluctuations but not non-linear turbulence (as defined by mode–mode coupling). Vertical averaging also allows for monitoring the evolution of the large-scale vertical field and we find that a feedback from horizontal low wavenumber MRI modes provides a clue as to why the large-scale vertical field sustains against turbulent diffusion in the non-linear saturation regime. We compute the terms in the mean field equations to identify the individual contributions to large-scale field growth for both types of averaging. The large-scale fields obtained from vertical averaging are found to compare well with global simulations and quasi-linear analytical analysis from a previous study by Ebrahimi & Blackman. We discuss the potential implications of these new results for understanding the large-scale MRI dynamo saturation and turbulence.« less

Energy flux determines magnetic field strength of planets and stars.

PubMed

Christensen, Ulrich R; Holzwarth, Volkmar; Reiners, Ansgar

2009-01-08

The magnetic fields of Earth and Jupiter, along with those of rapidly rotating, low-mass stars, are generated by convection-driven dynamos that may operate similarly (the slowly rotating Sun generates its field through a different dynamo mechanism). The field strengths of planets and stars vary over three orders of magnitude, but the critical factor causing that variation has hitherto been unclear. Here we report an extension of a scaling law derived from geodynamo models to rapidly rotating stars that have strong density stratification. The unifying principle in the scaling law is that the energy flux available for generating the magnetic field sets the field strength. Our scaling law fits the observed field strengths of Earth, Jupiter, young contracting stars and rapidly rotating low-mass stars, despite vast differences in the physical conditions of the objects. We predict that the field strengths of rapidly rotating brown dwarfs and massive extrasolar planets are high enough to make them observable.

Dynamo generation of magnetic fields in three-dimensional space - Solar cycle main flux tube formation and reversals

NASA Astrophysics Data System (ADS)

Yoshimura, H.

1983-08-01

The case of the solar magnetic cycle is investigated as a prototype of the dynamo processes involved in the generation of magnetic fields in astrophysics. Magnetohydrodynamic (MHD) equations are solved using a numerical method with a prescribed velocity field in order follow the movement and deformation. It is shown that a simple combination of differential rotation and global convection, given by a linear analysis of fluid dynamics in a rotating sphere, can perpetually create and reverse great magnetic flux tubes encircling the sun. These main flux tubes of the solar cycle are the progenitors of small-scale flux ropes of the solar activity. These findings indicate that magnetic fields can be generated by fluid motions and that MHD equations have a new type of oscillatory solution. It is shown that the solar cycle can be identified with one of these oscillatory solutions. It is proposed that the formation of magnetic flux tubes by streaming plasma flows is a universal mechanism of flux tube formation in astrophysics.

Summary of the Madison Dynamo Experiment

NASA Astrophysics Data System (ADS)

Kendrick, R. D.; Spence, E. J.; Nornberg, M. D.; Forest, C. B.

2001-10-01

A spherical dynamo experiment has been constructed at the University of Wisconsin's liquid sodium facility. The goals of the experiment are to observe and understand magnetic instabilities driven by flow shear in MHD systems, investigate MHD turbulence for magnetic Reynolds numbers of ~100, and understand the role of fluid turbulence in current generation. Magnetic field generation is possible for only specific flow geometries. We have studied and achieved simple roll flow geometries in a full scale water experiment. Results from this experiment have guided the design of the sodium experiment. The experiment consists of a 1 m diameter, spherical stainless steel vessel filled with liquid sodium at 110 Celsius. Two 100 Hp motors with impellers drive flows in the liquid sodium with flow velocities ~ 15 m/s. A gaussian grid of Hall probes on the surface of the sodium vessel measure the generated external magnetic field. Hall probe feed-thru arrays measure the internal field. Preliminary investigations include measurements of the turbulent electromotive force and excitation of magnetic eigenmodes.

Design of the Madison Dynamo Experiment

NASA Astrophysics Data System (ADS)

Kendrick, R. D.; Bayliss, R. A.; Forest, C. B.; Nornberg, M. D.; O'Connell, R.; Spence, E. J.

2003-10-01

A spherical dynamo experiment has been constructed at the University of Wisconsin's liquid sodium facility. The goals of the experiment are to observe and understand magnetic instabilities driven by flow shear in MHD systems, investigate MHD turbulence for magnetic Reynolds numbers of ˜100, and understand the role of fluid turbulence in current generation. Magnetic field generation is possible for only specific flow geometries. We have studied and achieved simple roll flow geometries in a full scale water experiment. Results from this experiment have guided the design of the sodium experiment. The experiment consists of a 1 m diameter, spherical stainless steel vessel filled with liquid sodium at 110 Celsius. Two 100 Hp motors with impellers drive flows in the liquid sodium with flow velocities ˜ 15 m/s. A grid of Hall probes on the surface of the sodium vessel measure the generated external magnetic field. Hall probe feed-thru arrays measure the internal field. Preliminary investigations include measurements of the turbulent electromotive force and excitation of magnetic eigenmodes.

Modeling of the coupled magnetospheric and neutral wind dynamos

NASA Technical Reports Server (NTRS)

Thayer, Jeff P.

1993-01-01

The solar wind interaction with the earth's magnetosphere generates electric fields and currents that flow from the magnetosphere to the ionosphere at high latitudes. Consequently, the neutral atmosphere is subject to the dissipation and conversion of this electrical energy to thermal and mechanical energy through Joule heating and Lorentz forcing. As a result of the mechanical energy stored within the neutral wind (caused in part by Lorentz--and pressure gradient--forces set up by the magnetospheric flux of electrical energy), electric currents and fields can be generated in the ionosphere through the neutral wind dynamo mechanism. At high latitudes this source of electrical energy has been largely ignored in past studies, owing to the assumed dominance of the solar wind/magnetospheric dynamo as an electrical energy source to the ionosphere. However, other researchers have demonstrated that the available electrical energy provided by the neutral wind is significant at high latitudes, particularly in the midnight sector of the polar cap and in the region of the magnetospheric convection reversal. As a result, the conclusions of a number of broad ranging high-latitude investigations may be modified if the neutral-wind contribution to high-latitude electrodynamics is properly accounted for. These include the following: studies assessing solar wind-magnetospheric coupling by comparing the cross polar cap potential with solar wind parameters; research based on the alignment of particle precipitation with convection or field aligned current boundaries; and synoptic investigations attributing seasonal variations in the observed electric field and current patterns to external sources. These research topics have been initiated by satellite and ground-based observations and have been attributed to magnetospheric causes. However, the contribution of the neutral wind to the high-latitude electric field and current systems and their seasonal and local time dependence has yet to be quantitatively evaluated. In this program, we are evaluating the coupled magnetospheric and neutral wind dynamos at high latitudes under various conditions. In addition to examining the impact of seasonal variations, we are investigating the consequences of the separate dynamos having pure current-source or voltage-source behaviors.

Multiple scale dynamo

PubMed Central

Le Mouël, Jean-Louis; Allègre, Claude J.; Narteau, Clément

1997-01-01

A scaling law approach is used to simulate the dynamo process of the Earth’s core. The model is made of embedded turbulent domains of increasing dimensions, until the largest whose size is comparable with the site of the core, pervaded by large-scale magnetic fields. Left-handed or right-handed cyclones appear at the lowest scale, the scale of the elementary domains of the hierarchical model, and disappear. These elementary domains then behave like electromotor generators with opposite polarities depending on whether they contain a left-handed or a right-handed cyclone. To transfer the behavior of the elementary domains to larger ones, a dynamic renormalization approach is used. A simple rule is adopted to determine whether a domain of scale l is a generator—and what its polarity is—in function of the state of the (l − 1) domains it is made of. This mechanism is used as the main ingredient of a kinematic dynamo model, which displays polarity intervals, excursions, and reversals of the geomagnetic field. PMID:11038547

The relation between magnetic and material arms in models for spiral galaxies

NASA Astrophysics Data System (ADS)

Moss, D.; Beck, R.; Sokoloff, D.; Stepanov, R.; Krause, M.; Arshakian, T. G.

2013-08-01

Context. Observations of polarized radio emission show that large-scale (regular) magnetic fields in spiral galaxies are not fully axisymmetric, but generally stronger in interarm regions. In some nearby galaxies such as NGC 6946 they are organized in narrow magnetic arms situated between the material spiral arms. Aims: The phenomenon of magnetic arms and their relation to the optical spiral arms (the material arms) calls for an explanation in the framework of galactic dynamo theory. Several possibilities have been suggested but are not completely satisfactory; here we attempt a consistent investigation. Methods: We use a 2D mean-field dynamo model in the no-z approximation and add injections of small-scale magnetic field, taken to result from supernova explosions, to represent the effects of dynamo action on smaller scales. This injection of small scale field is situated along the spiral arms, where star-formation mostly occurs. Results: A straightforward explanation of magnetic arms as a result of modulation of the dynamo mechanism by material arms struggles to produce pronounced magnetic arms, at least with realistic parameters, without introducing new effects such as a time lag between Coriolis force and α-effect. In contrast, by taking into account explicitly the small-scale magnetic field that is injected into the arms by the action of the star forming regions that are concentrated there, we can obtain dynamo models with magnetic structures of various forms that can be compared with magnetic arms. These are rather variable entities and their shape changes significantly on timescales of a few 100 Myr. Properties of magnetic arms can be controlled by changing the model parameters. In particular, a lower injection rate of small-scale field makes the magnetic configuration smoother and eliminates distinct magnetic arms. Conclusions: We conclude that magnetic arms can be considered as coherent magnetic structures generated by large-scale dynamo action, and associated with spatially modulated small-scale magnetic fluctuations, caused by enhanced star formation rates within the material arms.

Solar Activity Heading for a Maunder Minimum?

NASA Astrophysics Data System (ADS)

Schatten, K. H.; Tobiska, W. K.

2003-05-01

Long-range (few years to decades) solar activity prediction techniques vary greatly in their methods. They range from examining planetary orbits, to spectral analyses (e.g. Fourier, wavelet and spectral analyses), to artificial intelligence methods, to simply using general statistical techniques. Rather than concentrate on statistical/mathematical/numerical methods, we discuss a class of methods which appears to have a "physical basis." Not only does it have a physical basis, but this basis is rooted in both "basic" physics (dynamo theory), but also solar physics (Babcock dynamo theory). The class we discuss is referred to as "precursor methods," originally developed by Ohl, Brown and Williams and others, using geomagnetic observations. My colleagues and I have developed some understanding for how these methods work and have expanded the prediction methods using "solar dynamo precursor" methods, notably a "SODA" index (SOlar Dynamo Amplitude). These methods are now based upon an understanding of the Sun's dynamo processes- to explain a connection between how the Sun's fields are generated and how the Sun broadcasts its future activity levels to Earth. This has led to better monitoring of the Sun's dynamo fields and is leading to more accurate prediction techniques. Related to the Sun's polar and toroidal magnetic fields, we explain how these methods work, past predictions, the current cycle, and predictions of future of solar activity levels for the next few solar cycles. The surprising result of these long-range predictions is a rapid decline in solar activity, starting with cycle #24. If this trend continues, we may see the Sun heading towards a "Maunder" type of solar activity minimum - an extensive period of reduced levels of solar activity. For the solar physicists, who enjoy studying solar activity, we hope this isn't so, but for NASA, which must place and maintain satellites in low earth orbit (LEO), it may help with reboost problems. Space debris, and other aspects of objects in LEO will also be affected. This research is supported by the NSF and NASA.

Derivation of dynamo current drive in a closed-current volume and stable current sustainment in the HIT-SI experiment

NASA Astrophysics Data System (ADS)

Hossack, A. C.; Sutherland, D. A.; Jarboe, T. R.

2017-02-01

A derivation is given showing that the current inside a closed-current volume can be sustained against resistive dissipation by appropriately phased magnetic perturbations. Imposed-dynamo current drive theory is used to predict the toroidal current evolution in the helicity injected torus with steady inductive helicity injection (HIT-SI) experiment as a function of magnetic fluctuations at the edge. Analysis of magnetic fields from a HIT-SI discharge shows that the injector-imposed fluctuations are sufficient to sustain the measured toroidal current without instabilities whereas the small, plasma-generated magnetic fluctuations are not sufficiently large to sustain the current.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Garcia de Andrade, L. C.

Vishik's anti-dynamo theorem is applied to a nonstretched twisted magnetic flux tube in Riemannian space. Marginal or slow dynamos along curved (folded), torsioned (twisted), and nonstretching flux tubes plasma flows are obtained. Riemannian curvature of the twisted magnetic flux tube is computed in terms of the Frenet curvature in the thin tube limit. It is shown that, for nonstretched filaments, fast dynamo action in the diffusive case cannot be obtained, in agreement with Vishik's argument that fast dynamos cannot be obtained in nonstretched flows. Instead of a fast dynamo, a nonuniform stretching slow dynamo is obtained. An example is given,more » which generalizes plasma dynamo laminar flows, recently presented by Wang et al. [Phys Plasmas 9, 1491 (2002)], in the case of low magnetic Reynolds number Re{sub m}{>=}210. Curved and twisting Riemannian heliotrons, where nondynamo modes are found even when stretching is present, shows that the simple presence of stretching is not enough for the existence of dynamo action. In this paper, folding plays the role of Riemannian curvature and can be used to cancel magnetic fields, not enhancing the dynamo action. Nondynamo modes are found for certain values of torsion, or Frenet curvature (folding) in the spirit of the anti-dynamo theorem. It is also shown that curvature and stretching are fundamental for the existence of fast dynamos in plasmas.« less

Influence of large-scale zonal flows on the evolution of stellar and planetary magnetic fields

NASA Astrophysics Data System (ADS)

Petitdemange, Ludovic; Schrinner, Martin; Dormy, Emmanuel; ENS Collaboration

2011-10-01

Zonal flows and magnetic field are present in various objects as accretion discs, stars and planets. Observations show a huge variety of stellar and planetary magnetic fields. Of particular interest is the understanding of cyclic field variations, as known from the sun. They are often explained by an important Ω-effect, i.e., by the stretching of field lines because of strong differential rotation. We computed the dynamo coefficients for an oscillatory dynamo model with the help of the test-field method. We argue that this model is of α2 Ω -type and here the Ω-effect alone is not responsible for its cyclic time variation. More general conditions which lead to dynamo waves in global direct numerical simulations are presented. Zonal flows driven by convection in planetary interiors may lead to secondary instabilities. We showed that a simple, modified version of the MagnetoRotational Instability, i.e., the MS-MRI can develop in planteray interiors. The weak shear yields an instability by its constructive interaction with the much larger rotation rate of planets. We present results from 3D simulations and show that 3D MS-MRI modes can generate wave pattern at the surface of the spherical numerical domain. Zonal flows and magnetic field are present in various objects as accretion discs, stars and planets. Observations show a huge variety of stellar and planetary magnetic fields. Of particular interest is the understanding of cyclic field variations, as known from the sun. They are often explained by an important Ω-effect, i.e., by the stretching of field lines because of strong differential rotation. We computed the dynamo coefficients for an oscillatory dynamo model with the help of the test-field method. We argue that this model is of α2 Ω -type and here the Ω-effect alone is not responsible for its cyclic time variation. More general conditions which lead to dynamo waves in global direct numerical simulations are presented. Zonal flows driven by convection in planetary interiors may lead to secondary instabilities. We showed that a simple, modified version of the MagnetoRotational Instability, i.e., the MS-MRI can develop in planteray interiors. The weak shear yields an instability by its constructive interaction with the much larger rotation rate of planets. We present results from 3D simulations and show that 3D MS-MRI modes can generate wave pattern at the surface of the spherical numerical domain. The first author thanks DFG and PlanetMag project for financial support.

The nuclear dynamo; Can a nuclear tornado annihilate nations

DOE Office of Scientific and Technical Information (OSTI.GOV)

McNally, J.R. Jr.

1991-01-01

This paper reports on the development of the hypothesis of a nuclear dynamo for a controlled nuclear fusion reactor. This dynamo hypothesis suggests properties for a nuclear tornado that could annihilate nations if accidentally triggered by a single high yield to weight nuclear weapon detonation. The formerly classified reports on ignition of the atmosphere, the properties of a nuclear dynamo, methods to achieve a nuclear dynamo in the laboratory, and the analogy of a nuclear dynamo to a nuclear tornado are discussed. An unclassified international study of this question is urged.

Magnetic flux pumping in 3D nonlinear magnetohydrodynamic simulations

DOE PAGES

Krebs, I.; Jardin, S. C.; Gunter, S.; ...

2017-09-27

A self-regulating magnetic flux pumping mechanism in tokamaks that maintains the core safety factor at q≈1, thus preventing sawteeth, is analyzed in nonlinear 3D magnetohydrodynamic simulations using the M3D-C1 code. In these simulations, the most important mechanism responsible for the flux pumping is that a saturated (m=1,n=1) quasi-interchange instability generates an effective negative loop voltage in the plasma center via a dynamo effect. It is shown that sawtoothing is prevented in the simulations if β is sufficiently high to provide the necessary drive for the (m=1,n=1) instability that generates the dynamo loop voltage. In conclusion, the necessary amount of dynamomore » loop voltage is determined by the tendency of the current density profile to centrally peak which, in our simulations, is controlled by the peakedness of the applied heat source profile.« less

Overview of the Madison Dynamo Experiment

NASA Astrophysics Data System (ADS)

Kendrick, R. D.; Spence, E. J.; Nornberg, M. D.; Jacobson, C. M.; Parada, C. A.; Forest, C. B.

2006-10-01

A spherical dynamo experiment has been constructed at the University of Wisconsin-Madison's liquid-sodium facility. The experiment is designed to self-generate magnetic fields from flows of conducting metal. The apparatus consists of a 1 m diameter, spherical stainless steel vessel filled with liquid sodium. Two 100 Hp motors drive impellers which generate the flow. The motors have been operated up to 1300 RPM (70% of design specification), achieving a magnetic Reynolds number of 130, based on impeller tip speed. Various polarizations of external magnetic fields have been applied to the sodium, and the induced magnetic field has been measured by both internal and external Hall probe arrays. The voltage induced across the sphere by the turbulent flow has been measured. Techniques for using ultrasound Doppler velocimetry have been explored in the water model of the experiment, including the use of high-pressure bubbles as seed particles.

Energy coupling between the solar wind and the magnetosphere

NASA Technical Reports Server (NTRS)

Akasofu, S.-I.

1981-01-01

A description is given of the path leading to the first approximation expression for the solar wind-magnetosphere energy coupling function (epsilon), which correlates well with the total energy consumption rate (U sub T) of the magnetosphere. It is shown that epsilon is the primary factor controlling the time development of magnetospheric substorms and storms. The finding of this particular expression epsilon indicates how the solar wind couples its energy to the magnetosphere; the solar wind and the magnetosphere make up a dynamo. In fact, the power generated by the dynamo can be identified as epsilon through the use of a dimensional analysis. In addition, the finding of epsilon suggests that the magnetosphere is closer to a directly driven system than to an unloading system which stores the generated energy before converting it to substorm and storm energies. The finding of epsilon and its implications is considered to have significantly advanced and improved the understanding of magnetospheric processes.

Chandra Observations of Magnetic White Dwarfs and Their Theoretical Implications

NASA Technical Reports Server (NTRS)

Musielak, Z. E.; Noble, M.; Porter, J. G.; Winget, D. E.; Six, N. Frank (Technical Monitor)

2002-01-01

Observations of cool DA and DB white dwarfs have not yet been successful in detecting coronal X-ray emission but observations of late-type dwarfs and giants show that coronae are common for these stars. To produce coronal X-rays, a star must have dynamo-generated surface magnetic fields and a well-developed convection zone. There is strong observational evidence that the DA star LHS 1038 and the DB star GD 358 have weak and variable surface magnetic fields. Since these fields are likely to be generated by dynamo action and since both stars have well-developed convection zones, theory predicts detectable levels of coronal X-rays from these white dwarfs. However, we present analysis of Chandra observations of both stars showing no detectable X-ray emission. The derived upper limits for the X-ray fluxes provide strong constraints on theories of formation of coronae around magnetic white dwarfs.

The magnetic tides of Honolulu

USGS Publications Warehouse

Love, Jeffrey J.; Rigler, Erin Joshua

2013-01-01

We review the phenomenon of time-stationary, periodic quiet-time geomagnetic tides. These are generated by the ionospheric and oceanic dynamos, and, to a lesser-extent, by the quiet-time magnetosphere, and they are affected by currents induced in the Earth's electrically conducting interior. We examine historical time series of hourly magnetic-vector measurements made at the Honolulu observatory. We construct high-resolution, frequency-domain Lomb-periodogram and maximum-entropy power spectra that reveal a panorama of stationary harmonics across periods from 0.1 to 10000.0-d, including harmonics that result from amplitude and phase modulation. We identify solar-diurnal tides and their annual and solar-cycle sideband modulations, lunar semi-diurnal tides and their solar-diurnal sidebands, and tides due to precession of lunar eccentricity and nodes. We provide evidence that a method intended for separating the ionospheric and oceanic dynamo signals by midnight subsampling of observatory data time series is prone to frequency-domain aliasing. The tidal signals we summarize in this review can be used to test our fundamental understanding of the dynamics of the quiet-time ionosphere and magnetosphere, induction in the ocean and in the electrically conducting interior of the Earth, and they are useful for defining a quiet-time baseline against which magnetospheric-storm intensity is measured.

Kinematic dynamo action in square and hexagonal patterns.

PubMed

Favier, B; Proctor, M R E

2013-11-01

We consider kinematic dynamo action in rapidly rotating Boussinesq convection just above onset. The velocity is constrained to have either a square or a hexagonal pattern. For the square pattern, large-scale dynamo action is observed at onset, with most of the magnetic energy being contained in the horizontally averaged component. As the magnetic Reynolds number increases, small-scale dynamo action becomes possible, reducing the overall growth rate of the dynamo. For the hexagonal pattern, the breaking of symmetry between up and down flows results in an effective pumping velocity. For intermediate rotation rates, this additional effect can prevent the growth of any mean-field dynamo, so that only a small-scale dynamo is eventually possible at large enough magnetic Reynolds number. For very large rotation rates, this pumping term becomes negligible, and the dynamo properties of square and hexagonal patterns are qualitatively similar. These results hold for both perfectly conducting and infinite magnetic permeability boundary conditions.

Large-scale vortices in compressible turbulent medium with the magnetic field

NASA Astrophysics Data System (ADS)

Gvaramadze, V. V.; Dimitrov, B. G.

1990-08-01

An averaged equation which describes the large scale vortices and Alfven waves generation in a compressible helical turbulent medium with a constant magnetic field is presented. The presence of the magnetic field leads to anisotropization of the vortex generation. Possible applications of the anisotropic vortex dynamo effect are accretion disks of compact objects.

Reinforcement of double dynamo waves as a source of solar activity and its prediction on millennium timescale

NASA Astrophysics Data System (ADS)

Popova, E.; Zharkova, V. V.; Shepherd, S. J.; Zharkov, S.

2016-12-01

Using the principal components of solar magnetic field variations derived from the synoptic maps for solar cycles 21-24 with Principal Components Analysis (PCA) (Zharkova et al, 2015) we confirm our previous prediction of the upcoming Maunder minimum to occur in cycles 25-27, or in 2020-2055. We also use a summary curve of the two eigen vectors of solar magnetic field oscillations (or two dynamo waves) to extrapolate solar activity backwards to the three millennia and to compare it with relevant historic and Holocene data. Extrapolation of the summary curve confirms the eight grand cycles of 350-400-years superimposed on 22 year-cycles caused by beating effect of the two dynamo waves generated in the two (deep and shallow) layers of the solar interior. The grand cycles in different periods comprise a different number of individual 22-year cycles; the longer the grand cycles the larger number of 22 year cycles and the smaller their amplitudes. We also report the super-grand cycle of about 2000 years often found in solas activity with spectral analysis. Furthermore, the summary curve reproduces a remarkable resemblance to the sunspot and terrestrial activity reported in the past: the recent Maunder Minimum (1645-1715), Dalton minimum (1790-1815), Wolf minimum (1200), Homeric minimum (800-900 BC), the Medieval Warmth Period (900-1200), the Roman Warmth Period (400-10BC) and so on. Temporal variations of these dynamo waves are modelled with the two layer mean dynamo model with meridional circulation revealing a remarkable resemblance of the butterfly diagram to the one derived for the last Maunder minimum in 17 century and predicting the one for the upcoming Maunder minimum in 2020-2055.

Ab Initio Simulations of a Supernova-driven Galactic Dynamo in an Isolated Disk Galaxy

DOE PAGES

Butsky, Iryna; Zrake, Jonathan; Kim, Ji-hoon; ...

2017-07-10

Here, we study the magnetic field evolution of an isolated spiral galaxy, using isolated Milky Way–mass galaxy formation simulations and a novel prescription for magnetohydrodynamic (MHD) supernova feedback. Our main result is that a galactic dynamo can be seeded and driven by supernova explosions, resulting in magnetic fields whose strength and morphology are consistent with observations. In our model, supernovae supply thermal energy and a low-level magnetic field along with their ejecta. The thermal expansion drives turbulence, which serves a dual role by efficiently mixing the magnetic field into the interstellar medium and amplifying it by means of a turbulentmore » dynamo. The computational prescription for MHD supernova feedback has been implemented within the publicly available ENZO code and is fully described in this paper. This improves upon ENZO's existing modules for hydrodynamic feedback from stars and active galaxies. We find that the field attains microgauss levels over gigayear timescales throughout the disk. The field also develops a large-scale structure, which appears to be correlated with the disk's spiral arm density structure. We find that seeding of the galactic dynamo by supernova ejecta predicts a persistent correlation between gas metallicity and magnetic field strength. We also generate all-sky maps of the Faraday rotation measure from the simulation-predicted magnetic field, and we present a direct comparison with observations.« less

Kinematic dynamo, supersymmetry breaking, and chaos

NASA Astrophysics Data System (ADS)

Ovchinnikov, Igor V.; Enßlin, Torsten A.

2016-04-01

The kinematic dynamo (KD) describes the growth of magnetic fields generated by the flow of a conducting medium in the limit of vanishing backaction of the fields onto the flow. The KD is therefore an important model system for understanding astrophysical magnetism. Here, the mathematical correspondence between the KD and a specific stochastic differential equation (SDE) viewed from the perspective of the supersymmetric theory of stochastics (STS) is discussed. The STS is a novel, approximation-free framework to investigate SDEs. The correspondence reported here permits insights from the STS to be applied to the theory of KD and vice versa. It was previously known that the fast KD in the idealistic limit of no magnetic diffusion requires chaotic flows. The KD-STS correspondence shows that this is also true for the diffusive KD. From the STS perspective, the KD possesses a topological supersymmetry, and the dynamo effect can be viewed as its spontaneous breakdown. This supersymmetry breaking can be regarded as the stochastic generalization of the concept of dynamical chaos. As this supersymmetry breaking happens in both the diffusive and the nondiffusive cases, the necessity of the underlying SDE being chaotic is given in either case. The observed exponentially growing and oscillating KD modes prove physically that dynamical spectra of the STS evolution operator that break the topological supersymmetry exist with both real and complex ground state eigenvalues. Finally, we comment on the nonexistence of dynamos for scalar quantities.

Ab Initio Simulations of a Supernova-driven Galactic Dynamo in an Isolated Disk Galaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Butsky, Iryna; Zrake, Jonathan; Kim, Ji-hoon

We study the magnetic field evolution of an isolated spiral galaxy, using isolated Milky Way–mass galaxy formation simulations and a novel prescription for magnetohydrodynamic (MHD) supernova feedback. Our main result is that a galactic dynamo can be seeded and driven by supernova explosions, resulting in magnetic fields whose strength and morphology are consistent with observations. In our model, supernovae supply thermal energy and a low-level magnetic field along with their ejecta. The thermal expansion drives turbulence, which serves a dual role by efficiently mixing the magnetic field into the interstellar medium and amplifying it by means of a turbulent dynamo.more » The computational prescription for MHD supernova feedback has been implemented within the publicly available ENZO code and is fully described in this paper. This improves upon ENZO 's existing modules for hydrodynamic feedback from stars and active galaxies. We find that the field attains microgauss levels over gigayear timescales throughout the disk. The field also develops a large-scale structure, which appears to be correlated with the disk’s spiral arm density structure. We find that seeding of the galactic dynamo by supernova ejecta predicts a persistent correlation between gas metallicity and magnetic field strength. We also generate all-sky maps of the Faraday rotation measure from the simulation-predicted magnetic field, and we present a direct comparison with observations.« less

Ab Initio Simulations of a Supernova-driven Galactic Dynamo in an Isolated Disk Galaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Butsky, Iryna; Zrake, Jonathan; Kim, Ji-hoon

Here, we study the magnetic field evolution of an isolated spiral galaxy, using isolated Milky Way–mass galaxy formation simulations and a novel prescription for magnetohydrodynamic (MHD) supernova feedback. Our main result is that a galactic dynamo can be seeded and driven by supernova explosions, resulting in magnetic fields whose strength and morphology are consistent with observations. In our model, supernovae supply thermal energy and a low-level magnetic field along with their ejecta. The thermal expansion drives turbulence, which serves a dual role by efficiently mixing the magnetic field into the interstellar medium and amplifying it by means of a turbulentmore » dynamo. The computational prescription for MHD supernova feedback has been implemented within the publicly available ENZO code and is fully described in this paper. This improves upon ENZO's existing modules for hydrodynamic feedback from stars and active galaxies. We find that the field attains microgauss levels over gigayear timescales throughout the disk. The field also develops a large-scale structure, which appears to be correlated with the disk's spiral arm density structure. We find that seeding of the galactic dynamo by supernova ejecta predicts a persistent correlation between gas metallicity and magnetic field strength. We also generate all-sky maps of the Faraday rotation measure from the simulation-predicted magnetic field, and we present a direct comparison with observations.« less

Resonant fast dynamo

NASA Technical Reports Server (NTRS)

Strauss, H. R.

1986-01-01

A resonant fast dynamo is found in chaotic shear flows. The dynamo effect is produced by resonant perturbations of the velocity field, similar to resonant diffusion in plasma physics. The dynamo is called fast because the flow produces an electric field independent of the fluid resistivity.

Impact of time-dependent nonaxisymmetric velocity perturbations on dynamo action of von Kármán-like flows.

PubMed

Giesecke, André; Stefani, Frank; Burguete, Javier

2012-12-01

We present numerical simulations of the kinematic induction equation in order to examine the dynamo efficiency of an axisymmetric von Kármán-like flow subject to time-dependent nonaxisymmetric velocity perturbations. The numerical model is based on the setup of the French von Kármán-sodium dynamo (VKS) and on the flow measurements from a water experiment conducted at the University of Navarra in Pamplona, Spain. The principal experimental observations that are modeled in our simulations are nonaxisymmetric vortexlike structures which perform an azimuthal drift motion in the equatorial plane. Our simulations show that the interactions of these periodic flow perturbations with the fundamental drift of the magnetic eigenmode (including the special case of nondrifting fields) essentially determine the temporal behavior of the dynamo state. We find two distinct regimes of dynamo action that depend on the (prescribed) drift frequency of an (m=2) vortexlike flow perturbation. For comparatively slowly drifting vortices we observe a narrow window with enhanced growth rates and a drift of the magnetic eigenmode that is synchronized with the perturbation drift. The resonance-like enhancement of the growth rates takes place when the vortex drift frequency roughly equals the drift frequency of the magnetic eigenmode in the unperturbed system. Outside of this small window, the field generation is hampered compared to the unperturbed case, and the field amplitude of the magnetic eigenmode is modulated with approximately twice the vortex drift frequency. The abrupt transition between the resonant regime and the modulated regime is identified as a spectral exceptional point where eigenvalues (growth rates and frequencies) and eigenfunctions of two previously independent modes collapse. In the actual configuration the drift frequencies of the velocity perturbations that are observed in the water experiment are much larger than the fundamental drift frequency of the magnetic eigenmode that is obtained from our numerical simulations. Hence, we conclude that the fulfillment of the resonance condition might be unlikely in present day dynamo experiments. However, a possibility to increase the dynamo efficiency in the VKS experiment might be realized by an application of holes or fingers on the outer boundary in the equatorial plane. These mechanical distortions provoke an anchorage of the vortices at fixed positions thus allowing an adjustment of the temporal behavior of the nonaxisymmetric flow perturbations.

Lifetime of the Lunar Dynamo Constrained by the Young Apollo Regolith Breccia 15015

NASA Astrophysics Data System (ADS)

Wang, H.; Weiss, B. P.

2016-12-01

Paleomagnetic studies have shown that a dynamo magnetic field of tens of µT likely existed on the surface of the Moon from at least 4.5 to 3.6 Ga and declined to several µT by 3.3 Ga [Weiss and Tikoo, 2014]. Furthermore, a recent analysis of lunar regolith breccia 15498 found that the lunar surface field was still 5 µT at 1-2.5 Ga [Tikoo et al., 2015]. However, a key unknown is when the dynamo finally ceased. To address this, we studied the melt glass matrix of Apollo lunar regolith breccia 15015. 40Ar/39Ar measurements suggest that the glass formed at 1.0 ± 0.2 Ga [Eglinton et al., 1974], consistent with its trapped 40Ar/36Ar model age of 0.5 ± 0.4 Ga [Fagan et al. 2014]. Hysteresis data indicate a predominately pseudo-single domain grain size, making 15015 an exceptional paleomagnetic recorder among lunar rocks. Alternating field (AF) demagnetization and anhysteretic remanence (ARM) paleointensity experiments found that 15015 subsamples with faces exposed to band-saw cutting at Johnson Space Center contain highly stable natural remanence (NRM) (>420 mT) and yield paleointensities up to 60 µT, but have NRM directions that are highly non-unidirectional across the parent sample. Subsamples taken away from the saw-cut faces (>5 mm depth) contain no stable NRM and formed in a paleofield <0.1 µT (Fig. 1). Thermal demagnetization of band-sawed samples found that their AF-stable NRM demagnetizes by 150ºC, indicating that their stable NRMs are in fact partial thermoremanence (TRM) overprints from the band-saw cutting process, rather than true lunar total TRM. Thus, the lunar surface paleomagnetic field recorded by 15015 was apparently extremely weak (<0.1 µT) at 1.0 Ga. For typically assumed lunar interior parameters, essentially all published models of the lunar dynamo predict surface fields >0.1 µT for > 90% of the time period while the dynamo is active. Such a minimum field is comparable to estimates of the strongest lunar crustal surface fields and below even the weakest known dynamo surface field in the solar system today. Therefore, our 0.1 µT upper limit indicates that the lunar dynamo likely turned off sometime between 2.5 Ga and 1.0 Ga. This timing appears to be consistent with both thermochemical convection due to core crystallization and mantle precession as the major power sources for the late lunar dynamo.

Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?

NASA Astrophysics Data System (ADS)

Riols, A.; Rincon, F.; Cossu, C.; Lesur, G.; Ogilvie, G. I.; Longaretti, P.-Y.

2017-02-01

In Keplerian accretion disks, turbulence and magnetic fields may be jointly excited through a subcritical dynamo mechanisminvolving magnetorotational instability (MRI). This dynamo may notably contribute to explaining the time-variability of various accreting systems, as high-resolution simulations of MRI dynamo turbulence exhibit statistical self-organization into large-scale cyclic dynamics. However, understanding the physics underlying these statistical states and assessing their exact astrophysical relevance is theoretically challenging. The study of simple periodic nonlinear MRI dynamo solutions has recently proven useful in this respect, and has highlighted the role of turbulent magnetic diffusion in the seeming impossibility of a dynamo at low magnetic Prandtl number (Pm), a common regime in disks. Arguably though, these simple laminar structures may not be fully representative of the complex, statistically self-organized states expected in astrophysical regimes. Here, we aim at closing this seeming discrepancy by reporting the numerical discovery of exactly periodic, yet semi-statistical "chimeral MRI dynamo states" which are the organized outcome of a succession of MRI-unstable, non-axisymmetric dynamical stages of different forms and amplitudes. Interestingly, these states, while reminiscent of the statistical complexity of turbulent simulations, involve the same physical principles as simpler laminar cycles, and their analysis further confirms the theory that subcritical turbulent magnetic diffusion impedes the sustainment of an MRI dynamo at low Pm. Overall, chimera dynamo cycles therefore offer an unprecedented dual physical and statistical perspective on dynamos in rotating shear flows, which may prove useful in devising more accurate, yet intuitive mean-field models of time-dependent turbulent disk dynamos. Movies associated to Fig. 1 are available at http://www.aanda.org

Do steady fast magnetic dynamos exist?

NASA Technical Reports Server (NTRS)

Finn, John M.; Ott, Edward; Hanson, James D.; Kan, Ittai

1989-01-01

This paper considers the question of the existense of a steady fast kinematic magnetic dynamo for a conducting fluid with a steady velocity field and vanishingly small electrical resistivity. The analysis of examples of steady dynamos, found by considering the zero-resistivity dynamics, indicated that, for sufficiently small resistivity, dynamo action can indeed occur in steady smooth three-dimensional chaotic fluid flows and that fast dynamos should consequently be a typical occurrence for such flows.

Neutron star dynamos and the origins of pulsar magnetism

NASA Technical Reports Server (NTRS)

Thompson, Christopher; Duncan, Robert C.

1993-01-01

Neutron star convection is a transient phenomenon and has an extremely high magnetic Reynolds number. In this sense, a neutron star dynamo is the quintessential fast dynamo. The convective motions are only mildly turbulent on scales larger than the approximately 100 cm neutrino mean free path, but the turbulence is well developed on smaller scales. Several fundamental issues in the theory of fast dynamos are raised in the study of a neutron star dynamo, in particular the possibility of dynamo action in mirror-symmetric turbulence. It is argued that in any high magnetic Reynolds number dynamo, most of the magnetic energy becomes concentrated in thin flux ropes when the field pressure exceeds the turbulent pressure at the smallest scale of turbulence. In addition, the possibilities for dynamo action during the various (pre-collapse) stages of convective motion that occur in the evolution of a massive star are examined, and the properties of white dwarf and neutron star progenitors are contrasted.

Comparison of solar wind driving of the aurora in the two hemispheres due to the solar wind dynamo

NASA Astrophysics Data System (ADS)

Reistad, Jone Peter; Østgaard, Nikolai; Magnus Laundal, Karl; Haaland, Stein; Tenfjord, Paul; Oksavik, Kjellmar

2014-05-01

Event studies of simultaneous global imaging of the aurora in both hemispheres have suggested that an asymmetry of the solar wind driving between the two hemispheres could explain observations of non-conjugate aurora during specific driving conditions. North-South asymmetries in energy transfer from the solar wind across the magnetopause is believed to depend upon the dipole tilt angle and the x-component of the interplanetary magnetic field (IMF). Both negative tilt (winter North) and negative IMF Bx is expected to enhance the efficiency of the solar wind dynamo in the Northern Hemisphere. By the same token, positive tilt and IMF Bx is expected to enhance the solar wind dynamo efficiency in the Southern Hemisphere. We show a statistical study of the auroral response from both hemispheres using global imaging where we compare results during both favourable and not favourable conditions in each hemisphere. By this study we will address the question of general impact on auroral hemispheric asymmetries by this mechanism - the asymmetric solar wind dynamo. We use data from the Wideband Imaging Camera on the IMAGE spacecraft which during its lifetime from 2000-2005 covered both hemispheres. To ease comparison of the two hemispheres, seasonal differences in auroral brightness is removed as far as data coverage allows by only using events having small dipole tilt angles. Hence, the IMF Bx is expected to be the controlling parameter for the hemispheric preference of strongest solar wind dynamo efficiency in our dataset. Preliminary statistical results indicate the expected opposite behaviour in the two hemispheres, however, the effect is believed to be weak.

The effect of collisionality and diamagnetism on the plasma dynamo

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ji, H.; Yagi, Y.; Hattori, K.

1995-04-28

Fluctuation-induced dynamo forces are measured over a wide range of electron collisionality in the edge of TPE-1RM20 Reversed-Field Pinch (RFP). In the collisionless region the Magnetohydrodynamic (MHD) dynamo alone can sustain the parallel current, while in the collisional region a new dynamo mechanism resulting from the fluctuations in the electron diamagnetic drift becomes dominant. A comprehensive picture of the RFP dynamo emerges by combining with earlier results from MST and REPUTE RFPs.

Evolution in Cloud Population Statistics of the MJO: From AMIE Field Observations to Global Cloud-Permiting Models

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Chidong

Motivated by the success of the AMIE/DYNAMO field campaign, which collected unprecedented observations of cloud and precipitation from the tropical Indian Ocean in Octber 2011 – March 2012, this project explored how such observations can be applied to assist the development of global cloud-permitting models through evaluating and correcting model biases in cloud statistics. The main accomplishment of this project were made in four categories: generating observational products for model evaluation, using AMIE/DYNAMO observations to validate global model simulations, using AMIE/DYNAMO observations in numerical studies of cloud-permitting models, and providing leadership in the field. Results from this project provide valuablemore » information for building a seamless bridge between DOE ASR program’s component on process level understanding of cloud processes in the tropics and RGCM focus on global variability and regional extremes. In particular, experience gained from this project would be directly applicable to evaluation and improvements of ACME, especially as it transitions to a non-hydrostatic variable resolution model.« less

The generation of magnetic fields in astrophysical bodies. X - Magnetic buoyancy and the solar dynamo

NASA Technical Reports Server (NTRS)

Parker, E. N.

1975-01-01

The magnetic field appearing as bipolar magnetic regions at the surface of the sun represents the lines of force from a general azimuthal field of the order of 100 gauss somewhere beneath the surface. The amplification time, as a consequence of the nonuniform rotation, is of the order of 10 years. But magnetic buoyancy brings the azimuthal field up through much of the convective zone in a time rather less than 10 years, raising the question of where the azimuthal field can be retained long enough to be amplified. We show that magnetic fields can be retained for long periods of time in the stable radiative region beneath the convective zone, but unfortunately the solar dynamo cannot function there because turbulent diffusion is an essential part of its operation. The only possible conclusion appears to be that the dynamo operates principally in the very lowest levels of the convective zone at depths of 150,000 km or more, where the gas density is 0.1 g/cu cm, and the fields are limited to 50 gauss.

A Well-Known but Still Surprising Generator

ERIC Educational Resources Information Center

Haugland, Ole Anton

2014-01-01

The bicycle generator is often mentioned as an example of a method to produce electric energy. It is cheap and easily accessible, so it is a natural example to use in teaching. There are different types, but I prefer the old side-wall dynamo. The most common explanation of its working principle seems to be something like the illustration in Fig.…

Planetary Dynamos

NASA Astrophysics Data System (ADS)

Gaur, Vinod K.

The article begins with a reference to the first rational approaches to explaining the earth's magnetic field notably Elsasser's application of magneto-hydrodynamics, followed by brief outlines of the characteristics of planetary magnetic fields and of the potentially insightful homopolar dynamo in illuminating the basic issues: theoretical requirements of asymmetry and finite conductivity in sustaining the dynamo process. It concludes with sections on Dynamo modeling and, in particular, the Geo-dynamo, but not before some of the evocative physical processes mediated by the Lorentz force and the behaviour of a flux tube embedded in a perfectly conducting fluid, using Alfvén theorem, are explained, as well as the traditional intermediate approaches to investigating dynamo processes using the more tractable Kinematic models.

A long-lived lunar core dynamo.

PubMed

Shea, Erin K; Weiss, Benjamin P; Cassata, William S; Shuster, David L; Tikoo, Sonia M; Gattacceca, Jérôme; Grove, Timothy L; Fuller, Michael D

2012-01-27

Paleomagnetic measurements indicate that a core dynamo probably existed on the Moon 4.2 billion years ago. However, the subsequent history of the lunar core dynamo is unknown. Here we report paleomagnetic, petrologic, and (40)Ar/(39)Ar thermochronometry measurements on the 3.7-billion-year-old mare basalt sample 10020. This sample contains a high-coercivity magnetization acquired in a stable field of at least ~12 microteslas. These data extend the known lifetime of the lunar dynamo by 500 million years. Such a long-lived lunar dynamo probably required a power source other than thermochemical convection from secular cooling of the lunar interior. The inferred strong intensity of the lunar paleofield presents a challenge to current dynamo theory.

Emergence of magnetic flux generated in a solar convective dynamo

NASA Astrophysics Data System (ADS)

Chen, Feng; Rempel, Feng, Matthias; Fan, Yuhong

2016-10-01

We present a realistic numerical model of sunspot and active region formation through the emergence of flux tubes generated in a solar convective dynamo. The magnetic and velocity fields in a horizontal layer near the top boundary of the solar convective dynamo simulation are used as a time-dependent bottom boundary to drive the radiation magnetohydrodynamic simulations of the emergence of the flux tubes through the upper most layer of the convection zone to the photosphere. The emerging flux tubes interact with the convection and break into small scale magnetic elements that further rise to the photosphere. At the photosphere, several bipolar pairs of sunspots are formed through the coalescence of the small scale magnetic elements. The sunspot pairs in the simulation successfully reproduce the fundamental observed properties of solar active regions, including the more coherent leading spots with a stronger field strength, and the correct tilts of the bipolar pairs. These asymmetries originate from the intrinsic asymmetries in the emerging fields imposed at the bottom boundary, where the horizontal fields are already tilted. The leading sides of the emerging flux tubes are up against the downdraft lanes of the giant cells and strongly sheared downward. This leads to the stronger field strength of the leading polarity fields. We find a prograde flow in the emerging flux tube, which is naturally inherited from the solar convective dynamo simulation. The prograde flow gradually becomes a diverging flow as the flux tube rises. The emerging speed is similar to upflow speed of convective motions. The azimuthal average of the flows around a (leading) sunspot reveals a predominant down flow inside the sunspots and a large-scale horizontal inflow at the depth of about 10 Mm. The inflow pattern becomes an outflow in upper most convection zone in the vicinity of the sunspot, which could be considered as moat flows.

A potential thermal dynamo and its astrophysical applications

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lingam, Manasvi, E-mail: mlingam@princeton.edu; Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544; Mahajan, Swadesh M., E-mail: mahajan@mail.utexas.edu

2016-05-15

It is shown that thermal turbulence, not unlike the standard kinetic and magnetic turbulence, can be an effective driver of a mean-field dynamo. In simple models, such as hydrodynamics and magnetohydrodynamics, both vorticity and induction equations can have strong thermal drives that resemble the α and γ effects in conventional dynamo theories; the thermal drives are likely to be dominant in systems that are endowed with subsonic, low-β turbulence. A pure thermal dynamo is quite different from the conventional dynamo in which the same kinetic/magnetic mix in the ambient turbulence can yield a different ratio of macroscopic magnetic/vortical fields. Themore » possible implications of the similarities and differences between the thermal and non-thermal dynamos are discussed. The thermal dynamo is shown to be highly important in the stellar and planetary context, and yields results broadly consistent with other theoretical and experimental approaches.« less

Dynamo onset as a first-order transition: lessons from a shell model for magnetohydrodynamics.

PubMed

Sahoo, Ganapati; Mitra, Dhrubaditya; Pandit, Rahul

2010-03-01

We carry out systematic and high-resolution studies of dynamo action in a shell model for magnetohydrodynamic (MHD) turbulence over wide ranges of the magnetic Prandtl number PrM and the magnetic Reynolds number ReM. Our study suggests that it is natural to think of dynamo onset as a nonequilibrium first-order phase transition between two different turbulent, but statistically steady, states. The ratio of the magnetic and kinetic energies is a convenient order parameter for this transition. By using this order parameter, we obtain the stability diagram (or nonequilibrium phase diagram) for dynamo formation in our MHD shell model in the (PrM-1,ReM) plane. The dynamo boundary, which separates dynamo and no-dynamo regions, appears to have a fractal character. We obtain a hysteretic behavior of the order parameter across this boundary and suggestions of nucleation-type phenomena.

Elliptical instability in stably stratified fluid interiors

NASA Astrophysics Data System (ADS)

Vidal, J.; Hollerbach, R.; Schaeffer, N.; Cebron, D.

2016-12-01

Self-sustained magnetic fields in celestial bodies (planets, moons, stars) are due to flows in internal electrically conducting fluids. These fluid motions are often attributed to convection, as it is the case for the Earth's liquid core and the Sun. However some past or present liquid cores may be stably stratified. Alternative mechanisms may thus be needed to understand the dynamo process in these celestial objects. Turbulent flows driven by mechanical forcings, such as tides or precession, seem very promising since they are dynamo capable. However the effect of density stratification is not clear, because it can stabilize or destabilize mechanically-driven flows.To mimic an elliptical distortion due to tidal forcing in spherical geometry (full sphere and shell), we consider a theoretical base flow with elliptical streamlines and an associated density profile. It allows to keep the numerical efficiency of spectral methods in this geometry. The flow satisfies the stress-free boundary condition. We perform the stability analysis of the base state using three-dimensional simulations to study both the linear and nonlinear regimes. Stable and unstable density profiles are considered. A complementary local stability analysis (WKB) is also performed. We show that elliptical instability can still grow upon a stable stratification. We also study the mixing of the stratification by the elliptical instability. Finally we look at the dynamo capability of these flows.

Evidence for a Second Martian Dynamo from Electron Reflection Magnetometry

NASA Technical Reports Server (NTRS)

Lillis, R. J.; Manga, M.; Mitchell, D. L.; Lin, R. P.; Acuna, M. H.

2005-01-01

Present-day Mars does not possess an active core dynamo and associated global magnetic field. However, the discovery of intensely magnetized crust in Mars Southern hemisphere implies that a Martian dynamo has existed in the past. Resolving the history of the Martian core dynamo is important for understanding the evolution of the planet's interior. Moreover, because the global magnetic field provided by an active dynamo can shield the atmosphere from erosion by the solar wind, it may have influenced past Martian climate. Additional information is included in the original extended abstract.

Transition from large-scale to small-scale dynamo.

PubMed

Ponty, Y; Plunian, F

2011-04-15

The dynamo equations are solved numerically with a helical forcing corresponding to the Roberts flow. In the fully turbulent regime the flow behaves as a Roberts flow on long time scales, plus turbulent fluctuations at short time scales. The dynamo onset is controlled by the long time scales of the flow, in agreement with the former Karlsruhe experimental results. The dynamo mechanism is governed by a generalized α effect, which includes both the usual α effect and turbulent diffusion, plus all higher order effects. Beyond the onset we find that this generalized α effect scales as O(Rm(-1)), suggesting the takeover of small-scale dynamo action. This is confirmed by simulations in which dynamo occurs even if the large-scale field is artificially suppressed.

Dynamo transition in low-dimensional models.

PubMed

Verma, Mahendra K; Lessinnes, Thomas; Carati, Daniele; Sarris, Ioannis; Kumar, Krishna; Singh, Meenakshi

2008-09-01

Two low-dimensional magnetohydrodynamic models containing three velocity and three magnetic modes are described. One of them (nonhelical model) has zero kinetic and current helicity, while the other model (helical) has nonzero kinetic and current helicity. The velocity modes are forced in both these models. These low-dimensional models exhibit a dynamo transition at a critical forcing amplitude that depends on the Prandtl number. In the nonhelical model, dynamo exists only for magnetic Prandtl number beyond 1, while the helical model exhibits dynamo for all magnetic Prandtl number. Although the model is far from reproducing all the possible features of dynamo mechanisms, its simplicity allows a very detailed study and the observed dynamo transition is shown to bear similarities with recent numerical and experimental results.

Magnetostrophic balance in planetary dynamos - Predictions for Neptune's magnetosphere

NASA Technical Reports Server (NTRS)

Curtis, S. A.; Ness, N. F.

1986-01-01

With the purpose of estimating Neptune's magnetic field and its implications for nonthermal Neptune radio emissions, a new scaling law for planetary magnetic fields was developed in terms of externally observable parameters (the planet's mean density, radius, mass, rotation rate, and internal heat source luminosity). From a comparison of theory and observations by Voyager it was concluded that planetary dynamos are two-state systems with either zero intrinsic magnetic field (for planets with low internal heat source) or (for planets with the internal heat source sufficiently strong to drive convection) a magnetic field near the upper bound determined from magnetostrophic balance. It is noted that mass loading of the Neptune magnetosphere by Triton may play an important role in the generation of nonthermal radio emissions.

Design of the Madison Dynamo Experiment

NASA Astrophysics Data System (ADS)

Kendrick, R. D.; Forest, C. B.; O'Connell, R.; Nornberg, M. D.; Spence, E. J.

2004-11-01

A spherical dynamo experiment has been constructed at the University of Wisconsin's liquid-sodium facility. The goals of the experiment are to observe and understand magnetic instabilities driven by flow shear in MHD systems, investigate MHD turbulence for magnetic Reynolds numbers of 100, and understand the role of fluid turbulence in current generation. Magnetic field generation is possible for only specific flow geometries. The experiment consists of a 1 m diameter, spherical stainless steel vessel filled with liquid sodium at 110 Celsius. The temperature of the vessel is maintained through an actively-heated-and-cooled oil heat-exchange system. Two 100 Hp motors with impellers drive flows in the liquid sodium with flow velocities near 15 m/s. Each shaft is sealed with an oil-buffered dual mechanical cartridge seal. The experiment is automated for remote operation and data logging. The melting and transfer of one metric ton of sodium to a storage vessel is discussed. Operating parameters and performance of the experiment are presented.

Sodium Handling Technology and Engineering Design of the Madison Dynamo Experiment.

NASA Astrophysics Data System (ADS)

Kendrick, R.; Forest, C. B.; O'Connell, R.; Wright, A.; Robinson, K.

1998-11-01

A new liquid metal MHD experiment is being constructed at the University of Wisconsin to test several key predictions of dynamo theory: magnetic instabilities driven by sheared flow, the effects of turbulence on current generation, and the back-reaction of the self-generated magnetic field on the fluid motion which brings saturation. This presentation describes the engineering design of the experiment, which is a 0.5 m radius spherical vessel, filled with liquid sodium at 150 degrees Celsius. The experiment is designed to achieve a magnetic Reynolds number in excess of 100, which requires approximately 80 Hp of mechanical drive, producing flow velocities in sodium of 15 m/s through impellers. Handling liquid sodium offers a number of technical challenges, but routine techniques have been developed over the past several decades for safely handling large quantities for the fast breeder reactor. The handling strategy is discussed, technical details concerning seals and pressurazation are presented, and safety elements are highlighted.

Initial operation with sodium in the Madison Dynamo Experiment.

NASA Astrophysics Data System (ADS)

Kendrick, R.; Spence, Ej; Forest, C. B.; O'Connell, R.; Nornberg, Md; Canary, Hw; Wright, A.; Robinson, K.

1999-11-01

A new liquid metal MHD experiment has been constructed at the University of Wisconsin to test several key predictions of dynamo theory: magnetic instabilities driven by sheared flow, the effects of turbulence on current generation, and the back-reaction of the self-generated magnetic field on the fluid motion which brings saturation. This presentation describes the engineering design of the experiment, which is a 0.5 m radius spherical vessel, filled with liquid sodium at 150 ^circC. The experiment is designed to achieve a magnetic Reynolds number in excess of 100, which requires approximately 80 Hp of mechanical drive, producing flow velocities in sodium of 15 m/s through impellers. Handling liquid sodium offers a number of technical challenges, but routine techniques have been developed over the past several decades for safely handling large quantities for the fast breeder reactor. The handling strategy is discussed, technical details concerning seals and pressurization are presented, and safety elements are highlighted.

Babcock-Leighton solar dynamo: the role of downward pumping and the equatorward propagation of activity

NASA Astrophysics Data System (ADS)

Karak, Bidya Binay; Cameron, Robert

2016-05-01

We investigate the role of downward magnetic pumping near the surface using a kinematic Babcock-Leighton model. We find that the pumping causes the poloidal field to become predominately radial in the near-surface shear layer. This allows the negative radial shear in the near-surface layer to effectively act on the radial field to produce a toroidal field. Consequently, we observe a clear equatorward migration of the toroidal field at low latitudes even when there is no meridional flow in the deep CZ. We show a case where the period of a dynamo wave solution is approximately 11 years. Flux transport models are also shown with periods close to 11 years. Both the dynamo wave and flux transport dynamo are thus able to reproduce some of the observed features of solar cycle. The main difference between the two types of dynamo is the value of $\\alpha$ required to produce dynamo action. In both types of dynamo, the surface meridional flow helps to advect and build the polar field in high latitudes, while in flux transport dynamo the equatorward flow near the bottom of CZ advects toroidal field to cause the equatorward migration in butterfly wings and this advection makes the dynamo easier by transporting strong toroidal field to low latitudes where $\\alpha$ effect works. Another conclusion of our study is that the magnetic pumping suppresses the diffusion of fields through the photospheric surface which helps to achieve the 11-year dynamo cycle at a moderately larger value of magnetic diffusivity than has previously been used.

On the possibility of an alpha-sq omega-type dynamo in a thin layer inside the sun

NASA Technical Reports Server (NTRS)

Choudhuri, Arnab Rai

1990-01-01

If the solar dynamo operates in a thin layer of 10,000-km thickness at the interface between the convection zone and the radiative core, using the facts that the dynamo should have a period of 22 years and a half-wavelength of 40 deg in the theta-direction, it is possible to impose restrictions on the values which various dynamo parameters are allowed to have. It is pointed out that the dynamo should be of alpha-sq omega nature, and kinematical calculations are presented for free dynamo waves and for dynamos in thin rectangular slabs with appropriate boundary conditions. An alpha-sq omega dynamo is expected to produce a significant poloidal field which does not leak to the solar surface. It is found that the turbulent diffusity eta and alpha-coefficient are restricted to values within about a factor of 10, the median values being eta of about 10 to the 10th sq cm/sec and alpha of about 10 cm/sec. On the basis of mixing length theory, it is pointed out that such values imply a reasonable turbulent velocity of the order 30 m/s, but rather small turbulent length scales like 300 km.

A THEORETICAL STUDY OF THE BUILD-UP OF THE SUN’S POLAR MAGNETIC FIELD BY USING A 3D KINEMATIC DYNAMO MODEL

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hazra, Gopal; Choudhuri, Arnab Rai; Miesch, Mark S., E-mail: ghazra@physics.iisc.ernet.in, E-mail: arnab@physics.iisc.ernet.in, E-mail: miesch@ucar.edu

2017-01-20

We develop a three-dimensional kinematic self-sustaining model of the solar dynamo in which the poloidal field generation is from tilted bipolar sunspot pairs placed on the solar surface above regions of strong toroidal field by using the SpotMaker algorithm, and then the transport of this poloidal field to the tachocline is primarily caused by turbulent diffusion. We obtain a dipolar solution within a certain range of parameters. We use this model to study the build-up of the polar magnetic field and show that some insights obtained from surface flux transport models have to be revised. We present results obtained bymore » putting a single bipolar sunspot pair in a hemisphere and two symmetrical sunspot pairs in two hemispheres. We find that the polar fields produced by them disappear due to the upward advection of poloidal flux at low latitudes, which emerges as oppositely signed radial flux and which is then advected poleward by the meridional flow. We also study the effect that a large sunspot pair, violating Hale’s polarity law, would have on the polar field. We find that there would be some effect—especially if the anti-Hale pair appears at high latitudes in the mid-phase of the cycle—though the effect is not very dramatic.« less

Could giant basin-forming impacts have killed Martian dynamo?

PubMed Central

Kuang, W; Jiang, W; Roberts, J; Frey, H V

2014-01-01

The observed strong remanent crustal magnetization at the surface of Mars suggests an active dynamo in the past and ceased to exist around early to middle Noachian era, estimated by examining remagnetization strengths in extant and buried impact basins. We investigate whether the Martian dynamo could have been killed by these large basin-forming impacts, via numerical simulation of subcritical dynamos with impact-induced thermal heterogeneity across the core-mantle boundary. We find that subcritical dynamos are prone to the impacts centered on locations within 30° of the equator but can easily survive those at higher latitudes. Our results further suggest that magnetic timing places a strong constraint on postimpact polar reorientation, e.g., a minimum 16° polar reorientation is needed if Utopia is the dynamo killer. PMID:26074641

Could Giant Basin-Forming Impacts Have Killed Martian Dynamo?

NASA Technical Reports Server (NTRS)

Kuang, W.; Jiang, W.; Roberts, J.; Frey, H. V.

2014-01-01

The observed strong remanent crustal magnetization at the surface of Mars suggests an active dynamo in the past and ceased to exist around early to middle Noachian era, estimated by examining remagnetization strengths in extant and buried impact basins. We investigate whether the Martian dynamo could have been killed by these large basin-forming impacts, via numerical simulation of subcritical dynamos with impact-induced thermal heterogeneity across the core-mantle boundary. We find that subcritical dynamos are prone to the impacts centered on locations within 30 deg of the equator but can easily survive those at higher latitudes. Our results further suggest that magnetic timing places a strong constraint on postimpact polar reorientation, e.g., a minimum 16 deg polar reorientation is needed if Utopia is the dynamo killer.

Could giant basin-forming impacts have killed Martian dynamo?

PubMed

Kuang, W; Jiang, W; Roberts, J; Frey, H V

2014-11-28

The observed strong remanent crustal magnetization at the surface of Mars suggests an active dynamo in the past and ceased to exist around early to middle Noachian era, estimated by examining remagnetization strengths in extant and buried impact basins. We investigate whether the Martian dynamo could have been killed by these large basin-forming impacts, via numerical simulation of subcritical dynamos with impact-induced thermal heterogeneity across the core-mantle boundary. We find that subcritical dynamos are prone to the impacts centered on locations within 30° of the equator but can easily survive those at higher latitudes. Our results further suggest that magnetic timing places a strong constraint on postimpact polar reorientation, e.g., a minimum 16° polar reorientation is needed if Utopia is the dynamo killer.

Limited role of spectra in dynamo theory: coherent versus random dynamos.

PubMed

Tobias, Steven M; Cattaneo, Fausto

2008-09-19

We discuss the importance of phase information and coherence times in determining the dynamo properties of turbulent flows. We compare the kinematic dynamo properties of three flows with the same energy spectrum. The first flow is dominated by coherent structures with nontrivial phase information and long eddy coherence times, the second has random phases and long-coherence time, the third has nontrivial phase information, but short coherence time. We demonstrate that the first flow is the most efficient kinematic dynamo, owing to the presence of sustained stretching and constructive folding. We argue that these results place limitations on the possible inferences of the dynamo properties of flows from the use of spectra alone, and that the role of coherent structures must always be accounted for.

The Hottest Hot Jupiters May Host Atmospheric Dynamos

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rogers, T. M.; McElwaine, J. N.

2017-06-01

Hot Jupiters have proven themselves to be a rich class of exoplanets that test our theories of planetary evolution and atmospheric dynamics under extreme conditions. Here, we present three-dimensional magnetohydrodynamic simulations and analytic results that demonstrate that a dynamo can be maintained in the thin, stably stratified atmosphere of a hot Jupiter, independent of the presumed deep-seated dynamo. This dynamo is maintained by conductivity variations arising from strong asymmetric heating from the planets’ host star. The presence of a dynamo significantly increases the surface magnetic field strength and alters the overall planetary magnetic field geometry, possibly affecting star–planet magnetic interactions.

Bistability and chaos in the Taylor-Green dynamo.

PubMed

Yadav, Rakesh K; Verma, Mahendra K; Wahi, Pankaj

2012-03-01

Using direct numerical simulations, we study dynamo action under Taylor-Green forcing for a magnetic Prandtl number of 0.5. We observe bistability with weak- and strong-magnetic-field branches. Both the dynamo branches undergo subcritical dynamo transition. We also observe a host of dynamo states including constant, periodic, quasiperiodic, and chaotic magnetic fields. One of the chaotic states originates through a quasiperiodic route with phase locking, while the other chaotic attractor appears to follow the Newhouse-Ruelle-Takens route to chaos. We also observe intermittent transitions between quasiperiodic and chaotic states for a given Taylor-Green forcing.

Fluctuation reduction and enhanced confinement in the MST reversed-field pinch

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chapman, Brett Edward

1997-10-01

Plasmas with a factor of ≥3 improvement in energy confinement have been achieved in the MST reversed-field pinch (RFP). These plasmas occur spontaneously, following sawtooth crashes, subject to constraints on, eg, toroidal magnetic field reversal and wall conditioning. Possible contributors to the improved confinement include a reduction of core-resonant, global magnetic fluctuations and a reduction of electrostatic fluctuations over the entire plasma edge. One feature of these plasmas is a region of strong ExB flow shear in the edge. Never before observed in conjunction with enhanced confinement in the RFP, such shear is common in enhanced confinement discharges in tokamaks and stellarators. Another feature of these plasmas is a new type of discrete dynamo event. Like sawtooth crashes, a common form of discrete dynamo, these events correspond to bursts of edge parallel current. The reduction of electrostatic fluctuations in these plasmas occurs within and beyond the region of strong ExB flow shear, similar to what is observed in tokamaks and stellarators. However, the reductions in the MST include fluctuations whose correlation lengths are larger than the width of the shear region. The reduction of the global magnetic fluctuations is most likely due to flattening of the μ=μ 0more » $$\\vec{J}$$∙$$\\vec{B}$$/B 2 profile. Flattening can occur, eg, due to the new type of discrete dynamo event and reduced edge resistivity. Enhanced confinement plasmas are also achieved in the MST when auxiliary current is applied to flatten the μ profile and reduce magnetic fluctuations. Unexpectedly, these plasmas also exhibit a region (broader than in the case above) of strong ExB flow shear in the edge, an edge-wide reduction of electrostatic fluctuations, and the new type of discrete dynamo event. Auxiliary current drive has historically been viewed as the principal route to fusion reactor viability for the RFP.« less

Manifestations of the MHD and kinetic dynamo through soft x-rays

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chartas, G.A.

1991-08-01

The underlying mechanisms that produce and sustain the reversed toroidal field in RFP's are investigated by analyzing 2Dx-ray emissivity reconstruction and by correlating the evolution of the hot electron properties to the reversed toroidal magnetic field. Reconnection of emissivity surfaces as seen in soft x-ray (SXR) reconstructing occur near the predicted resonant surface for the m=1, n=5, 6,-7 resistive tearing modes. Two distinct rates of reversed magnetic field generation are observed. First, in the MHD relaxation phase a sudden increase in B{sub t}(a) is detected. This event coincides with a large increase in the edge hot electron current density. Themore » second mode of flux generation is observed t have a slower rate and occurs during the diffusion phase. A variation of the edge hot electron current density by a factor of four produced only a small change in the measured B{sub t}(a), implying the contributions of the hot electrons to the dynamo during the diffusion phase is small. {tilde T}{sub e}, / was measured to be approximately 60%, which is much larger than the corresponding quantity for the bulk component which is about 30%. Scaling of the magnetic Reynolds number with the diffusion and MHD relaxation time, {tau}{sub MHD} indicated that the {tau}{sub MHD} does not have a strong dependence on the Spitzer resistivity whereas the diffusion time does depend on the classical resistivity. SXR emission mode analysis during the transition from a rotating to a locked plasma shows a decrease in the m=1 Fourier Bastille component of the emissivity. This is due to the flattening of the emissivity profile as seen in the SXR reconstructions.« less

Predictability and Coupled Dynamics of MJO During DYNAMO

DTIC Science & Technology

2013-09-30

1 DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited. Predictability and Coupled Dynamics of MJO During DYNAMO ... DYNAMO time period. APPROACH We are working as a team to study MJO dynamics and predictability using several models as team members of the ONR DRI...associated with the DYNAMO experiment. This is a fundamentally collaborative proposal that involves close collaboration with Dr. Hyodae Seo of the

Paleomagnetism of Hadean and Archean Detrital Zircons from the Jack Hills, Western Australia

NASA Astrophysics Data System (ADS)

Weiss, B. P.; Lima, E. A.; Alexander, E.; Bell, E. A.; Boehnke, P.; Wielicki, M. M.; Harrison, M.; Fu, R. R.; Kehayias, P.; Glenn, D. R.; Walsworth, R. L.; Araujo, J. F. D.; Einsle, J. F.; Harrison, R.; Trail, D.; Watson, E. B.

2016-12-01

Determining the history of Earth's dynamo prior to the oldest known well-preserved rock record is one of the ultimate challenges in the field of paleomagnetism. The dynamo's early history has major implications for the evolution of the core, the initiation of plate tectonics, the physics of magnetic field generation, and the habitability of the early Earth. The only known minerals that might retain paleomagnetic records from well before 3.5 billion years ago (Ga) are detrital zircon crystals found in sedimentary rocks in Western Australia. Ranging up to 4.38 Ga in age, they are the oldest known terrestrial minerals. Tarduno et al. (2015) argued that detrital zircons contain records of an active dynamo dating back to 4.2 Ga. However, it has not been demonstrated that the zircons have escaped remagnetization during the intervening time since their formation (Weiss et al. 2016). Therefore, the age of magnetization in the Jack Hills zircons and the existence of a dynamo prior to 3.5 Ga have yet to be established. To address this issue, we have been studying the magnetism and thermal and aqueous alteration histories of single Archean and Hadean Jack Hills zircon crystals. Peak unblocking temperatures combined with electron backscatter diffraction indicate that the zircons contain inclusions of magnetite and hematite. Electron microscopy, X-ray tomography, and quantum diamond magnetometry indicate that much of the iron oxides in the zircons are associated with cracks and are therefore likely secondary. However, our newly developed Li-in-zircon geospeedometry technique shows for the first time that a small fraction of Hadean zircons retain sharp gradients in Li concentration (see figure), indicating they likely have never heated above the magnetite Curie temperature since their formation at >4 Ga. We describe thermal demagnetization and Thellier-Thellier paleointensity studies of these zircons and implications for the existence of a Hadean dynamo.

Magnetic fields driven by tidal mixing in radiative stars

NASA Astrophysics Data System (ADS)

Vidal, Jérémie; Cébron, David; Schaeffer, Nathanaël; Hollerbach, Rainer

2018-04-01

Stellar magnetism plays an important role in stellar evolution theory. Approximatively 10 per cent of observed main sequence (MS) and pre-main-sequence (PMS) radiative stars exhibit surface magnetic fields above the detection limit, raising the question of their origin. These stars host outer radiative envelopes, which are stably stratified. Therefore, they are assumed to be motionless in standard models of stellar structure and evolution. We focus on rapidly rotating, radiative stars which may be prone to the tidal instability, due to an orbital companion. Using direct numerical simulations in a sphere, we study the interplay between a stable stratification and the tidal instability, and assess its dynamo capability. We show that the tidal instability is triggered regardless of the strength of the stratification (Brunt-Väisälä frequency). Furthermore, the tidal instability can lead to both mixing and self-induced magnetic fields in stably stratified layers (provided that the Brunt-Väisälä frequency does not exceed the stellar spin rate in the simulations too much). The application to stars suggests that the resulting magnetic fields could be observable at the stellar surfaces. Indeed, we expect magnetic field strengths up to several Gauss. Consequently, tidally driven dynamos should be considered as a (complementary) dynamo mechanism, possibly operating in radiative MS and PMS stars hosting orbital companions. In particular, tidally driven dynamos may explain the observed magnetism of tidally deformed and rapidly rotating Vega-like stars.

Magnetic Helicity and Planetary Dynamos

NASA Technical Reports Server (NTRS)

Shebalin, John V.

2012-01-01

A model planetary dynamo based on the Boussinesq approximation along with homogeneous boundary conditions is considered. A statistical theory describing a large-scale MHD dynamo is found, in which magnetic helicity is the critical parameter

Magnetized Turbulent Dynamo in Protogalaxies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Leonid Malyshkin; Russell M. Kulsrud

The prevailing theory for the origin of cosmic magnetic fields is that they have been amplified to their present values by the turbulent dynamo inductive action in the protogalactic and galactic medium. Up to now, in calculation of the turbulent dynamo, it has been customary to assume that there is no back reaction of the magnetic field on the turbulence, as long as the magnetic energy is less than the turbulent kinetic energy. This assumption leads to the kinematic dynamo theory. However, the applicability of this theory to protogalaxies is rather limited. The reason is that in protogalaxies the temperaturemore » is very high, and the viscosity is dominated by magnetized ions. As the magnetic field strength grows in time, the ion cyclotron time becomes shorter than the ion collision time, and the plasma becomes strongly magnetized. As a result, the ion viscosity becomes the Braginskii viscosity. Thus, in protogalaxies the back reaction sets in much earlier, at field strengths much lower than those which correspond to field-turbulence energy equipartition, and the turbulent dynamo becomes what we call the magnetized turbulent dynamo. In this paper we lay the theoretical groundwork for the magnetized turbulent dynamo. In particular, we predict that the magnetic energy growth rate in the magnetized dynamo theory is up to ten times larger than that in the kinematic dynamo theory. We also briefly discuss how the Braginskii viscosity can aid the development of the inverse cascade of magnetic energy after the energy equipartition is reached.« less

A Comparative Analysis of the Magnetic Field Signals over Impact Structures on the Earth, Mars and the Moon

NASA Technical Reports Server (NTRS)

Isac, Anca; Mandea, Mioara; Purucker, Michael; Langlais, Benoit

2015-01-01

An improved description of magnetic fields of terrestrial bodies has been obtained from recent space missions, leading to a better characterization of the internal fields including those of crustal origin. One of the striking differences in their crustal magnetic field is the signature of large impact craters. A comparative analysis of the magnetic characteristics of these structures can shed light on the history of their respective planetary-scale magnetic dynamos. This has motivated us to identify impact craters and basins, first by their quasi-circular features from the most recent and detailed topographic maps and then from available global magnetic field maps. We have examined the magnetic field observed above 27 complex craters on the Earth, 34 impact basins on Mars and 37 impact basins on the Moon. For the first time, systematic trends in the amplitude and frequency of the magnetic patterns, inside and outside of these structures are observed for all three bodies. The demagnetization effects due to the impact shock wave and excavation processes have been evaluated applying the Equivalent Source Dipole forward modeling approach. The main characteristics of the selected impact craters are shown. The trends in their magnetic signatures are indicated, which are related to the presence or absence of a planetary-scale dynamo at the time of their formation and to impact processes. The low magnetic field intensity at center can be accepted as the prime characteristic of a hypervelocity impact and strongly associated with the mechanics of impact crater formation. In the presence of an active internal field, the process of demagnetization due to the shock impact is associated with post-impact remagnetization processes, generating a more complex magnetic signature.

Dramatically Enhanced Spin Dynamo with Plasmonic Diabolo Cavity.

PubMed

Gou, Peng; Qian, Jie; Xi, Fuchun; Zou, Yuexin; Cao, Jun; Yu, Haochi; Zhao, Ziyi; Yang, Le; Xu, Jie; Wang, Hengliang; Zhang, Lijian; An, Zhenghua

2017-07-13

The applications of spin dynamos, which could potentially power complex nanoscopic devices, have so far been limited owing to their extremely low energy conversion efficiencies. Here, we present a unique plasmonic diabolo cavity (PDC) that dramatically improves the spin rectification signal (enhancement of more than three orders of magnitude) under microwave excitation; further, it enables an energy conversion efficiency of up to ~0.69 mV/mW, compared with ~0.27 μV/mW without a PDC. This remarkable improvement arises from the simultaneous enhancement of the microwave electric field (~13-fold) and the magnetic field (~195-fold), which cooperate in the spin precession process generates photovoltage (PV) efficiently under ferromagnetic resonance (FMR) conditions. The interplay of the microwave electromagnetic resonance and the ferromagnetic resonance originates from a hybridized mode based on the plasmonic resonance of the diabolo structure and Fabry-Perot-like modes in the PDC. Our work sheds light on how more efficient spin dynamo devices for practical applications could be realized and paves the way for future studies utilizing both artificial and natural magnetism for applications in many disciplines, such as for the design of future efficient wireless energy conversion devices, high frequent resonant spintronic devices, and magnonic metamaterials.

Predictability and Coupled Dynamics of MJO During DYNAMO

DTIC Science & Technology

2013-09-30

1 DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited. Predictability and Coupled Dynamics of MJO During DYNAMO ...Model (LIM) for MJO predictions and apply it in retrospective cross-validated forecast mode to the DYNAMO time period. APPROACH We are working as...a team to study MJO dynamics and predictability using several models as team members of the ONR DRI associated with the DYNAMO experiment. This is a

Magnetic Eigenmodes in the Madison Dynamo Experiment

NASA Astrophysics Data System (ADS)

Nornberg, M. D.; Bayliss, R. A.; Forest, C. B.; Kendrick, R. D.; O'Connell, R.; Spence, E. J.

2002-11-01

A spherical dynamo experiment has been constructed at the University of Wisconsin's liquid sodium facility. The goals of the experiment are to observe and understand magnetic instabilities driven by flow shear in MHD systems, investigate MHD turbulence for magnetic Reynolds numbers of 100, and understand the role of fluid turbulence in current generation. Magnetic field generation is only possible for specific flow geometries. We have studied and achieved simple roll flow geometries in a full scale water experiment. Results from the water experiment have guided the design of the sodium experiment. The experiment consists of a 1 m diameter, spherical stainless steel vessel filled with liquid sodium at 110 Celsius. Two 100 Hp motors with impellers drive flows in the liquid sodium with flow velocities of 15 m/s. A gaussian grid of 66 Hall probes on the surface of the sodium vessel measure the generated external magnetic field. Hall probe feed-thru arrays measure the internal field. A pair of magnetic field coils produce a roughly uniform field inside the sphere with a centerline field strength of 100 gauss. Preliminary investigations include measurements of the turbulent electromotive force and excitation of magnetic eigenmodes.

Dynamos in asymptotic-giant-branch stars as the origin of magnetic fields shaping planetary nebulae.

PubMed

Blackman, E G; Frank, A; Markiel, J A; Thomas, J H; Van Horn, H M

2001-01-25

Planetary nebulae are thought to be formed when a slow wind from the progenitor giant star is overtaken by a subsequent fast wind generated as the star enters its white dwarf stage. A shock forms near the boundary between the winds, creating the relatively dense shell characteristic of a planetary nebula. A spherically symmetric wind will produce a spherically symmetric shell, yet over half of known planetary nebulae are not spherical; rather, they are elliptical or bipolar in shape. A magnetic field could launch and collimate a bipolar outflow, but the origin of such a field has hitherto been unclear, and some previous work has even suggested that a field could not be generated. Here we show that an asymptotic-giant-branch (AGB) star can indeed generate a strong magnetic field, having as its origin a dynamo at the interface between the rapidly rotating core and the more slowly rotating envelope of the star. The fields are strong enough to shape the bipolar outflows that produce the observed bipolar planetary nebulae. Magnetic braking of the stellar core during this process may also explain the puzzlingly slow rotation of most white dwarf stars.

SPONTANEOUS FORMATION OF SURFACE MAGNETIC STRUCTURE FROM LARGE-SCALE DYNAMO IN STRONGLY STRATIFIED CONVECTION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Masada, Youhei; Sano, Takayoshi, E-mail: ymasada@auecc.aichi-edu.ac.jp, E-mail: sano@ile.osaka-u.ac.jp

We report the first successful simulation of spontaneous formation of surface magnetic structures from a large-scale dynamo by strongly stratified thermal convection in Cartesian geometry. The large-scale dynamo observed in our strongly stratified model has physical properties similar to those in earlier weakly stratified convective dynamo simulations, indicating that the α {sup 2}-type mechanism is responsible for the dynamo. In addition to the large-scale dynamo, we find that large-scale structures of the vertical magnetic field are spontaneously formed in the convection zone (CZ) surface only in cases with a strongly stratified atmosphere. The organization of the vertical magnetic field proceedsmore » in the upper CZ within tens of convective turnover time and band-like bipolar structures recurrently appear in the dynamo-saturated stage. We consider several candidates to be possibly be the origin of the surface magnetic structure formation, and then suggest the existence of an as-yet-unknown mechanism for the self-organization of the large-scale magnetic structure, which should be inherent in the strongly stratified convective atmosphere.« less

Energy Flow Exciting Field-Aligned Current at Substorm Expansion Onset

NASA Astrophysics Data System (ADS)

Ebihara, Y.; Tanaka, T.

2017-12-01

At substorm expansion onset, upward field-aligned currents (FACs) increase abruptly, and a large amount of electromagnetic energy starts to consume in the polar ionosphere. A question arises as to where the energy comes from. Based on the results obtained by the global magnetohydrodynamics simulation, we present energy flow and energy conversion associated with the upward FACs that manifest the onset. Our simulations show that the cusp/mantle region transmits electromagnetic energy to almost the entire region of the magnetosphere when the interplanetary magnetic field is southward. Integral curve of the Poynting flux shows a spiral moving toward the ionosphere, probably suggesting the pathway of electromagnetic energy from the cusp/mantle dynamo to the ionosphere. The near-Earth reconnection initiates three-dimensional redistribution of the magnetosphere. Flow shear in the near-Earth region results in the generation of the near-Earth dynamo and the onset FACs. The onset FACs are responsible to transport the electromagnetic energy toward the Earth. In the near-Earth region, the electromagnetic energy coming from the cusp/mantle dynamo is converted to the kinetic energy (known as bursty bulk flow) and the thermal energy (associated with high-pressure region in the inner magnetosphere). Then, they are converted to the electromagnetic energy associated with the onset FACs. A part of electromagnetic energy is stored in the lobe region during the growth phase. The release of the stored energy, together with the continuously supplied energy from the cusp/mantle dynamo, contributes to the energy supply to the ionosphere during the expansion phase.

Tidal excitation of elliptical instability in the Martian core: Possible mechanism for generating the core dynamo

NASA Astrophysics Data System (ADS)

Arkani-Hamed, J.; Seyed-Mahmoud, B.; Aldridge, K. D.; Baker, R. E.

2008-06-01

We propose a causal relationship between the creation of the giant impact basins on Mars by a large asteroid, ruptured when it entered the Roche limit, and the excitation of the Martian core dynamo. Our laboratory experiments indicate that the elliptical instability of the Martian core can be excited if the asteroid continually exerts tidal forces on Mars for ~20,000 years. Our numerical experiments suggest that the growth-time of the instability was 5,000-15,000 years when the asteroid was at a distance of 50,000-75,000 km. We demonstrate the stability of the orbital motion of an asteroid captured by Mars at a distance of 100,000 km in the presence of the Sun and Jupiter. We also present our results for the tidal interaction of the asteroid with Mars. An asteroid captured by Mars in prograde fashion can survive and excite the elliptical instability of the core for only a few million years, whereas a captured retrograde asteroid can excite the elliptical instability for hundreds of millions of years before colliding with Mars. The rate at which tidal energy dissipates in Mars during this period is over two orders of magnitude greater than the rate at which magnetic energy dissipates. If only 1% of the tidal energy dissipation is partitioned to the core, sufficient energy would be available to maintain the core dynamo. Accordingly, a retrograde asteroid is quite capable of exciting an elliptical instability in the Martian core, thus providing a candidate process to drive a core dynamo.

On the dynamo generation of flux ropes in the Venus ionosphere

NASA Technical Reports Server (NTRS)

Luhmann, J. G.; Elphic, R. C.

1985-01-01

Small scale magnetic field structures or 'flux ropes' observed in the ionosphere of Venus can be interpreted as the result of a kinematic dynamo process acting on weak seed fields. The seed fields result from the prevailing downward convection of magnetic flux from the vicinity of the ionopause, while small scale fluctuations in the velocity of the ionospheric plasma, which can be caused by collisional coupling to gravity waves in the neutral atmosphere, provide the mechanism by which the field is twisted and redistributed into features of similar scale. This mechanism naturally explains some of the average properties of flux ropes such as the variation of their characteristics with altitude and solar zenith angle. It also elucidates the relationship between the large scale and small scale ionospheric magnetic fields.

Processing Doppler Lidar and Cloud Radar Observations for Analysis of Convective Mass Flux Parameterizations Using DYNAMO Direct Observations

DTIC Science & Technology

2014-09-30

for Analysis of Convective Mass Flux Parameterizations Using DYNAMO Direct Observations R. Michael Hardesty CIRES/University of Colorado/NOAA 325...the RV-Revell during legs 2 & 3 of the DYNAMO experiement to help characterize vertical transport through the boundary layer and to build statistics...obtained during DYNAMO , and to investigate whether cold pools that emanate from convection organize the interplay between humidity and convection and

Feasibility Study for a Plasma Dynamo Facility to Investigate Fundamental Processes in Plasma Astrophysics. Final report

DOE Office of Scientific and Technical Information (OSTI.GOV)

Forest, Cary B.

The scientific equipment purchased on this grant was used on the Plasma Dynamo Prototype Experiment as part of Professor Forest's feasibility study for determining if it would be worthwhile to propose building a larger plasma physics experiment to investigate various fundamental processes in plasma astrophysics. The initial research on the Plasma Dynamo Prototype Experiment was successful so Professor Forest and Professor Ellen Zweibel at UW-Madison submitted an NSF Major Research Instrumentation proposal titled "ARRA MRI: Development of a Plasma Dynamo Facility for Experimental Investigations of Fundamental Processes in Plasma Astrophysics." They received funding for this project and the Plasma Dynamomore » Facility also known as the "Madison Plasma Dynamo Experiment" was constructed. This experiment achieved its first plasma in the fall of 2012 and U.S. Dept. of Energy Grant No. DE-SC0008709 "Experimental Studies of Plasma Dynamos," now supports the research.« less

Solar-wind/magnetospheric dynamos: MHD-scale collective entry of the solar wind energy, momentum and mass into the magnetosphere

NASA Technical Reports Server (NTRS)

Song, Yan; Lysak, Robert L.

1992-01-01

A quasi open MHD (Magnetohydrodynamic) scale anomalous transport controlled boundary layer model is proposed, where the MHD collective behavior of magnetofluids (direct dynamo effect, anomalous viscous interaction and anomalous diffusion of the mass and the magnetic field) plays the main role in the conversion of the Solar Wind (SW) kinetic and magnetic energy into electromagnetic energy in the Magnetosphere (MSp). The so called direct and indirect dynamo effects are based on inductive and purely dissipative energy conversion, respectively. The self organization ability of vector fields in turbulent magnetofluids implies an inductive response of the plasma, which leads to the direct dynamo effect. The direct dynamo effect describes the direct formation of localized field aligned currents and the transverse Alfven waves and provides a source for MHD scale anomalous diffusivity and viscosity. The SW/MSp coupling depends on the dynamo efficiency.

Galerkin analysis of kinematic dynamos in the von Kármán geometry

NASA Astrophysics Data System (ADS)

Marié, L.; Normand, C.; Daviaud, F.

2006-01-01

We investigate dynamo action by solving the kinematic dynamo problem for velocity fields of the von Kármán type between two coaxial counter-rotating propellers in a cylinder. A Galerkin method is implemented that takes advantage of the symmetries of the flow and their subsequent influence on the nature of the magnetic field at the dynamo threshold. Distinct modes of instability have been identified that differ by their spatial and temporal behaviors. Our calculations give the result that a stationary and antisymmetric mode prevails at the dynamo threshold. We then present a quantitative analysis of the results based on the parametric study of four interaction coefficients obtained by reduction of our initially large eigenvalue problem. We propose these coefficients to measure the relative importance of the different mechanisms at play in the von Kármán kinematic dynamo.

The stability of nonlinear dynamos and the limited role of kinematic growth rates

NASA Astrophysics Data System (ADS)

Brandenburg, A.; Krause, F.; Meinel, R.; Moss, D.; Tuominen, I.

1989-04-01

The growth rate behavior of several kinematic dynamo models was investigated in the context of the observation that, as a rule, a magnetic field of a single symmetry dominates in the sun and other cosmic objects. For all dynamo models considered, it is shown that, as the dynamo numbers increase, the kinematic growth rates of fields of different parities are asymptotically equal, indicating that growth rates do not dominate the final state of the field. The possibility that the stability of different solutions of nonlinear dynamos determines the final state was then investigated. Dynamo models in spherical geometry were found in which both symmetric and antisymmetric solutions are stable. The kind of symmetry finally established depends in these cases on the initial conditions, i.e., on the history of the object. It is noted that the basic mechanism stabilizing or destabilizing different solutions is not well understood.

Role of large-scale velocity fluctuations in a two-vortex kinematic dynamo.

PubMed

Kaplan, E J; Brown, B P; Rahbarnia, K; Forest, C B

2012-06-01

This paper presents an analysis of the Dudley-James two-vortex flow, which inspired several laboratory-scale liquid-metal experiments, in order to better demonstrate its relation to astrophysical dynamos. A coordinate transformation splits the flow into components that are axisymmetric and nonaxisymmetric relative to the induced magnetic dipole moment. The reformulation gives the flow the same dynamo ingredients as are present in more complicated convection-driven dynamo simulations. These ingredients are currents driven by the mean flow and currents driven by correlations between fluctuations in the flow and fluctuations in the magnetic field. The simple model allows us to isolate the dynamics of the growing eigenvector and trace them back to individual three-wave couplings between the magnetic field and the flow. This simple model demonstrates the necessity of poloidal advection in sustaining the dynamo and points to the effect of large-scale flow fluctuations in exciting a dynamo magnetic field.

Effect of the Lorentz force on on-off dynamo intermittency.

PubMed

Alexakis, Alexandros; Ponty, Yannick

2008-05-01

An investigation of the dynamo instability close to the threshold produced by an ABC forced flow is presented. We focus on the on-off intermittency behavior of the dynamo and the countereffect of the Lorentz force in the nonlinear stage of the dynamo. The Lorentz force drastically alters the statistics of the turbulent fluctuations of the flow and reduces their amplitude. As a result, much longer bursts (on phases) are observed than is expected based on the amplitude of the fluctuations in the kinematic regime of the dynamo. For large Reynolds numbers, the duration time of the on phase follows a power law distribution, while for smaller Reynolds numbers the Lorentz force completely kills the noise and the system transits from a chaotic state into a laminar time periodic flow. The behavior of the on-off intermittency as the Reynolds number is increased is also examined. The connections with dynamo experiments and theoretical modeling are discussed.

The Role of Convective Shell Thickness on Dynamo Scaling Laws for Magnetic Field Morphology: Implications for the Ice Giants and Future Earth

NASA Astrophysics Data System (ADS)

Stanley, S.; Tian, B. Y.

2016-12-01

Previous dynamo scaling law studies (Christensen and Aubert, 2006) have demonstrated that the morphology of a planet's magnetic field is determined by the local Rossby number (Rol): a non-dimensional diagnostic variable that quantifies the ratio of inertial forces to Coriolis forces on the average length scale of the flow. Dynamos with Rol < 0.1 produce dipolar dominated magnetic fields whereas dynamos with Rol > 0.1 produce multipolar magnetic fields. Scaling studies have also determined the dependence of the local Rossby number on non-dimensional parameters governing the system - specifically the Ekman, Prandtl, magnetic Prandtl and flux-based Rayleigh numbers (Olson and Christensen, 2006). However, those studies focused on the specific convective shell thickness of the Earth's core and hence could not determine the influence of convective shell thickness on the local Rossby number. Aubert et al. (2009) investigated the role of convective shell thickness on dynamo scaling laws in order to investigate the palaeo-evolution of the geodynamo. Due to the focus of that study, they varied the ratio of the inner to outer core radii (rio) from 0 to 0.35 and found Rol scales with (1+rio). Here we consider a larger range of convective shell thicknesses and find an exponential dependence of rio on the local Rossby number. Our results are consistent with Aubert et al. (2009) for their small rio values. With this new scaling dependence on convective shell thickness, we find that Uranus and Neptune reside deeply in the multipolar regime, whereas without the dependence on rio, they resided near Rol =0.1; i.e. on the boundary between dipolar and multipolar fields and close to where Earth resides in the parameter space. We also find that Earth will reside more deeply in the multipolar regime, and hence not produce a stable dipolar field once the inner core has grown such that rio = 0.4.

Poynting Robertson Battery and the Chiral Magnetic Fields of AGN Jets

NASA Technical Reports Server (NTRS)

Kazanas, Demosthenes

2010-01-01

We propose that the magnetic fields in the accretion disks of active galactic nuclei (AGNs) are generated by azimuthal electric currents due to the difference between the plasma electron and ion velocities that arises when the electrons are retarded by interactions with the AGN photons (the Poynting Robertson battery). This process provides a unique relation between the polarity of the poloidal B field to the angular velocity Omega of the accretion disk (B is parallel to Omega), a relation absent in the more popular dynamo B-field generation. This then leads to a unique direction for the toroidal B field induced by disk rotation. Observations of the toroidal fields of 29 AGN jets revealed by parsec-scale Faraday rotation measurements show a clear asymmetry that is consistent with this model, with the probability that this asymmetry comes about by chance being approx.0.06 %. This lends support to the hypothesis that the universe is seeded by B fields that are generated in AGNs via this mechanism and subsequently injected into intergalactic space by the jet outflows.

Simulating Cyclic Evolution of Coronal Magnetic Fields using a Potential Field Source Surface Model Coupled with a Dynamo Model

NASA Astrophysics Data System (ADS)

Suresh, A.; Dikpati, M.; Burkepile, J.; de Toma, G.

2013-12-01

The structure of the Sun's corona varies with solar cycle, from a near spherical symmetry at solar maximum to an axial dipole at solar minimum. Why does this pattern occur? It is widely accepted that large-scale coronal structure is governed by magnetic fields, which are most likely generated by the dynamo action in the solar interior. In order to understand the variation in coronal structure, we couple a potential field source surface model with a cyclic dynamo model. In this coupled model, the magnetic field inside the convection zone is governed by the dynamo equation and above the photosphere these dynamo-generated fields are extended from the photosphere to the corona by using a potential field source surface model. Under the assumption of axisymmetry, the large-scale poloidal fields can be written in terms of the curl of a vector potential. Since from the photosphere and above the magnetic diffusivity is essentially infinite, the evolution of the vector potential is given by Laplace's Equation, the solution of which is obtained in the form of a first order Associated Legendre Polynomial. By taking linear combinations of these polynomial terms, we find solutions that match more complex coronal structures. Choosing images of the global corona from the Mauna Loa Solar Observatory at each Carrington rotation over half a cycle (1986-1991), we compute the coefficients of the Associated Legendre Polynomials up to degree eight and compare with observation. We reproduce some previous results that at minimum the dipole term dominates, but that this term fades with the progress of the cycle and higher order multipole terms begin to dominate. We find that the amplitudes of these terms are not exactly the same in the two limbs, indicating that there is some phi dependence. Furthermore, by comparing the solar minimum corona during the past three minima (1986, 1996, and 2008), we find that, while both the 1986 and 1996 minima were dipolar, the minimum in 2008 was unusual, as there was departure from a dipole. In order to investigate the physical cause of this departure from dipole, we implement north-south asymmetry in the surface source of the magnetic fields in our model, and find that such n/s asymmetry in solar cycle could be one of the reasons for this departure. This work is partially supported by NASA's LWS grant with award number NNX08AQ34G. NCAR is sponsored by the NSF.

Searching for the fastest dynamo: laminar ABC flows.

PubMed

Alexakis, Alexandros

2011-08-01

The growth rate of the dynamo instability as a function of the magnetic Reynolds number R(M) is investigated by means of numerical simulations for the family of the Arnold-Beltrami-Childress (ABC) flows and for two different forcing scales. For the ABC flows that are driven at the largest available length scale, it is found that, as the magnetic Reynolds number is increased: (a) The flow that results first in a dynamo is the 2 1/2-dimensional flow for which A=B and C=0 (and all permutations). (b) The second type of flow that results in a dynamo is the one for which A=B≃2C/5 (and permutations). (c) The most symmetric flow, A=B=C, is the third type of flow that results in a dynamo. (d) As R(M) is increased, the A=B=C flow stops being a dynamo and transitions from a local maximum to a third-order saddle point. (e) At larger R(M), the A=B=C flow reestablishes itself as a dynamo but remains a saddle point. (f) At the largest examined R(M), the growth rate of the 2 1/2-dimensional flows starts to decay, the A=B=C flow comes close to a local maximum again, and the flow A=B≃2C/5 (and permutations) results in the fastest dynamo with growth rate γ≃0.12 at the largest examined R(M). For the ABC flows that are driven at the second largest available length scale, it is found that (a) the 2 1/2-dimensional flows A=B,C=0 (and permutations) are again the first flows that result in a dynamo with a decreased onset. (b) The most symmetric flow, A=B=C, is the second type of flow that results in a dynamo. It is, and it remains, a local maximum. (c) At larger R(M), the flow A=B≃2C/5 (and permutations) appears as the third type of flow that results in a dynamo. As R(M) is increased, it becomes the flow with the largest growth rate. The growth rates appear to have some correlation with the Lyapunov exponents, but constructive refolding of the field lines appears equally important in determining the fastest dynamo flow.

Key characteristics of the Fe-snow regime in Ganymede's core

NASA Astrophysics Data System (ADS)

Rückriemen, Tina; Breuer, Doris; Spohn, Tilman

2014-05-01

Ganymede shows signs of an internally produced dipolar magnetic field (|Bdip|≡719 nT) [1]. For small planetary bodies such as Ganymede the Fe-snow regime, i.e. the top-down solidification of iron, has been suggested to play an important role in the core cooling history [2,3]. In that regime, iron crystals form first at the core-mantle boundary (CMB) due to shallow or negative slopes of the melting temperature [2,3]. The solid iron particles are heavier than the surrounding Fe-FeS fluid, i.e. a snow zone forms, settle to deeper core regions, where the core temperature is higher than the melting temperature, and remelt again. As a consequence, a stable chemical gradient in the Fe-FeS fluid arises within the snow zone. We speculate this style of convection via sedimentation to be small scale, therefore it lacks an important criterion necessary for dynamo action [4]. Below this zone, whose thickness increases with time, the process of remelting of iron creates a gravitationally unstable situation. We propose that this could be the driving mechanism for a potential dynamo. However, dynamo action would be restricted to the time period the snow zone needs to grow across the core. With a 1D thermo-chemical evolution model, we investigate key characteristics of the Fe-snow regime within Ganymede's core: the compositional density gradient of the fluid Fe-FeS within the snow zone and the time period necessary to grow the snow zone across the core. Additionally, we determine the dipolar magnetic field strength associated with a dynamo in Ganymede's deeper fluid core. We vary important input paramters such as the initial sulfur concentration (7-19 wt.%), the core heat flux (2-6 mW/m2) and the thermal conductivity (20-60 W/mK) with the nominal model being: xs=10 wt.%, qcmb=4 mW/m2, kc=32 W/mK. We find, that heat fluxes higher than 6 or 22 mW/m2 are required for double-diffusive or overturning convection to overcome the compositional density gradient within the snow zone, respectively. Since Ganymede's core heat flux does not exceed values of 4 mW/m2 [2], we consider the snow zone to be stable against thermal convection. The time necessary to grow the snow zone across the core is between 230-1900 Myr. For representative models we calculate the temporal evolution of the surface dipolar magnetic field strength according to [5]. All models show surface dipolar magnetic field strengths during the evolution of the snow zone that match the observed value of |Bdip|≡719 nT. In conclusion, we find that the Fe-snow regime produces a stably-stratified liquid layer in the snow zone below which a magnetic field of observed strength can be generated. Such a chemical dynamo is restricted in time and stops as soon as an inner solid core starts to grow suggesting the absence of such an inner core in Ganymede. The present model further suggests a core with high initial sulfur concentration, because this leads to a late start and a long duration of the dynamo necessary to explain the present magnetic field. References [1] Kivelson, M et al. (1996), Nature, 384(6609), [2] Hauck II, S. et al. (2006), JGR, 111(E9), [3] Williams, Q. (2009), EPSL, 284(3), [4] Christensen, U. and J. Wicht (2007), Treatise of Geophysics, Elsevier, [5] Christensen, U., and J. Aubert (2006), GJI, 166(1)

A nonmagnetic differentiated early planetary body

DOE PAGES

Weiss, Benjamin P.; Wang, Huapei; Sharp, Thomas G.; ...

2017-06-19

Paleomagnetic studies of meteorites have shown that the solar nebula was likely magnetized and that many early planetary bodies generated dynamo magnetic fields in their advecting metallic cores. The surface fields on these bodies were recorded by a diversity of chondrites and achondrites, ranging in intensity from several μT to several hundred μT. In fact, an achondrite parent body without evidence for paleomagnetic fields has yet to be confidently identified, hinting that early solar system field generation and the dynamo process in particular may have been common. Here we present paleomagnetic measurements of the ungrouped achondrite NWA 7325 indicating thatmore » it last cooled in a near-zero field (<~1.7μT), estimated to have occurred at 4563.09 ± 0.26 million years ago (Ma) from Al–Mg chronometry. Because NWA 7325 is highly depleted in siderophile elements, its parent body nevertheless underwent large-scale metal-silicate differentiation and likely formed a metallic core. This makes NWA 7325 the first recognized example of an essentially unmagnetized igneous rock from a differentiated early solar system body. These results indicate that all magnetic fields, including those from any core dynamo on the NWA 7325 parent body, the solar nebula, young Sun, and solar wind, were <1.7 μT at the location of NWA 7325 at 4563 Ma. Finally, this supports a recent conclusion that the solar nebula had dissipated by ~4 million years after solar system formation. NWA 7325 also serves as an experimental control that gives greater confidence in the positive identification of remanent magnetization in other achondrites.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Weiss, Benjamin P.; Wang, Huapei; Sharp, Thomas G.

Paleomagnetic studies of meteorites have shown that the solar nebula was likely magnetized and that many early planetary bodies generated dynamo magnetic fields in their advecting metallic cores. The surface fields on these bodies were recorded by a diversity of chondrites and achondrites, ranging in intensity from several μT to several hundred μT. In fact, an achondrite parent body without evidence for paleomagnetic fields has yet to be confidently identified, hinting that early solar system field generation and the dynamo process in particular may have been common. Here we present paleomagnetic measurements of the ungrouped achondrite NWA 7325 indicating thatmore » it last cooled in a near-zero field (<~1.7μT), estimated to have occurred at 4563.09 ± 0.26 million years ago (Ma) from Al–Mg chronometry. Because NWA 7325 is highly depleted in siderophile elements, its parent body nevertheless underwent large-scale metal-silicate differentiation and likely formed a metallic core. This makes NWA 7325 the first recognized example of an essentially unmagnetized igneous rock from a differentiated early solar system body. These results indicate that all magnetic fields, including those from any core dynamo on the NWA 7325 parent body, the solar nebula, young Sun, and solar wind, were <1.7 μT at the location of NWA 7325 at 4563 Ma. Finally, this supports a recent conclusion that the solar nebula had dissipated by ~4 million years after solar system formation. NWA 7325 also serves as an experimental control that gives greater confidence in the positive identification of remanent magnetization in other achondrites.« less

A nonmagnetic differentiated early planetary body

NASA Astrophysics Data System (ADS)

Weiss, Benjamin P.; Wang, Huapei; Sharp, Thomas G.; Gattacceca, Jérôme; Shuster, David L.; Downey, Brynna; Hu, Jinping; Fu, Roger R.; Kuan, Aaron T.; Suavet, Clément; Irving, Anthony J.; Wang, Jun; Wang, Jiajun

2017-06-01

Paleomagnetic studies of meteorites have shown that the solar nebula was likely magnetized and that many early planetary bodies generated dynamo magnetic fields in their advecting metallic cores. The surface fields on these bodies were recorded by a diversity of chondrites and achondrites, ranging in intensity from several μT to several hundred μT. In fact, an achondrite parent body without evidence for paleomagnetic fields has yet to be confidently identified, hinting that early solar system field generation and the dynamo process in particular may have been common. Here we present paleomagnetic measurements of the ungrouped achondrite NWA 7325 indicating that it last cooled in a near-zero field (<∼1.7 μT), estimated to have occurred at 4563.09 ± 0.26 million years ago (Ma) from Al-Mg chronometry. Because NWA 7325 is highly depleted in siderophile elements, its parent body nevertheless underwent large-scale metal-silicate differentiation and likely formed a metallic core. This makes NWA 7325 the first recognized example of an essentially unmagnetized igneous rock from a differentiated early solar system body. These results indicate that all magnetic fields, including those from any core dynamo on the NWA 7325 parent body, the solar nebula, young Sun, and solar wind, were <1.7 μT at the location of NWA 7325 at 4563 Ma. This supports a recent conclusion that the solar nebula had dissipated by ∼4 million years after solar system formation. NWA 7325 also serves as an experimental control that gives greater confidence in the positive identification of remanent magnetization in other achondrites.

Numerical simulation of the 6 day wave effects on the ionosphere: Dynamo modulation

NASA Astrophysics Data System (ADS)

Gan, Quan; Wang, Wenbing; Yue, Jia; Liu, Hanli; Chang, Loren C.; Zhang, Shaodong; Burns, Alan; Du, Jian

2016-10-01

The Thermosphere-Ionosphere-Mesosphere Electrodynamics General Circulation Model (TIME-GCM) is used to theoretically study the 6 day wave effects on the ionosphere. By introducing a 6 day perturbation with zonal wave number 1 at the model lower boundary, the TIME-GCM reasonably reproduces the 6 day wave in temperature and horizontal winds in the mesosphere and lower thermosphere region during the vernal equinox. The E region wind dynamo exhibits a prominent 6 day oscillation that is directly modulated by the 6 day wave. Meanwhile, significant local time variability (diurnal and semidiurnal) is also seen in wind dynamo as a result of altered tides due to the nonlinear interaction between the 6 day wave and migrating tides. More importantly, the perturbations in the E region neutral winds (both the 6 day oscillation and tidal-induced short-term variability) modulate the polarization electric fields, thus leading to the perturbations in vertical ion drifts and ionospheric F2 region peak electron density (NmF2). Our modeling work shows that the 6 day wave couples with the ionosphere via both the direct neutral wind modulation and the interaction with atmospheric tides.

Can Superflares Occur on the Sun? A View from Dynamo Theory

NASA Astrophysics Data System (ADS)

Katsova, M. M.; Kitchatinov, L. L.; Livshits, M. A.; Moss, D. L.; Sokoloff, D. D.; Usoskin, I. G.

2018-01-01

Recent data from the Kepler mission has revealed the occurrence of superflares in Sun-like stars which exceed by far any observed solar flares in released energy. Radionuclide data do not provide evidence for occurrence of superflares on the Sun over the past eleven millennia. Stellar data for a subgroup of superflaring Kepler stars are analysed in an attempt to find possible progenitors of their abnormal magnetic activity. A natural idea is that the dynamo mechanism in superflaring stars differs in some respect from that in the Sun. We search for a difference in the dynamo-related parameters between superflaring stars and the Sun to suggest a dynamo mechanism as close as possible to the conventional solar/stellar dynamo but capable of providing much higher magnetic energy. Dynamo based on joint action of differential rotation and mirror asymmetric motions can in principle result in excitation of two types of magnetic fields. First of all, it is well-known in solar physics dynamo waves. The point is that another magnetic configuration with initial growth and further stabilisation can also be excited. For comparable conditions, magnetic field of second configuration is much stronger than that of the first one just because dynamo does not spend its energy for periodic magnetic field inversions but uses it for magnetic field growth. We analysed available data from the Kepler mission concerning the superflaring stars in order to find tracers of anomalous magnetic activity. As suggested in a recent paper [1], we find that anti-solar differential rotation or anti-solar sign of the mirror-asymmetry of stellar convection can provide the desired strong magnetic field in dynamo models. We confirm this concept by numerical models of stellar dynamos with corresponding governing parameters. We conclude that the proposed mechanism can plausibly explain the superflaring events at least for some cool stars, including binaries, subgiants and, possibly, low-mass stars and young rapid rotators.

Influence of Mercury

NASA Astrophysics Data System (ADS)

Tackley, P. J.; Aurnou, J. M.; Aubert, J.

2009-04-01

Due to the absence of an atmosphere and proximity to the Sun, Mercury's surface temperature varies laterally by several 100s K, even when averaged over long time periods. The dominant variation in time-averaged surface T occurs from pole to equator (~225 K) [1]. The resonant relationship between Mercury's orbit and rotation results in a smaller longitudinal variation (~100 K) [1]. Here we demonstrate, using models of mantle convection in a 3-D spherical shell, that this stationary lateral variation in surface temperature has a small but significant influence on mantle convection and on the lateral variation of heat flux across the core-mantle boundary (CMB). We evaluate the possible observational signature of this laterally-varying convection in terms of boundary topography, stress distribution, gravity and moment of inertia tensor. We furthermore test whether the lateral variation in CMB flux is capable of driving a thermal wind dynamo, i.e., weak dynamo action with no internally-driven core convective motions. For Mercury's mantle we assume a dry olivine rheology including both diffusion creep and disclocation creep with rheological parameters such as activation energy and volume taken from the synthesis of [2]. We assume decaying radiogenic heat sources with the same concentration as in the bulk silicate Earth, and a parameterised model of core cooling. The models are run for 4.5 Ga from a relatively hot initial state with random initial perturbations. We use the code StagYY, which uses a finite-volume discretization on a spherical yin-yang grid and a multigrid solver [3]. Results in spherical axisymmetric geometry, compare a case with constant surface temperature to one with a latitude-dependent surface temperature. The system forms about 3 convection cells from pole to equator. Although the results look similar to first order, in the latitude-dependent case the convection is noticably more sluggish and colder towards the pole. In CMB flux, both cases display large oscillations due to convection cells. A pole-to-equator trend is superimposed on this for the case with laterally-varying surface temperature. Although the amplitude of this long-wavelength variation is smaller than that of the within-cell variation, its long-wavelength nature might be effective in driving thermal winds in the core. Results in a full 3-D spherical shell indicate that convection adopts a cellular structure with a polygonal network of downwellings and plume-like upwellings, as is usually obtained for stagnant lid convection, for example, in the recent 3-D spherical Mercury models of [4]. This is in notable contrast to the models of [5], in which linear upwellings were obtained. This difference could be because the initial perturbations used by [5] used a small number of low-order spherical harmonics, i.e., a long-wavelength pattern with particular symmetries, whereas our initial perturbations are random white noise. The origin of this difference requires further investigation. The pattern of CMB heat flux shows a strong l=2, m=0 pattern, again with superimposed small-scale variations due to convection cells. The surface geoid displays an very dominant (2,0) pattern, which would be a strong diagnostic of this behaviour. These models are being further analysed for boundary topography and stress distribution. Models of planetary dynamos have traditionally depended upon the concept that secular cooling and internal radioactive decay are responsible for genererating convective fluid motions within the core [e.g. 6]. Some models, of Earth's dynamo in particular, also include thermal winds --shear flows driven by heat flux variations along the core-mantle boundary -- that modify the dynamo process [e.g. 7]. We have now shown, following the work of [8], that thermal winds themselves are capable of driving dynamo action in planetary cores (Fig. 4). In fully self-consistent, three-dimensional models, we find that thermal wind dynamos do not require a net heat flux to emanate from the core and can operate even when the core fluid is neutrally stratified. In these models, the dynamo is powered externally by thermal energy stored in the mantle. This dynamo mechanism can occur on planetary bodies, such as Mercury, which are likely to have weak net heat fluxes from their cores but possess significant core-mantle boundary heat flux variations (Figures 1 - 3). We plan to use the pattern of CMB heat flux from the mantle models as a boundary condition for core models, in order to determine the feasibility of thermal wind dynamo action occurring in Mercury's core. References [1] Aharonson, O., et al. (2004) EPSL, 218, 261-268. [2] Karato, S. and Wu, P. (1993) Sci., 260, 771-778. [3] Tackley, P. J. (2008) PEPI, doi: 10.1016/j.pepi.2008.08.005.. [4] Breuer, D. et al. (2007) Sp. Sci. Rev., 132, 229-260. [5] King, S. D. (2008) Nature Geoscience, 1, 229-232. [5] Heimpel, M. H. et al. (2005) EPSL, 236, 542-557. [7] Willis, A., et al. (2007) PEPI, 165, 83-92. [8] Sarson, G., (2003) PRSL A, 459, 1241-1259. [9] Aubert, J., et al. (2008) GJI, 172, 945-956.

Varying the forcing scale in low Prandtl number dynamos

NASA Astrophysics Data System (ADS)

Brandenburg, A.; Haugen, N. E. L.; Li, Xiang-Yu; Subramanian, K.

2018-06-01

Small-scale dynamos are expected to operate in all astrophysical fluids that are turbulent and electrically conducting, for example the interstellar medium, stellar interiors, and accretion disks, where they may also be affected by or competing with large-scale dynamos. However, the possibility of small-scale dynamos being excited at small and intermediate ratios of viscosity to magnetic diffusivity (the magnetic Prandtl number) has been debated, and the possibility of them depending on the large-scale forcing wavenumber has been raised. Here we show, using four values of the forcing wavenumber, that the small-scale dynamo does not depend on the scale-separation between the size of the simulation domain and the integral scale of the turbulence, i.e., the forcing scale. Moreover, the spectral bottleneck in turbulence, which has been implied as being responsible for raising the excitation conditions of small-scale dynamos, is found to be invariant under changing the forcing wavenumber. However, when forcing at the lowest few wavenumbers, the effective forcing wavenumber that enters in the definition of the magnetic Reynolds number is found to be about twice the minimum wavenumber of the domain. Our work is relevant to future studies of small-scale dynamos, of which several applications are being discussed.

Measurements of dynamo electric field and momentum transport induced by fluctuations on HIST

NASA Astrophysics Data System (ADS)

Hirono, H.; Hanao, T.; Hyobu, T.; Ito, K.; Matsumoto, K.; Nakayama, T.; Kikuchi, Y.; Fukumoto, N.; Nagata, M.

2012-10-01

Coaxial Helicity injection (CHI) is an efficient current-drive method used in spheromak and spherical torus (ST) experiments. It is an important issue to investigate dynamo effect to explore CHI current drive mechanisms. To establish the dynamo model with two-fluid Hall effects, we verify the parallel mean-field Ohm's law balance. The spatial profiles of the MHD/Hall dynamo electric fields are measured by using Mach probe and Hall probe involving 3-axis magnetic pick-up coils. The MHD/Hall fluctuation-induced electromotive forces are large enough to sustain the mean toroidal current against the resistive decay. We have measured the electron temperature and the density with great accuracy by using a new electrostatic probe with voltage sweeping. The result shows that the electron temperature is high in the core region and low in the central open flux column (OFC), and the electron density is highest in the OFC region. The Hall dynamo becomes more dominant in a lower density region compared to the MHD dynamo. In addition, the fluctuation-induced Maxwell and Reynolds stresses are calculated to examine the fast radial transport of momentum from the OFC to the core region during the dynamo drive.

Dynamic motif occupancy (DynaMO) analysis identifies transcription factors and their binding sites driving dynamic biological processes

PubMed Central

Kuang, Zheng; Ji, Zhicheng

2018-01-01

Abstract Biological processes are usually associated with genome-wide remodeling of transcription driven by transcription factors (TFs). Identifying key TFs and their spatiotemporal binding patterns are indispensable to understanding how dynamic processes are programmed. However, most methods are designed to predict TF binding sites only. We present a computational method, dynamic motif occupancy analysis (DynaMO), to infer important TFs and their spatiotemporal binding activities in dynamic biological processes using chromatin profiling data from multiple biological conditions such as time-course histone modification ChIP-seq data. In the first step, DynaMO predicts TF binding sites with a random forests approach. Next and uniquely, DynaMO infers dynamic TF binding activities at predicted binding sites using their local chromatin profiles from multiple biological conditions. Another landmark of DynaMO is to identify key TFs in a dynamic process using a clustering and enrichment analysis of dynamic TF binding patterns. Application of DynaMO to the yeast ultradian cycle, mouse circadian clock and human neural differentiation exhibits its accuracy and versatility. We anticipate DynaMO will be generally useful for elucidating transcriptional programs in dynamic processes. PMID:29325176

Iron snow in the Martian Core?

NASA Astrophysics Data System (ADS)

Davies, C. J.; Pommier, A.

2017-12-01

The decline of Mars' global magnetic field some 3.8-4.1 billion years ago is thought to reflect the demise of the dynamo that operated in its liquid core. The termination of the dynamo is intimately tied to the thermochemical evolution of the core-mantle system and therefore to the present-day physical state of the Martian core. The standard model predicts that the Martian dynamo failed because thermal convection stopped and the core remained entirely liquid until the present. Here we consider an alternative hypothesis that the Martian core crystallized from the top down in the so-called iron snow regime. We derive energy-entropy equations describing the long-timescale thermal and magnetic evolution of the core that incorporate the self-consistent formation of a snow layer that freezes out pure iron and is assumed to be on the liquidus; the iron sinks and remelts in the deeper core, acting as a possible source for magnetic field generation. Compositions are in the FeS system, with a sulfur content up to 16 wt%. The values of the different parameters (core radius, density and CMB pressure) are varied within bounds set by recent internal structure models that satisfy existing geodetic constraints (planetary mass, moment of inertia and tidal Love number). The melting curve and adiabat, CMB heat flow and thermal conductivity were also varied, based on previous experimental and numerical works. We observe that the formation of snow zones occurs for a wide range of interior and thermal structure properties and depends critically on the initial sulfur concentration. Gravitational energy release and latent heat effects arising during growth of the snow zone do not generate sufficient entropy to restart the dynamo unless the snow zone occupies a significant fraction of the core. Our results suggest that snow zones can be 1.5-2 Gyrs old, though thermal stratification of the uppermost core, not included in our model, likely delays onset. Models that match the available magnetic and geodetic constraints have an initial S concentration of about 10wt.% and snow zones that occupy approximately the top 100 km of the present-day Martian core.

Anelastic spherical dynamos with radially variable electrical conductivity

NASA Astrophysics Data System (ADS)

Dietrich, W.; Jones, C. A.

2018-05-01

A series of numerical simulations of the dynamo process operating inside gas giant planets has been performed. We use an anelastic, fully nonlinear, three-dimensional, benchmarked MHD code to evolve the flow, entropy and magnetic field. Our models take into account the varying electrical conductivity, high in the ionised metallic hydrogen region, low in the molecular outer region. Our suite of electrical conductivity profiles ranges from Jupiter-like, where the outer hydrodynamic region is quite thin, to Saturn-like, where there is a thick non-conducting shell. The rapid rotation leads to the formation of two distinct dynamical regimes which are separated by a magnetic tangent cylinder - mTC. Outside the mTC there are strong zonal flows, where Reynolds stress balances turbulent viscosity, but inside the mTC Lorentz force reduces the zonal flow. The dynamic interaction between both regions induces meridional circulation. We find a rich diversity of magnetic field morphologies. There are Jupiter-like steady dipolar fields, and a belt of quadrupolar dominated dynamos spanning the range of models between Jupiter-like and Saturn-like conductivity profiles. This diversity may be linked to the appearance of reversed sign helicity in the metallic regions of our dynamos. With Saturn-like conductivity profiles we find models with dipolar magnetic fields, whose axisymmetric components resemble those of Saturn, and which oscillate on a very long time-scale. However, the non-axisymmetric field components of our models are at least ten times larger than those of Saturn, possibly due to the absence of any stably stratified layer.

Large-scale dynamos in rapidly rotating plane layer convection

NASA Astrophysics Data System (ADS)

Bushby, P. J.; Käpylä, P. J.; Masada, Y.; Brandenburg, A.; Favier, B.; Guervilly, C.; Käpylä, M. J.

2018-05-01

Context. Convectively driven flows play a crucial role in the dynamo processes that are responsible for producing magnetic activity in stars and planets. It is still not fully understood why many astrophysical magnetic fields have a significant large-scale component. Aims: Our aim is to investigate the dynamo properties of compressible convection in a rapidly rotating Cartesian domain, focusing upon a parameter regime in which the underlying hydrodynamic flow is known to be unstable to a large-scale vortex instability. Methods: The governing equations of three-dimensional non-linear magnetohydrodynamics (MHD) are solved numerically. Different numerical schemes are compared and we propose a possible benchmark case for other similar codes. Results: In keeping with previous related studies, we find that convection in this parameter regime can drive a large-scale dynamo. The components of the mean horizontal magnetic field oscillate, leading to a continuous overall rotation of the mean field. Whilst the large-scale vortex instability dominates the early evolution of the system, the large-scale vortex is suppressed by the magnetic field and makes a negligible contribution to the mean electromotive force that is responsible for driving the large-scale dynamo. The cycle period of the dynamo is comparable to the ohmic decay time, with longer cycles for dynamos in convective systems that are closer to onset. In these particular simulations, large-scale dynamo action is found only when vertical magnetic field boundary conditions are adopted at the upper and lower boundaries. Strongly modulated large-scale dynamos are found at higher Rayleigh numbers, with periods of reduced activity (grand minima-like events) occurring during transient phases in which the large-scale vortex temporarily re-establishes itself, before being suppressed again by the magnetic field.

DOUBLE DYNAMO SIGNATURES IN A GLOBAL MHD SIMULATION AND MEAN-FIELD DYNAMOS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Beaudoin, Patrice; Simard, Corinne; Cossette, Jean-François

The 11 year solar activity cycle is the most prominent periodic manifestation of the magnetohydrodynamical (MHD) large-scale dynamo operating in the solar interior, yet longer and shorter (quasi-) periodicities are also present. The so-called “quasi-biennial” signal appearing in many proxies of solar activity has been gaining increasing attention since its detection in p -mode frequency shifts, which suggests a subphotospheric origin. A number of candidate mechanisms have been proposed, including beating between co-existing global dynamo modes, dual dynamos operating in spatially separated regions of the solar interior, and Rossby waves driving short-period oscillations in the large-scale solar magnetic field producedmore » by the 11 year activity cycle. In this article, we analyze a global MHD simulation of solar convection producing regular large-scale magnetic cycles, and detect and characterize shorter periodicities developing therein. By constructing kinematic mean-field α {sup 2}Ω dynamo models incorporating the turbulent electromotive force (emf) extracted from that same simulation, we find that dual-dynamo behavior materializes in fairly wide regions of the model’s parameters space. This suggests that the origin of the similar behavior detected in the MHD simulation lies with the joint complexity of the turbulent emf and differential rotation profile, rather that with dynamical interactions such as those mediated by Rossby waves. Analysis of the simulation also reveals that the dual dynamo operating therein leaves a double-period signature in the temperature field, consistent with a dual-period helioseismic signature. Order-of-magnitude estimates for the magnitude of the expected frequency shifts are commensurate with helioseismic measurements. Taken together, our results support the hypothesis that the solar quasi-biennial oscillations are associated with a secondary dynamo process operating in the outer reaches of the solar convection zone.« less

Solar and planetary dynamos: comparison and recent developments

NASA Astrophysics Data System (ADS)

Petrovay, K.

2009-03-01

While obviously having a common root, solar and planetary dynamo theory have taken increasingly divergent routes in the last two or three decades, and there are probably few experts now who can claim to be equally versed in both. Characteristically, even in the fine and comprehensive book “The magnetic Universe” (Rudiger & Hollerbach 2004), the chapters on planets and on the Sun were written by different authors. Separate reviews written on the two topics include Petrovay (2000), Charbonneau (2005), Choudhuri (2008) on the solar dynamo and Glatzmaier (2002), Stevenson (2003) on the planetary dynamo. In the following I will try to make a systematic comparison between solar and planetary dynamos, presenting analogies and differences, and highlighting some interesting recent results.

27. View looking to port from ship's centerline toward main ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

27. View looking to port from ship's centerline toward main electrical control panel, behind which is DC-AC motor-generator set. DC dynamo appears at lower right of image, waste water overflow pipe from hot well appears in upper right of image. - Ferry TICONDEROGA, Route 7, Shelburne, Chittenden County, VT

Chandra Observations of Magnetic White Dwarfs and their Theoretical Implications

NASA Technical Reports Server (NTRS)

Musielak, Z. E.; Noble, M.; Porter, J. G.; Winget, D. E.

2003-01-01

Observations of cool DA and DB white dwarfs have not yet been successful in detecting coronal X-ray emission, but observations of late-type dwarfs and giants show that coronae are common for these stars. To produce coronal X-rays, a star must have dynamo-generated surface magnetic fields and a well-developed convection zone. There is some observational evidence that the DA star LHS 1038 and the DB star GD 358 have weak and variable surface magnetic fields. It has been suggested that such fields can be generated by dynamo action, and since both stars have well-developed convection zones, theory predicts detectable levels of coronal X-rays from these white dwarfs. However, we present analysis of Chandra observations of both stars showing no detectable X-ray emission. The derived upper limits for the X-ray fluxes provide strong constraints on theories of formation of coronae around magnetic white dwarfs. Another important implication of our negative Chandra observations is the possibility that the magnetic fields of LHS 1038 and GD 358 are fossil fields.

Statistical theory of dynamo

NASA Astrophysics Data System (ADS)

Kim, E.; Newton, A. P.

2012-04-01

One major problem in dynamo theory is the multi-scale nature of the MHD turbulence, which requires statistical theory in terms of probability distribution functions. In this contribution, we present the statistical theory of magnetic fields in a simplified mean field α-Ω dynamo model by varying the statistical property of alpha, including marginal stability and intermittency, and then utilize observational data of solar activity to fine-tune the mean field dynamo model. Specifically, we first present a comprehensive investigation into the effect of the stochastic parameters in a simplified α-Ω dynamo model. Through considering the manifold of marginal stability (the region of parameter space where the mean growth rate is zero), we show that stochastic fluctuations are conductive to dynamo. Furthermore, by considering the cases of fluctuating alpha that are periodic and Gaussian coloured random noise with identical characteristic time-scales and fluctuating amplitudes, we show that the transition to dynamo is significantly facilitated for stochastic alpha with random noise. Furthermore, we show that probability density functions (PDFs) of the growth-rate, magnetic field and magnetic energy can provide a wealth of useful information regarding the dynamo behaviour/intermittency. Finally, the precise statistical property of the dynamo such as temporal correlation and fluctuating amplitude is found to be dependent on the distribution the fluctuations of stochastic parameters. We then use observations of solar activity to constrain parameters relating to the effect in stochastic α-Ω nonlinear dynamo models. This is achieved through performing a comprehensive statistical comparison by computing PDFs of solar activity from observations and from our simulation of mean field dynamo model. The observational data that are used are the time history of solar activity inferred for C14 data in the past 11000 years on a long time scale and direct observations of the sun spot numbers obtained in recent years 1795-1995 on a short time scale. Monte Carlo simulations are performed on these data to obtain PDFs of the solar activity on both long and short time scales. These PDFs are then compared with predicted PDFs from numerical simulation of our α-Ω dynamo model, where α is assumed to have both mean α0 and fluctuating α' parts. By varying the correlation time of fluctuating α', the ratio of the amplitude of the fluctuating to mean alpha <α'2>/α02 (where angular brackets <> denote ensemble average), and the ratio of poloidal to toroidal magnetic fields, we show that the results from our stochastic dynamo model can match the PDFs of solar activity on both long and short time scales. In particular, a good agreement is obtained when the fluctuation in alpha is roughly equal to the mean part with a correlation time shorter than the solar period.

A GLOBAL GALACTIC DYNAMO WITH A CORONA CONSTRAINED BY RELATIVE HELICITY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Prasad, A.; Mangalam, A., E-mail: avijeet@iiap.res.in, E-mail: mangalam@iiap.res.in

We present a model for a global axisymmetric turbulent dynamo operating in a galaxy with a corona that treats the parameters of turbulence driven by supernovae and by magneto-rotational instability under a common formalism. The nonlinear quenching of the dynamo is alleviated by the inclusion of small-scale advective and diffusive magnetic helicity fluxes, which allow the gauge-invariant magnetic helicity to be transferred outside the disk and consequently to build up a corona during the course of dynamo action. The time-dependent dynamo equations are expressed in a separable form and solved through an eigenvector expansion constructed using the steady-state solutions ofmore » the dynamo equation. The parametric evolution of the dynamo solution allows us to estimate the final structure of the global magnetic field and the saturated value of the turbulence parameter α{sub m}, even before solving the dynamical equations for evolution of magnetic fields in the disk and the corona, along with α-quenching. We then solve these equations simultaneously to study the saturation of the large-scale magnetic field, its dependence on the small-scale magnetic helicity fluxes, and the corresponding evolution of the force-free field in the corona. The quadrupolar large-scale magnetic field in the disk is found to reach equipartition strength within a timescale of 1 Gyr. The large-scale magnetic field in the corona obtained is much weaker than the field inside the disk and has only a weak impact on the dynamo operation.« less

Data acquisition in a high-speed rotating frame for New Mexico Institute of Mining and Technology liquid sodium αω dynamo experiment.

PubMed

Si, Jiahe; Colgate, Stirling A; Li, Hui; Martinic, Joe; Westpfahl, David

2013-10-01

New Mexico Institute of Mining and Technology liquid sodium αω-dynamo experiment models the magnetic field generation in the universe as discussed in detail by Colgate, Li, and Pariev [Phys. Plasmas 8, 2425 (2001)]. To obtain a quasi-laminar flow with magnetic Reynolds number R(m) ~ 120, the dynamo experiment consists of two co-axial cylinders of 30.5 cm and 61 cm in diameter spinning up to 70 Hz and 17.5 Hz, respectively. During the experiment, the temperature of the cylinders must be maintained to 110 °C to ensure that the sodium remains fluid. This presents a challenge to implement a data acquisition (DAQ) system in such high temperature, high-speed rotating frame, in which the sensors (including 18 Hall sensors, 5 pressure sensors, and 5 temperature sensors, etc.) are under the centrifugal acceleration up to 376g. In addition, the data must be transmitted and stored in a computer 100 ft away for safety. The analog signals are digitized, converted to serial signals by an analog-to-digital converter and a field-programmable gate array. Power is provided through brush/ring sets. The serial signals are sent through ring/shoe sets capacitively, then reshaped with cross-talk noises removed. A microcontroller-based interface circuit is used to decode the serial signals and communicate with the data acquisition computer. The DAQ accommodates pressure up to 1000 psi, temperature up to more than 130 °C, and magnetic field up to 1000 G. First physics results have been analyzed and published. The next stage of the αω-dynamo experiment includes the DAQ system upgrade.

On geodynamo integrations conserving momentum flux

NASA Astrophysics Data System (ADS)

Wu, C.; Roberts, P. H.

2012-12-01

The equations governing the geodynamo are most often integrated by representing the magnetic field and fluid velocity by toroidal and poloidal scalars (for example, MAG code [1]). This procedure does not automatically conserve the momentum flux. The results can, particularly for flows with large shear, introduce significant errors, unless the viscosity is artificially increased. We describe a method that evades this difficulty, by solving the momentum equation directly while properly conserving momentum. It finds pressure by FFT and cyclic reduction, and integrates the governing equations on overlapping grids so avoiding the pole problem. The number of operations per time step is proportional to N3 where N is proportional to the number of grid points in each direction. This contrasts with the order N4 operations of standard spectral transform methods. The method is easily parallelized. It can also be easily adapted to schemes such as the Weighted Essentially Non-Oscillatory (WENO) method [2], a flux based procedure based on upwinding that is numerically stable even for zero explicit viscosity. The method has been successfully used to investigate the generation of magnetic fields by flows confined to spheroidal containers and driven by precessional and librational forcing [3, 4]. For spherical systems it satisfies dynamo benchmarks [5]. [1] MAG, http://www.geodynamics.org/cig/software/mag [2] Liu, XD, Osher, S and Chan, T, Weighted Essentially Nonoscillatory Schemes, J. Computational Physics, 115, 200-212, 1994. [3] Wu, CC and Roberts, PH, On a dynamo driven by topographic precession, Geophysical & Astrophysical Fluid Dynamics, 103, 467-501, (DOI: 10.1080/03091920903311788), 2009. [4] Wu, CC and Roberts, PH, On a dynamo driven topographically by longitudinal libration, Geophysical & Astrophysical Fluid Dynamics, DOI:10.1080/03091929.2012.682990, 2012. [5] Christensen, U, et al., A numerical dynamo benchmark, Phys. Earth Planet Int., 128, 25-34, 2001.

A NEW SIMPLE DYNAMO MODEL FOR STELLAR ACTIVITY CYCLE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yokoi, N.; Hamba, F.; Schmitt, D.

2016-06-20

A new simple dynamo model for stellar activity cycle is proposed. By considering an inhomogeneous flow effect on turbulence, it is shown that turbulent cross helicity (velocity–magnetic-field correlation) enters the expression of turbulent electromotive force as the coupling coefficient for the mean absolute vorticity. This makes the present model different from the current α –Ω-type models in two main ways. First, in addition to the usual helicity ( α ) and turbulent magnetic diffusivity ( β ) effects, we consider the cross-helicity effect as a key ingredient of the dynamo process. Second, the spatiotemporal evolution of cross helicity is solvedmore » simultaneously with the mean magnetic fields. The basic scenario is as follows. In the presence of turbulent cross helicity, the toroidal field is induced by the toroidal rotation. Then, as in usual models, the α effect generates the poloidal field from the toroidal one. This induced poloidal field produces a turbulent cross helicity whose sign is opposite to the original one (negative production). With this cross helicity of the reversed sign, a reversal in field configuration starts. Eigenvalue analyses of the simplest possible model give a butterfly diagram, which confirms the above scenario and the equatorward migrations, the phase relationship between the cross helicity and magnetic fields. These results suggest that the oscillation of the turbulent cross helicity is a key for the activity cycle. The reversal of the cross helicity is not the result of the magnetic-field reversal, but the cause of the latter. This new model is expected to open up the possibility of the mean-field or turbulence closure dynamo approaches.« less

Exploring a deep meridional flow hypothesis for a circulation dominated solar dynamo model

NASA Astrophysics Data System (ADS)

Guerrero, G. A.; Muñoz, J. D.; de Gouveia dal Pino, E. M.

2005-09-01

Circulation-dominated solar dynamo models, which employ a helioseismic rotation profile and a fixed meridional flow, give a good approximation to the large scale solar magnetic phenomena, such as the 11-year cycle or the so called Hale's law of polarities. Nevertheless, the larger amplitude of the radial shear ∂Ω/∂r at the high latitudes makes the dynamo to produce a strong toroidal magnetic field at high latitudes, in contradiction with the observations of the sunspots (Sporer's Law). A possible solution was proposed by Nandy and Choudhuri in which a deep meridional flow can conduct the magnetic field inside of a stable layer (the radiative core) and then allow that it erupts just at lower latitudes. Although they obtain good results, this hypothesis generates new problems like the mixture of elements in the radiative core (that alters the abundance of the elements) and the transfer of angular momentum. We have recently explored this hypothesis in a different approximation, using the magnetic buoyancy mechanism proposed by Dikpati and Charbonneau (1999) and found that a deep meridional flow pushes the maximum of the toroidal magnetic field towards the solar equator, but, in contrast to Nandy and Choudhuri (2002 ), a second zone of maximum fields remains at the poles. In that work, we have also introduced a bipolytropic density profile in order to better reproduce the stratification in the radiative zone. We here review these results and also discuss a new possible scenario where the tachocline has an ellipsoidal shape, following early helioseismologic observations, and find that the modification of the geometry of the tachocline can lead to results which are in good agreement with observations and opens the possibility to explore in more detail, through the dynamo model, the place where the magnetic field could be really stored.

Solar Cycle 24 and the Solar Dynamo

NASA Technical Reports Server (NTRS)

Pesnell, W. D.; Schatten, K.

2007-01-01

We will discuss the polar field precursor method for solar activity prediction, which predicts cycle 24 will be significantly lower than recent activity cycles, and some new ideas rejuvenating Babcock's shallow surface dynamo. The polar field precursor method is based on Babcock and Leighton's dynamo models wherein the polar field at solar minimum plays a major role in generating the next cycle's toroidal field and sunspots. Thus, by examining the polar fields of the Sun near solar minimum, a forecast for the next cycle's activity is obtained. With the current low value for the Sun's polar fields, this method predicts solar cycle 24 will be one of the lowest in recent times, with smoothed F10.7 radio flux values peaking near 135 plus or minus 35 (2 sigma), in the 2012-2013 timeframe (equivalent to smoothed Rz near 80 plus or minus 35 [2 sigma]). One may have to consider solar activity as far back as the early 20th century to find a cycle of comparable magnitude. We discuss unusual behavior in the Sun's polar fields that support this prediction. Normally, the solar precursor method is consistent with the geomagnetic precursor method, wherein geomagnetic variations are thought to be a good measure of the Sun's polar field strength. Because of the unusual polar field, the Earth does not appear to be currently bathed in the Sun's extended polar field (the interplanetary field), hence negating the primal cause behind the geomagnetic precursor technique. We also discuss how percolation may support Babcock's original shallow solar dynamo. In this process ephemeral regions from the solar magnetic carpet, guided by shallow surface fields, may collect to form pores and sunspots.

Sign-singular measures - Fast magnetic dynamos, and high-Reynolds-number fluid turbulence

NASA Astrophysics Data System (ADS)

Ott, Edward; Du, Yunson; Sreenivasan, K. R.; Juneja, A.; Suri, A. K.

1992-11-01

It is shown that sign-singular measures with nontrivial cancellation exponents occur in dynamos and fluid turbulence. A cancellation exponent is introduced to characterize such measures quantitatively. Examples from kinematic magnetic dynamos and fluid turbulence are used to illlustrate this kind of singular behavior.

Recent and future liquid metal experiments on homogeneous dynamo action and magnetic instabilities

NASA Astrophysics Data System (ADS)

Stefani, Frank; Gerbeth, Gunter; Giesecke, Andre; Gundrum, Thomas; Kirillov, Oleg; Seilmayer, Martin; Gellert, Marcus; Rüdiger, Günther; Gailitis, Agris

2011-10-01

The present status of the Riga dynamo experiment is summarized and the prospects for its future exploitation are evaluated. We further discuss the plans for a large-scale precession driven dynamo experiment to be set-up in the framework of the new installation DRESDYN (DREsden Sodium facility for dynamo and thermohydraulic studies) at Helmholtz-Zentrum Dresden-Rossendorf. We report recent investigations of the magnetorotational instability and the Tayler instability and sketch the plans for another large-scale liquid sodium facility devoted to the combined study of both effects.

Geophysics: Timing of the Martian dynamo

NASA Astrophysics Data System (ADS)

Schubert, G.; Russell, C. T.; Moore, W. B.

2000-12-01

On Mars, the strong magnetization in the highland crust of the southern hemisphere and the absence of magnetic anomalies at the Hellas and Argyre impact basins have been taken as signs that the core dynamo that once drove the planet's magnetic field turned off more than 4 billion years (Gyr) ago. Here, we argue instead that the Martian dynamo turned on less than 4 Gyr ago and turned off at an unknown time since then. High spatial resolution magnetometry in both Martian hemispheres is needed to reveal the true history of the Martian dynamo.

Kinematic α tensors and dynamo mechanisms in a von Kármán swirling flow.

PubMed

Ravelet, F; Dubrulle, B; Daviaud, F; Ratié, P-A

2012-07-13

We provide experimental and numerical evidence of in-blades vortices in the von Kármán swirling flow. We estimate the associated kinematic α-effect tensor and show that it is compatible with recent models of the von Kármán sodium (VKS) dynamo. We further show that depending on the relative frequency of the two impellers, the dominant dynamo mechanism may switch from α2 to α - Ω dynamo. We discuss some implications of these results for VKS experiments.

Investigation of the Air-Wave-Sea Interaction Modes Using an Airborne Doppler Wind Lidar: Analyses of the HRDL Data Taken using DYNAMO

DTIC Science & Technology

2013-10-07

Interaction Modes Using an Airborne Doppler Wind Lidar: Analyses of the HRDL data taken using DYNAMO 5a. CONTRACT NUMBER N0001411C0464 5b. GRANT...efficiency of energy, mass and momentum exchange at the bottom and top of the ABL. 15. SUBJECT TERMS DYNAMO , ABL 16. SECURITY CLASSIFICATION OF: 17...Investigation of the Air-Wave-Sea Interaction Modes Using an Airborne Doppler Wind Lidar: Analyses of the HRDL data taken during DYNAMO George

Cosmic Ray and Tev Gamma Ray Generation by Quasar Remnants

NASA Technical Reports Server (NTRS)

Boldt, Elihu; Loewenstein, Michael; White, Nicholas E. (Technical Monitor)

2000-01-01

Results from new broadband (radio to X-ray) high-resolution imaging studies of the dormant quasar remnant cores of nearby giant elliptical galaxies are now shown to permit the harboring of compact dynamos capable of generating the highest energy cosmic ray particles and associated curvature radiation of TeV photons. Confirmation would imply a global inflow of interstellar gas all the way to the accretion powered supermassive black hole at the center of the host galaxy.

MEAN-FIELD MODELING OF AN α{sup 2} DYNAMO COUPLED WITH DIRECT NUMERICAL SIMULATIONS OF RIGIDLY ROTATING CONVECTION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Masada, Youhei; Sano, Takayoshi, E-mail: ymasada@harbor.kobe-u.ac.jp, E-mail: sano@ile.osaka-u.ac.jp

2014-10-10

The mechanism of large-scale dynamos in rigidly rotating stratified convection is explored by direct numerical simulations (DNS) in Cartesian geometry. A mean-field dynamo model is also constructed using turbulent velocity profiles consistently extracted from the corresponding DNS results. By quantitative comparison between the DNS and our mean-field model, it is demonstrated that the oscillatory α{sup 2} dynamo wave, excited and sustained in the convection zone, is responsible for large-scale magnetic activities such as cyclic polarity reversal and spatiotemporal migration. The results provide strong evidence that a nonuniformity of the α-effect, which is a natural outcome of rotating stratified convection, canmore » be an important prerequisite for large-scale stellar dynamos, even without the Ω-effect.« less

A solar dynamo surface wave at the interface between convection and nonuniform rotation

NASA Technical Reports Server (NTRS)

Parker, E. N.

1993-01-01

A simple dynamo surface wave is presented to illustrate the basic principles of a dynamo operating in the thin layer of shear and suppressed eddy diffusion beneath the cyclonic convection in the convection zone of the sun. It is shown that the restriction of the shear delta(Omega)/delta(r) to a region below the convective zone provides the basic mode with a greatly reduced turbulent diffusion coefficient in the region of strong azimuthal field. The dynamo takes on the character of a surface wave tied to the lower surface z = 0 of the convective zone. There is a substantial body of evidence suggesting a fibril state for the principal flux bundles beneath the surface of the sun, with fundamental implications for the solar dynamo.

POSSIBLE CHROMOSPHERIC ACTIVITY CYCLES IN AD LEO

DOE Office of Scientific and Technical Information (OSTI.GOV)

Buccino, Andrea P.; Petrucci, Romina; Mauas, Pablo J. D.

2014-01-20

AD Leo (GJ 388) is an active dM3 flare star that has been extensively observed both in the quiescent and flaring states. Since this active star is near the fully convective boundary, studying its long-term chromospheric activity in detail could be an appreciable contribution to dynamo theory. Here, using the Lomb-Scargle periodogram, we analyze the Ca II K line-core fluxes derived from CASLEO spectra obtained between 2001 and 2013 and the V magnitude from the ASAS database between 2004 and 2010. From both of these totally independent time series, we obtain a possible activity cycle with a period of approximately seven yearsmore » and a less significant shorter cycle of approximately two years. A tentative interpretation is that a dynamo operating near the surface could be generating the longer cycle, while a second dynamo operating in the deep convection zone could be responsible for the shorter one. Based on the long duration of our observing program at CASLEO and the fact that we observe different spectral features simultaneously, we also analyze the relation between simultaneous measurements of the Na I index (R{sub D}{sup ′}), Hα, and Ca II K fluxes at different activity levels of AD Leo, including flares.« less

Latitudinal migration of sunspots based on the ESAI database

NASA Astrophysics Data System (ADS)

Zhang, Juan; Li, Fu-Yu; Feng, Wen

2018-01-01

The latitudinal migration of sunspots toward the equator, which implies there is propagation of the toroidal magnetic flux wave at the base of the solar convection zone, is one of the crucial observational bases for the solar dynamo to generate a magnetic field by shearing of the pre-existing poloidal magnetic field through differential rotation. The Extended time series of Solar Activity Indices (ESAI) elongated the Greenwich observation record of sunspots by several decades in the past. In this study, ESAI’s yearly mean latitude of sunspots in the northern and southern hemispheres during the years 1854 to 1985 is utilized to statistically test whether hemispherical latitudinal migration of sunspots in a solar cycle is linear or nonlinear. It is found that a quadratic function is statistically significantly better at describing hemispherical latitudinal migration of sunspots in a solar cycle than a linear function. In addition, the latitude migration velocity of sunspots in a solar cycle decreases as the cycle progresses, providing a particular constraint for solar dynamo models. Indeed, the butterfly wing pattern with a faster latitudinal migration rate should present stronger solar activity with a shorter cycle period, and it is located at higher latitudinal position, giving evidence to support the Babcock-Leighton dynamo mechanism.

Dynamo Tests for Stratification Below the Core-Mantle Boundary

NASA Astrophysics Data System (ADS)

Olson, P.; Landeau, M.

2017-12-01

Evidence from seismology, mineral physics, and core dynamics points to a layer with an overall stable stratification in the Earth's outer core, possibly thermal in origin, extending below the core-mantle boundary (CMB) for several hundred kilometers. In contrast, energetic deep mantle convection with elevated heat flux implies locally unstable thermal stratification below the CMB in places, consistent with interpretations of non-dipole geomagnetic field behavior that favor upwelling flows below the CMB. Here, we model the structure of convection and magnetic fields in the core using numerical dynamos with laterally heterogeneous boundary heat flux in order to rationalize this conflicting evidence. Strongly heterogeneous boundary heat flux generates localized convection beneath the CMB that coexists with an overall stable stratification there. Partially stratified dynamos have distinctive time average magnetic field structures. Without stratification or with stratification confined to a thin layer, the octupole component is small and the CMB magnetic field structure includes polar intensity minima. With more extensive stratification, the octupole component is large and the magnetic field structure includes intense patches or high intensity lobes in the polar regions. Comparisons with the time-averaged geomagnetic field are generally favorable for partial stratification in a thin layer but unfavorable for stratification in a thick layer beneath the CMB.

IMPORTANCE OF MERIDIONAL CIRCULATION IN FLUX TRANSPORT DYNAMO: THE POSSIBILITY OF A MAUNDER-LIKE GRAND MINIMUM

DOE Office of Scientific and Technical Information (OSTI.GOV)

Karak, Bidya Binay, E-mail: bidya_karak@physics.iisc.ernet.i

2010-12-01

Meridional circulation is an important ingredient in flux transport dynamo models. We have studied its importance on the period, the amplitude of the solar cycle, and also in producing Maunder-like grand minima in these models. First, we model the periods of the last 23 sunspot cycles by varying the meridional circulation speed. If the dynamo is in a diffusion-dominated regime, then we find that most of the cycle amplitudes also get modeled up to some extent when we model the periods. Next, we propose that at the beginning of the Maunder minimum the amplitude of meridional circulation dropped to amore » low value and then after a few years it increased again. Several independent studies also favor this assumption. With this assumption, a diffusion-dominated dynamo is able to reproduce many important features of the Maunder minimum remarkably well. If the dynamo is in a diffusion-dominated regime, then a slower meridional circulation means that the poloidal field gets more time to diffuse during its transport through the convection zone, making the dynamo weaker. This consequence helps to model both the cycle amplitudes and the Maunder-like minima. We, however, fail to reproduce these results if the dynamo is in an advection-dominated regime.« less

Dynamic motif occupancy (DynaMO) analysis identifies transcription factors and their binding sites driving dynamic biological processes.

PubMed

Kuang, Zheng; Ji, Zhicheng; Boeke, Jef D; Ji, Hongkai

2018-01-09

Biological processes are usually associated with genome-wide remodeling of transcription driven by transcription factors (TFs). Identifying key TFs and their spatiotemporal binding patterns are indispensable to understanding how dynamic processes are programmed. However, most methods are designed to predict TF binding sites only. We present a computational method, dynamic motif occupancy analysis (DynaMO), to infer important TFs and their spatiotemporal binding activities in dynamic biological processes using chromatin profiling data from multiple biological conditions such as time-course histone modification ChIP-seq data. In the first step, DynaMO predicts TF binding sites with a random forests approach. Next and uniquely, DynaMO infers dynamic TF binding activities at predicted binding sites using their local chromatin profiles from multiple biological conditions. Another landmark of DynaMO is to identify key TFs in a dynamic process using a clustering and enrichment analysis of dynamic TF binding patterns. Application of DynaMO to the yeast ultradian cycle, mouse circadian clock and human neural differentiation exhibits its accuracy and versatility. We anticipate DynaMO will be generally useful for elucidating transcriptional programs in dynamic processes. © The Author(s) 2017. Published by Oxford University Press on behalf of Nucleic Acids Research.

The ω{OMEGA} dynamo in accretion disks of rotating black holes.

NASA Astrophysics Data System (ADS)

Khanna, R.; Camenzind, M.

1996-03-01

We develop the kinematic theory of axisymmetric dynamo action in the innermost part of an accretion disk around a rotating black hole. The problem is formulated in the 3+1 split of Kerr spacetime. It turns out that the gravitomagnetic field of the hole gives rise to a dynamo current for the the poloidal magnetic field without any need of turbulent plasma motions even in axisymmetry. We show that Cowling's theorem does not apply in the Kerr metric. This gravitomagnetic dynamo effect (ω-effect) requires finite diffusivity and is enhanced by anomalous or turbulent magnetic diffusivity. The reformulation of the problem in the framework of mean field magnetohydrodynamics introduces the familiar α-effect. The dynamo equations are formally identical with their classical equivalents (i.e. equations for the α{OMEGA} dynamo in flat space), augmented by the general relativistic ω-effect-term as source. We have carried out time-dependent numerical simulations of the dynamo in a turbulent differentially rotating accretion disk using a finite element code with implicit time-stepping. The advection of the magnetic field with the plasma is fully included. Solutions are discussed for extremely and less rapidly rotating black holes. We observe growing dipolar, quadrupolar and mixed modes, the second being, however, dominant. A common feature of all our simulations of the ω{OMEGA} dynamo is that it will finally build up a stellar like magnetosphere around the black hole, which blends into the outer disk field topology in a transition region. This finding enforces the analogy in the models of jet formation in AGN and YSOs. An interesting feature occurs for less rapidly rotating holes. The frame dragging effect introduces a boundary layer in the plasma rotation, where the plasma is prone to resistive magnetohydrodynamical instabilities such as the rippling mode or the tearing mode and thus the boundary layer has to be regarded as a potential site of particle acceleration. We also present a simulation of the αω{OMEGA} dynamo. For a heuristic description of α in the 3+1 split of Kerr spacetime, the ω-effect is dominated by the α-effect. For the same parameters as in the simulations of the ω{OMEGA} dynamo, the αω{OMEGA} dynamo behaves much more dynamically. The simulation shows radially and vertically oscillating dipolar, quadrupolar and mixed modes.

Progress Towards a Time-Dependent Theory of Solar Meridional Flows

NASA Astrophysics Data System (ADS)

Shirley, James H.

2017-08-01

Large-scale meridional motions of solar materials play an important role in flux transport dynamo models. Meridional flows transport surface magnetic flux to polar regions of the Sun, where it may later be subducted and conveyed back towards the equatorial region by a deep return flow in the convection zone. The transported flux may thereafter lead to the generation of new toroidal fields, thereby completing the dynamo cycle. More than two decades of observations have revealed that meridional flow speeds vary substantially with time. Further, a complex morphological variability of meridional flow cells is now recognized, with multiple cell structures detected both in latitude and in depth. ‘Countercells’ with reversed flow directions have been detected at various times. Flow speeds are apparently influenced by the proximity of flows to active regions. This complexity represents a considerable challenge to dynamo modeling efforts. Flows morphology and speed changes may be arbitrarily prescribed in models, but physical realism of model outputs may be questionable, and elusive: The models are ‘trying to hit a moving target.’ Considerations such as these led Belucz et al. (2013; Ap. J. 806:169) to call for “time-dependent theories that can tell us theoretically how this circulation may change its amplitude and form in each hemisphere.” Such a theory now exists for planetary atmospheres (Shirley, 2017; Plan. Sp. Sci. 141, 1-16). Proof of concept for the non-tidal orbit-spin coupling hypothesis of Shirley (2017) was obtained through numerical modeling of the atmospheric circulation of Mars (Mischna & Shirley, 2017; Plan. Sp. Sci. 141, 45-72). Much-improved correspondence of numerical modeling outcomes with observations was demonstrated. In this presentation we will briefly review the physical hypothesis and some prior evidence of its possible role in solar dynamo excitation. We show a strong correlation between observed meridional flow speeds of magnetic features in Cycle 23 with the putative dynamical forcing function. We will also briefly discuss the potential for incorporating orbit-spin coupling accelerations within existing numerical solar dynamo models.

The role of large eddy fluctuations in the magnetic dynamics of the Madison Dynamo Experiment

NASA Astrophysics Data System (ADS)

Kaplan, Elliot

The Madison Dynamo Experiment (MDE), a liquid sodium magnetohydrodynamics experiment in a 1 m diameter sphere at the University of Wisconsin-Madison, had measured [in Spence et al., 2006] diamagnetic electrical currents in the experiment that violated an anti dynamo theorem for axisymmetric flow. The diamagnetic currents were instead attributed to nonaxisymmetric turbulent fluctuations. The experimental apparatus has been modified to reduce the strength of the large-scale turbulence driven by the shear layer in its flow. A 7.62 cm baffle was affixed to the equator of the machine to stabilize the shear layer. This reduction has correlated with a decrease in the magnetic fields, induced by the flow, which had been associated with the α and β effects of mean-field magnetohydrodynamics. The research presented herein presents the experimental evidence for reduced fluctuations and reduced mean field emfs, and provides a theoretical framework—based upon mean-field MHD—that connects the observations. The shapes of the large-scale velocity fluctuations are inferred by the spectra of induced magnetic fluctuations and measured in a kinematically similar water experiment. The Bullard and Gellman [1954] formalism demonstrates that the large-scale velocity fluctuations that are inhibited by the baffle can beat with the large-scale magnetic fluctuations that they produce to generate a mean-field emf of the sort measured in Spence et al. [2006]. This shows that the reduction of these large-scale eddies has brought the MDE closer to exciting a dynamo magnetic field. We also examine the mean-field like effects of large-scale (stable) eddies in the Dudley-James [1989] two-vortex dynamo (that the MDE was based upon). Rotating the axis of symmetry redefines the problem from one of an axisymmetric flow exciting a nonaxisymmetric field to one of a combination of axisymmetric and nonaxisymmetric flows exciting a predominantly axisymmetric magnetic eigenmode. As a result, specific interactions between large-scale velocity modes and large-scale magnetic modes are shown to correspond to the Ω effect and the mean-field α and β effects.

Evolution of dynamo-generated magnetic fields in accretion disks around compact and young stars

NASA Technical Reports Server (NTRS)

Stepinski, Tomasz F.

1994-01-01

Geometrically thin, optically thick, turbulent accretion disks are believed to surround many stars. Some of them are the compact components of close binaries, while the others are throught to be T Tauri stars. These accretion disks must be magnetized objects because the accreted matter, whether it comes from the companion star (binaries) or from a collapsing molecular cloud core (single young stars), carries an embedded magnetic field. In addition, most accretion disks are hot and turbulent, thus meeting the condition for the MHD turbulent dynamo to maintain and amplify any seed field magnetic field. In fact, for a disk's magnetic field to persist long enough in comparison with the disk viscous time it must be contemporaneously regenerated because the characteristic diffusion time of a magnetic field is typically much shorter than a disk's viscous time. This is true for most thin accretion disks. Consequently, studying magentic fields in thin disks is usually synonymous with studying magnetic dynamos, a fact that is not commonly recognized in the literature. Progress in studying the structure of many accretion disks was achieved mainly because most disks can be regarded as two-dimensional flows in which vertical and radial structures are largely decoupled. By analogy, in a thin disk, one may expect that vertical and radial structures of the magnetic field are decoupled because the magnetic field diffuses more rapidly to the vertical boundary of the disk than along the radius. Thus, an asymptotic method, called an adiabatic approximation, can be applied to accretion disk dynamo. We can represent the solution to the dynamo equation in the form B = Q(r)b(r,z), where Q(r) describes the field distribution along the radius, while the field distribution across the disk is included in the vector function b, which parametrically depends on r and is normalized by the condition max (b(z)) = 1. The field distribution across the disk is established rapidly, while the radial distribution Q(r) evolves on a considerably longer timescale. It is this evolution that is the subject of this paper.

Systematic parameter study of dynamo bifurcations in geodynamo simulations

NASA Astrophysics Data System (ADS)

Petitdemange, Ludovic

2018-04-01

We investigate the nature of the dynamo bifurcation in a configuration applicable to the Earth's liquid outer core, i.e. in a rotating spherical shell with thermally driven motions with no-slip boundaries. Unlike in previous studies on dynamo bifurcations, the control parameters have been varied significantly in order to deduce general tendencies. Numerical studies on the stability domain of dipolar magnetic fields found a dichotomy between non-reversing dipole-dominated dynamos and the reversing non-dipole-dominated multipolar solutions. We show that, by considering weak initial fields, the above transition disappears and is replaced by a region of bistability for which dipolar and multipolar dynamos coexist. Such a result was also observed in models with free-slip boundaries in which the geostrophic zonal flow can develop and participate to the dynamo mechanism for non-dipolar fields. We show that a similar process develops in no-slip models when viscous effects are reduced sufficiently. The following three regimes are distinguished: (i) Close to the onset of convection (Rac) with only the most critical convective mode (wave number) being present, dynamos set in supercritically in the Ekman number regime explored here and are dipole-dominated. Larger critical magnetic Reynolds numbers indicate that they are particularly inefficient. (ii) in the range 3 < Ra /Rac 10) , the relative importance of zonal flows increases with Ra in non-magnetic models. The field topology depends on the magnitude of the initial magnetic field. The dipolar branch has a subcritical behavior whereas the multipolar branch has a supercritical behavior. By approaching more realistic parameters, the extension of this bistable regime increases. A hysteretic behavior questions the common interpretation for geomagnetic reversals. Far above the dynamo threshold (by increasing the magnetic Prandtl number), Lorentz forces contribute to the first order force balance, as predicted for planetary dynamos. When Ra is sufficiently high, dipolar fields affect significantly the flow speed, the flow structure and heat transfer which is reduced by the Lorentz force regardless of the field strength. This physical regime seems to be relevant for studying geomagnetic processes.

A Nonmagnetic Differentiated Early Planetary Body

DOE Office of Scientific and Technical Information (OSTI.GOV)

Weiss, Benjamin P.; Wang, Jun

2017-06-15

Paleomagnetic studies of meteorites have shown that the solar nebula was likely magnetized and that many early planetary bodies generated dynamo magnetic fields in their advecting metallic cores. The surface fields on these bodies were recorded by a diversity of chondrites and achondrites, ranging in intensity from several μT to several hundred μT. In fact, an achondrite parent body without evidence for paleomagnetic fields has yet to be confidently identified, hinting that early solar system field generation and the dynamo process in particular may have been common. Here we present paleomagnetic measurements of the ungrouped achondrite NWA 7325 indicating thatmore » it last cooled in a near-zero field (<∼1.7μT), estimated to have occurred at 4563.09 ± 0.26 million years ago (Ma) from Al–Mg chronometry. Because NWA 7325 is highly depleted in siderophile elements, its parent body nevertheless underwent large-scale metal-silicate differentiation and likely formed a metallic core. This makes NWA 7325 the first recognized example of an essentially unmagnetized igneous rock from a differentiated early solar system body. These results indicate that all magnetic fields, including those from any core dynamo on the NWA 7325 parent body, the solar nebula, young Sun, and solar wind, were <1.7 μT at the location of NWA 7325 at 4563 Ma. This supports a recent conclusion that the solar nebula had dissipated by ∼4 million years after solar system formation. NWA 7325 also serves as an experimental control that gives greater confidence in the positive identification of remanent magnetization in other achondrites.« less

MAGNETOROTATIONAL TURBULENCE TRANSPORTS ANGULAR MOMENTUM IN STRATIFIED DISKS WITH LOW MAGNETIC PRANDTL NUMBER BUT MAGNETIC REYNOLDS NUMBER ABOVE A CRITICAL VALUE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Oishi, Jeffrey S.; Mac Low, Mordecai-Mark, E-mail: jsoishi@stanford.edu, E-mail: mordecai@amnh.org

2011-10-10

The magnetorotational instability (MRI) may dominate outward transport of angular momentum in accretion disks, allowing material to fall onto the central object. Previous work has established that the MRI can drive a mean-field dynamo, possibly leading to a self-sustaining accretion system. Recently, however, simulations of the scaling of the angular momentum transport parameter {alpha}{sub SS} with the magnetic Prandtl number Pm have cast doubt on the ability of the MRI to transport astrophysically relevant amounts of angular momentum in real disk systems. Here, we use simulations including explicit physical viscosity and resistivity to show that when vertical stratification is included,more » mean-field dynamo action operates, driving the system to a configuration in which the magnetic field is not fully helical. This relaxes the constraints on the generated field provided by magnetic helicity conservation, allowing the generation of a mean field on timescales independent of the resistivity. Our models demonstrate the existence of a critical magnetic Reynolds number Rm{sub crit}, below which transport becomes strongly Pm-dependent and chaotic, but above which the transport is steady and Pm-independent. Prior simulations showing Pm dependence had Rm < Rm{sub crit}. We conjecture that this steady regime is possible because the mean-field dynamo is not helicity-limited and thus does not depend on the details of the helicity ejection process. Scaling to realistic astrophysical parameters suggests that disks around both protostars and stellar mass black holes have Rm >> Rm{sub crit}. Thus, we suggest that the strong Pm dependence seen in recent simulations does not occur in real systems.« less

Magnetorotational Turbulence Transports Angular Momentum in Stratified Disks with Low Magnetic Prandtl Number but Magnetic Reynolds Number above a Critical Value

DOE Office of Scientific and Technical Information (OSTI.GOV)

Oishi, Jeffrey S.; /KIPAC, Menlo Park; Low, Mordecai-Mark Mac

2012-02-14

The magnetorotational instability (MRI) may dominate outward transport of angular momentum in accretion disks, allowing material to fall onto the central object. Previous work has established that the MRI can drive a mean-field dynamo, possibly leading to a self-sustaining accretion system. Recently, however, simulations of the scaling of the angular momentum transport parameter {alpha}{sub SS} with the magnetic Prandtl number Pm have cast doubt on the ability of the MRI to transport astrophysically relevant amounts of angular momentum in real disk systems. Here, we use simulations including explicit physical viscosity and resistivity to show that when vertical stratification is included,more » mean field dynamo action operates, driving the system to a configuration in which the magnetic field is not fully helical. This relaxes the constraints on the generated field provided by magnetic helicity conservation, allowing the generation of a mean field on timescales independent of the resistivity. Our models demonstrate the existence of a critical magnetic Reynolds number Rm{sub crit}, below which transport becomes strongly Pm-dependent and chaotic, but above which the transport is steady and Pm-independent. Prior simulations showing Pm-dependence had Rm < Rm{sub crit}. We conjecture that this steady regime is possible because the mean field dynamo is not helicity-limited and thus does not depend on the details of the helicity ejection process. Scaling to realistic astrophysical parameters suggests that disks around both protostars and stellar mass black holes have Rm >> Rm{sub crit}. Thus, we suggest that the strong Pm dependence seen in recent simulations does not occur in real systems.« less

Diagnostics of Turbulent Dynamo from the Flux Emergence Rate in Solar Active Regions

NASA Astrophysics Data System (ADS)

Abramenko, V. I.; Tikhonova, O. I.; Kutsenko, A. S.

2017-12-01

Line-of-sight magnetograms acquired by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamic Observatory (SDO) and by the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO) for 14 emerging ARs were used to study the derivative of the total unsigned flux-the flux emergence rate, R( t). We found that the emergence regime is not universal: each AR displays a unique emergence process. Nevertheless, two types of the emergence process can be identified. First type is a "regular" emergence with quasi-constant behavior of R( t) during a 1-3 day emergence interval with a rather low magnitude of the flux derivative, R max = (0.57 ± 0.22) × 1022 Mx day-1. The second type can be described as "accelerated" emergence with a long interval (>1 day) of the rapidly increasing flux derivative R( t) that result in a rather high magnitude of R max= (0.92 ± 0.29) × 1022 Mx day-1, which later changes to a very short (about a one third of day) interval of R( t) = const followed by a monotonous decrease of R( t). The first type events might be associated with emergence of a flux tube with a constant amount of flux that rises through the photosphere with a quasi-constant speed. Such events can be explained by the traditional largescale solar dynamo generating the toroidal flux deep in the convective zone. The second-type events can be interpreted as a signature of sub-surface turbulent dynamo action that generates additional magnetic flux (via turbulent motions) as the magnetic structure makes its way up to the solar surface.

77 FR 58124 - Dynamo Power LLC; Supplemental Notice That Initial Market-Based Rate Filing Includes Request for...

Federal Register 2010, 2011, 2012, 2013, 2014

2012-09-19

... DEPARTMENT OF ENERGY Federal Energy Regulatory Commission [Docket No. ER12-2614-000] Dynamo Power LLC; Supplemental Notice That Initial Market-Based Rate Filing Includes Request for Blanket Section 204 Authorization This is a supplemental notice in the above-referenced proceeding, of Dynamo Power...

77 FR 64978 - Dynamo Power LLC; Supplemental Notice That Initial Market-Based Rate Filing Includes Request for...

Federal Register 2010, 2011, 2012, 2013, 2014

2012-10-24

... DEPARTMENT OF ENERGY Federal Energy Regulatory Commission [Docket No. ER13-45-000] Dynamo Power LLC; Supplemental Notice That Initial Market-Based Rate Filing Includes Request for Blanket Section 204 Authorization This is a supplemental notice in the above-referenced proceeding of Dynamo Power LLC...

Experimental investigation of dynamo effect in the secondary pumps of the fast breeder reactor Superphenix

NASA Astrophysics Data System (ADS)

Alemany, A.; Marty, Ph.; Plunian, F.; Soto, J.

2000-01-01

The fast breeder reactors (FBR) BN600 (Russia) and Phenix (France) have been the subject of several experimental studies aimed at the observation of dynamo action. Though no dynamo effect has been identified, the possibility was raised for the FBR Superphenix (France) which has an electric power twice that of BN600 and five times larger than Phenix. We present the results of a series of experimental investigations on the secondary pumps of Superphenix. The helical sodium flow inside one pump corresponds to a maximum magnetic Reynolds number (Rm) of 25 in the experimental conditions (low temperature). The magnetic field was recorded in the vicinity of the pumps and no dynamo action has been identified. An estimate of the critical flow rate necessary to reach dynamo action has been found, showing that the pumps are far from producing dynamo action. The magnetic energy spectrum was also recorded and analysed. It is of the form k[minus sign]11/3, suggesting the existence of a large-scale magnetic field. Following Moffatt (1978), this spectrum slope is also justified by a phenomenological approach.

Numerical modeling of the Madison Dynamo Experiment.

NASA Astrophysics Data System (ADS)

Bayliss, R. A.; Wright, J. C.; Forest, C. B.; O'Connell, R.

2002-11-01

Growth, saturation and turbulent evolution of the Madison dynamo experiment is investigated numerically using a 3-D pseudo-spectral simulation of the MHD equations; results of the simulations will be compared to results obtained from the experiment. The code, Dynamo (Fortran90), allows for full evolution of the magnetic and velocity fields. The induction equation governing B and the curl of the momentum equation governing V are separately or simultaneously solved. The code uses a spectral representation via spherical harmonic basis functions of the vector fields in longitude and latitude, and fourth order finite differences in the radial direction. The magnetic field evolution has been benchmarked against the laminar kinematic dynamo predicted by M.L. Dudley and R.W. James (M.L. Dudley and R.W. James, Time-dependent kinematic dynamos with stationary flows, Proc. R. Soc. Lond. A 425, p. 407 (1989)). Power balance in the system has been verified in both mechanically driven and perturbed hydrodynamic, kinematic, and dynamic cases. Evolution of the vacuum magnetic field has been added to facilitate comparison with the experiment. Modeling of the Madison Dynamo eXperiment will be presented.

Whose Evidence Base? The Dynamic Effects of Ownership, Receptivity and Values on Collaborative Evidence-Informed Policy Making

ERIC Educational Resources Information Center

Shine, Kasey Treadwell; Bartley, Brendan

2011-01-01

This paper explores questions of ownership of and receptivity to research-based evidence and, in combination with often competing values, their effect on collaborative evidence-informed policy making (EIPM). We propose that these issues generate a "dynamo" of push-pull factors for policy makers, researchers and research managers. Through…

On MHD rotational transport, instabilities and dynamo action in stellar radiation zones

NASA Astrophysics Data System (ADS)

Mathis, Stéphane; Brun, A.-S.; Zahn, J.-P.

2009-04-01

Magnetic field and their related dynamical effects are thought to be important in stellar radiation zones. For instance, it has been suggested that a dynamo, sustained by a m = 1 MHD instability of toroidal magnetic fields (discovered by Tayler in 1973), could lead to a strong transport of angular momentum and of chemicals in such stable regions. We wish here to recall the different magnetic transport processes present in radiative zone and show how the dynamo can operate by recalling the conditions required to close the dynamo loop (BPol → BTor → BPol). Helped by high-resolution 3D MHD simulations using the ASH code in the solar case, we confirm the existence of the m = 1 instability, study its non-linear saturation, but we do not detect, up to a magnetic Reylnods number of 105, any dynamo action.

Sharp magnetic structures from dynamos with density stratification

NASA Astrophysics Data System (ADS)

Jabbari, Sarah; Brandenburg, Axel; Kleeorin, Nathan; Rogachevskii, Igor

2017-05-01

Recent direct numerical simulations (DNS) of large-scale turbulent dynamos in strongly stratified layers have resulted in surprisingly sharp bipolar structures at the surface. Here, we present new DNS of helically and non-helically forced turbulence with and without rotation and compare with corresponding mean-field simulations (MFS) to show that these structures are a generic outcome of a broader class of dynamos in density-stratified layers. The MFS agree qualitatively with the DNS, but the period of oscillations tends to be longer in the DNS. In both DNS and MFS, the sharp structures are produced by converging flows at the surface and might be driven in non-linear stage of evolution by the Lorentz force associated with the large-scale dynamo-driven magnetic field if the dynamo number is at least 2.5 times supercritical.

The dynamo of the diurnal tide and its effect on the thermospheric circulation

NASA Technical Reports Server (NTRS)

Mayr, H. G.; Harris, I.; Herrero, F. A.

1990-01-01

A theoretical multiconstituent model (including O, N2, and O2) which describes the interactions between neutral winds, dynamo electric fields, and ion drifts is used to interpret observations that revealed a dominance of the fundamental diurnal tide in the upper thermosphere and at equatorial latitudes, and its effect on the thermospheric circulation. The model is shown to reproduce reasonably well the magnitudes of the neutral winds, ion drift velocities, and the ratio between the two. A solution for the neutral winds in which the dynamo electric field is forced to zero shows that the dynamo-induced ion drift is very important in accelerating the neutral atmosphere at higher altitudes. The dynamo interaction primarily affects the curl component of the field; its effect on the temperature and density perturbations is small.

Revisiting the use of hyperdiffusivities in numerical dynamo models

NASA Astrophysics Data System (ADS)

Fournier, A.; Aubert, J.

2012-04-01

The groundbreaking numerical dynamo models of Glatzmaier & Roberts (1995) and Kuang & Bloxham (1997) received some criticism due to their use of hyperdiffusivities, whereby small scale processes artificially experience much stronger dissipation than large scale processes. This stronger dissipation they chose was anisotropic, in that it was only effective in the horizontal direction, and parameterized in spectral space using the following generic formula for any diffusive parameter ν ν(l) = ν0 ifl ≤ l0, ν(l) = ν0[1 + a(l- l0)n] ifl > l0, in which l is the spherical harmonic degree, ν0 is a reference value, l0 is the degree above which hyperdiffusivities start operating, and a and n are real numbers. Following the same choice as the studies mentioned above (which had most notably l0 = 0), Grote & Busse (2000) showed in a fully nonlinear context that the usage of hyperdiffusivities could lead to substantially different dynamics and magnetic field generation mechanisms. Without questioning the physical relevance of this parameterization of subgrid scale processes, we wish here to revisit the use of hyperdiffusivities (as defined mathematically above), on the account of the observation that today's models are run with a truncation at much larger spherical harmonic degree than early models. Consequently, they do not require hyperdiffusivities to kick in at the largest scales (l0 can be set to several tens). An exploration of those regions of parameter space less accessible to numerical models could therefore benefit from their use, provided they do not alter noticeably the largest scales of the dynamo (which are the ones expressing themselves in the record of the geomagnetic secular variation). We compare the statistics of a direct numerical simulation with the statistics of several hyperdiffusive simulations. In the prospect of exploring the parameter space and constructing statistics for their subsequent use for geomagnetic data assimilation practice, we conclude that a sensible use of hyperdiffusivities can lead to a much wanted decrease in computational cost, while not altering the nature of the solution.

Flux-transport Dynamos Driven by a Tachocline α -effect; a Solution to Magnetic Parity Selection in the Sun

NASA Astrophysics Data System (ADS)

Dikpati, M.; Gilman, P. A.

2001-05-01

We propose here an α Ω flux-transport dynamo driven by a tachocline α -effect, produced by the global hydrodynamic instability of tachocline differential rotation as calculated using a shallow-water model (Dikpati & Gilman, 2001, ApJ, Mar.20 issue). Growing, unstable shallow-water modes propagating longitudinally in the tachocline create alternate vortices which correlate with upward/downward radial motion of top boundary, associated with convergence/divergence of the disturbance flow to produce a longitude-averaged net kinetic helicity, and hence an α -effect. We show that a flux-transport dynamo driven by a tachocline α -effect is equally successful as a Babcock-Leighton flux-transport dynamo (Dikpati & Charbonneau 1999, ApJ, 518, 508) in reproducing many large-scale solar cycle features, including the most difficult feature of phase relationship between the subsurface toroidal field and surface radial field. In view of the success of flux-transport dynamos, whether the α -effect is at the surface or in the tachocline, we argue that the solar dynamo should be considered to involve three basic processes, rather than two (α -effect and Ω -effect only). The third important process is the advective transport of flux by meridional circulation. In reality, both α -effects (Babcock-Leighton type and tachocline α -effect) are likely to exist, but it is hard to estimate their relative magnitudes. We show, by extending the simulation in a full spherical shell model that a flux-transport dynamo driven by a tachocline α -effect selects toroidal field that is antisymmetric about the equator, while a Babcock-Leighton flux-transport dynamo selects symmetric toroidal field. Since our present Sun selects antisymmetric toroidal fields, we argue that the flux-transport solar dynamo is primarily driven by a tachocline α -effect. Acknowledgements: This work is supported by NASA grants W-19752 and S-10145-X. National Center for Atmospheric Research is sponsored by National Science Foundation.

Statistical simulation of the magnetorotational dynamo.

PubMed

Squire, J; Bhattacharjee, A

2015-02-27

Turbulence and dynamo induced by the magnetorotational instability (MRI) are analyzed using quasilinear statistical simulation methods. It is found that homogenous turbulence is unstable to a large-scale dynamo instability, which saturates to an inhomogenous equilibrium with a strong dependence on the magnetic Prandtl number (Pm). Despite its enormously reduced nonlinearity, the dependence of the angular momentum transport on Pm in the quasilinear model is qualitatively similar to that of nonlinear MRI turbulence. This demonstrates the importance of the large-scale dynamo and suggests how dramatically simplified models may be used to gain insight into the astrophysically relevant regimes of very low or high Pm.

Role of a continuous MHD dynamo in the formation of 3D equilibria in fusion plasmas

NASA Astrophysics Data System (ADS)

Piovesan, P.; Bonfiglio, D.; Cianciosa, M.; Luce, T. C.; Taylor, N. Z.; Terranova, D.; Turco, F.; Wilcox, R. S.; Wingen, A.; Cappello, S.; Chrystal, C.; Escande, D. F.; Holcomb, C. T.; Marrelli, L.; Paz-Soldan, C.; Piron, L.; Predebon, I.; Zaniol, B.; DIII-D, The; RFX-Mod Teams

2017-07-01

Stationary 3D equilibria can form in fusion plasmas via saturation of magnetohydrodynamic (MHD) instabilities or stimulated by external 3D fields. In these cases the current profile is anomalously broad due to magnetic flux pumping produced by the MHD modes. Flux pumping plays an important role in hybrid tokamak plasmas, maintaining the minimum safety factor above unity and thus removing sawteeth. It also enables steady-state hybrid operation, by redistributing non-inductive current driven near the center by electron cyclotron waves. A validated flux pumping model is not yet available, but it would be necessary to extrapolate hybrid operation to future devices. In this work flux pumping physics is investigated for helical core equilibria stimulated by external 3D fields in DIII-D hybrid plasmas. We show that flux pumping can be produced in a continuous way by an MHD dynamo emf. The same effect maintains helical equilibria in reversed-field pinch (RFP) plasmas. The effective MHD dynamo loop voltage is calculated for experimental 3D equilibrium reconstructions, by balancing Ohm’s law over helical flux surfaces, and is consistent with the expected current redistribution. Similar results are also obtained with more sophisticated nonlinear MHD simulations. The same modelling approach is applied to helical RFP states forming spontaneously in RFX-mod as the plasma current is raised above 0.8-1 MA. This comparison allows to identify the underlying physics common to tokamak and RFP: a helical core displacement modulates parallel current density along flux tubes, which requires a helical electrostatic potential to build up, giving rise to a helical MHD dynamo flow.

Role of a continuous MHD dynamo in the formation of 3D equilibria in fusion plasmas

DOE Office of Scientific and Technical Information (OSTI.GOV)

Piovesan, P.; Bonfiglio, D.; Cianciosa, M.

Stationary 3D equilibria can form in fusion plasmas via saturation of magnetohydrodynamic (MHD) instabilities or stimulated by external 3D fields. In these cases the current profile is anomalously broad due to magnetic flux pumping produced by the MHD modes. Flux pumping plays an important role in hybrid tokamak plasmas, maintaining the minimum safety factor above unity and thus removing sawteeth. It also enables steady-state hybrid operation, by redistributing non-inductive current driven near the center by electron cyclotron waves. A validated flux pumping model is not yet available, but it would be necessary to extrapolate hybrid operation to future devices. Inmore » this work flux pumping physics is investigated for helical core equilibria stimulated by external 3D fields in DIII-D hybrid plasmas. We show that flux pumping can be produced in a continuous way by an MHD dynamo emf. The same effect maintains helical equilibria in reversed-field pinch (RFP) plasmas. The effective MHD dynamo loop voltage is calculated for experimental 3D equilibrium reconstructions, by balancing Ohm’s law over helical flux surfaces, and is consistent with the expected current redistribution. Similar results are also obtained with more sophisticated nonlinear MHD simulations. The same modelling approach is applied to helical RFP states forming spontaneously in RFX-mod as the plasma current is raised above 0.8–1 MA. This comparison allows to identify the underlying physics common to tokamak and RFP: a helical core displacement modulates parallel current density along flux tubes, which requires a helical electrostatic potential to build up, giving rise to a helical MHD dynamo flow.« less

The Role of DYNAMO in Situ Observations in Improving NASA Ceres-like Daily Surface and Atmospheric Radiative Flux Estimates

NASA Technical Reports Server (NTRS)

Wang, Hailan; Su, Wenying; Loeb, Norman G.; Achuthavarier, Deepthi; Schubert, Siegfried D.

2017-01-01

The daily surface and atmospheric radiative fluxes from NASA Clouds and the Earths RadiantEnergy System (CERES) Synoptic 1 degree (SYN1deg) Ed3A are among the most widely used data to studycloud-radiative feedback. The CERES SYN1deg data are based on Fu-Liou radiative transfer computations thatuse specific humidity (Q) and air temperature (T) from NASA Global Modeling and Assimilation Office (GMAO)reanalyses as inputs and are therefore subject to the quality of those fields. This study uses in situ Q and Tobservations collected during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign toaugment the input stream used in the NASA GMAO reanalysis and assess the impact on the CERES dailysurface and atmospheric longwave estimates. The results show that the assimilation of DYNAMOobservations considerably improves the vertical profiles of analyzed Q and T over and near DYNAMO stationsby moistening and warming the lower troposphere and upper troposphere and drying and cooling themid-upper troposphere. As a result of these changes in Q and T, the computed CERES daily surface downwardlongwave flux increases by about 5 W m(exp -2), due mainly to the warming and moistening in the lowertroposphere; the computed daily top-of-atmosphere (TOA) outgoing longwave radiation increases by2-3 W m(exp -2) during dry periods only. Correspondingly, the estimated local atmospheric longwave radiativecooling enhances by about 5 W m(exp -2) (7-8 W m(exp -2)) during wet (dry) periods. These changes reduce the bias inthe CERES SYN1deg-like daily longwave estimates at both the TOA and surface and represent animprovement over the DYNAMO region.

Role of a continuous MHD dynamo in the formation of 3D equilibria in fusion plasmas

DOE PAGES

Piovesan, P.; Bonfiglio, D.; Cianciosa, M.; ...

2017-04-28

Stationary 3D equilibria can form in fusion plasmas via saturation of magnetohydrodynamic (MHD) instabilities or stimulated by external 3D fields. In these cases the current profile is anomalously broad due to magnetic flux pumping produced by the MHD modes. Flux pumping plays an important role in hybrid tokamak plasmas, maintaining the minimum safety factor above unity and thus removing sawteeth. It also enables steady-state hybrid operation, by redistributing non-inductive current driven near the center by electron cyclotron waves. A validated flux pumping model is not yet available, but it would be necessary to extrapolate hybrid operation to future devices. Inmore » this work flux pumping physics is investigated for helical core equilibria stimulated by external 3D fields in DIII-D hybrid plasmas. We show that flux pumping can be produced in a continuous way by an MHD dynamo emf. The same effect maintains helical equilibria in reversed-field pinch (RFP) plasmas. The effective MHD dynamo loop voltage is calculated for experimental 3D equilibrium reconstructions, by balancing Ohm’s law over helical flux surfaces, and is consistent with the expected current redistribution. Similar results are also obtained with more sophisticated nonlinear MHD simulations. The same modelling approach is applied to helical RFP states forming spontaneously in RFX-mod as the plasma current is raised above 0.8–1 MA. This comparison allows to identify the underlying physics common to tokamak and RFP: a helical core displacement modulates parallel current density along flux tubes, which requires a helical electrostatic potential to build up, giving rise to a helical MHD dynamo flow.« less

Thermosphere-Ionosphere-Mesosphere Modeling Using the TIE-GCM, TIME-GCM, and WACCM That Will Lead to the Development of a Seamless Model of the Whole Atmosphere

DTIC Science & Technology

2006-09-30

disturbances from the lower atmosphere and ocean affect the upper atmosphere and how this variability interacts with the variability generated by solar and...represents “ general circulation model.” Both models include self-consistent ionospheric electrodynamics, that is, a calculation of the electric fields...and currents generated by the ionospheric dynamo, and consideration of their effects on the neutral dynamics. The TIE-GCM is used for studies that

The connection between the peaks in velocity dispersion and star-forming clumps of turbulent galaxies

NASA Astrophysics Data System (ADS)

Oliva-Altamirano, P.; Fisher, D. B.; Glazebrook, K.; Wisnioski, E.; Bekiaris, G.; Bassett, R.; Obreschkow, D.; Abraham, R.

2018-02-01

We present Keck/OSIRIS adaptive optics observations with 150-400 pc spatial sampling of 7 turbulent, clumpy disc galaxies from the DYNAMO sample ($0.07

Optimal Length Scale for a Turbulent Dynamo.

PubMed

Sadek, Mira; Alexakis, Alexandros; Fauve, Stephan

2016-02-19

We demonstrate that there is an optimal forcing length scale for low Prandtl number dynamo flows that can significantly reduce the required energy injection rate. The investigation is based on simulations of the induction equation in a periodic box of size 2πL. The flows considered are the laminar and turbulent ABC flows forced at different forcing wave numbers k_{f}, where the turbulent case is simulated using a subgrid turbulence model. At the smallest allowed forcing wave number k_{f}=k_{min}=1/L the laminar critical magnetic Reynolds number Rm_{c}^{lam} is more than an order of magnitude smaller than the turbulent critical magnetic Reynolds number Rm_{c}^{turb} due to the hindering effect of turbulent fluctuations. We show that this hindering effect is almost suppressed when the forcing wave number k_{f} is increased above an optimum wave number k_{f}L≃4 for which Rm_{c}^{turb} is minimum. At this optimal wave number, Rm_{c}^{turb} is smaller by more than a factor of 10 than the case forced in k_{f}=1. This leads to a reduction of the energy injection rate by 3 orders of magnitude when compared to the case where the system is forced at the largest scales and thus provides a new strategy for the design of a fully turbulent experimental dynamo.

Efficiency Measurement Using a Motor-Dynamo Module

ERIC Educational Resources Information Center

Ng, Pun-hon; Wong, Siu-ling; Mak, Se-yuen

2009-01-01

In this article, we describe a simple method which can be used to measure the efficiency of a low power dc motor, a motor-converted dynamo and a coupled motor-dynamo module as a function of the speed of rotation. The result can also be used to verify Faraday's law of electromagnetic induction. (Contains 1 table and 8 figures.)

Stochastic Modelling, Analysis, and Simulations of the Solar Cycle Dynamic Process

NASA Astrophysics Data System (ADS)

Turner, Douglas C.; Ladde, Gangaram S.

2018-03-01

Analytical solutions, discretization schemes and simulation results are presented for the time delay deterministic differential equation model of the solar dynamo presented by Wilmot-Smith et al. In addition, this model is extended under stochastic Gaussian white noise parametric fluctuations. The introduction of stochastic fluctuations incorporates variables affecting the dynamo process in the solar interior, estimation error of parameters, and uncertainty of the α-effect mechanism. Simulation results are presented and analyzed to exhibit the effects of stochastic parametric volatility-dependent perturbations. The results generalize and extend the work of Hazra et al. In fact, some of these results exhibit the oscillatory dynamic behavior generated by the stochastic parametric additative perturbations in the absence of time delay. In addition, the simulation results of the modified stochastic models influence the change in behavior of the very recently developed stochastic model of Hazra et al.

Tsunami: ocean dynamo generator.

PubMed

Sugioka, Hiroko; Hamano, Yozo; Baba, Kiyoshi; Kasaya, Takafumi; Tada, Noriko; Suetsugu, Daisuke

2014-01-08

Secondary magnetic fields are induced by the flow of electrically conducting seawater through the Earth's primary magnetic field ('ocean dynamo effect'), and hence it has long been speculated that tsunami flows should produce measurable magnetic field perturbations, although the signal-to-noise ratio would be small because of the influence of the solar magnetic fields. Here, we report on the detection of deep-seafloor electromagnetic perturbations of 10-micron-order induced by a tsunami, which propagated through a seafloor electromagnetometer array network. The observed data extracted tsunami characteristics, including the direction and velocity of propagation as well as sea-level change, first to verify the induction theory. Presently, offshore observation systems for the early forecasting of tsunami are based on the sea-level measurement by seafloor pressure gauges. In terms of tsunami forecasting accuracy, the integration of vectored electromagnetic measurements into existing scalar observation systems would represent a substantial improvement in the performance of tsunami early-warning systems.

A basal magma ocean dynamo to explain the early lunar magnetic field

NASA Astrophysics Data System (ADS)

Scheinberg, Aaron L.; Soderlund, Krista M.; Elkins-Tanton, Linda T.

2018-06-01

The source of the ancient lunar magnetic field is an unsolved problem in the Moon's evolution. Theoretical work invoking a core dynamo has been unable to explain the magnitude of the observed field, falling instead one to two orders of magnitude below it. Since surface magnetic field strength is highly sensitive to the depth and size of the dynamo region, we instead hypothesize that the early lunar dynamo was driven by convection in a basal magma ocean formed from the final stages of an early lunar magma ocean; this material is expected to be dense, radioactive, and metalliferous. Here we use numerical convection models to predict the longevity and heat flow of such a basal magma ocean and use scaling laws to estimate the resulting magnetic field strength. We show that, if sufficiently electrically conducting, a magma ocean could have produced an early dynamo with surface fields consistent with the paleomagnetic observations.

The dynamo dilemma

NASA Technical Reports Server (NTRS)

Parker, E. N.

1987-01-01

The recent determination that the angular velocity Omega of the sun declines downward through the convective zone raises serious questions about the nature of the solar dynamo. The principal qualitative features of the sun are the azimuthal fields that migrate toward the equator in association with an oscillating poloidal field which reverses at about the time of maximum appearance of bipolar magnetic regions. If Omega decreases downward, or is negligible, the horizontal gradient in Omega produces a dynamo with some of these essential characteristics. There is reason to think that the dynamo is confined to the lower half of the convective zone, where alpha has the opposite sign from the usual (alpha of greater than 0 in the northern hemisphere) producing equatorward migration but reversing the sign of the associated poloidal field. Meridional circulation may play an essential role in shaping the dynamo. At the present time it is essential to measure Omega accurately and determine the nature of the meridional circulation.

Impact of Convection on Surface Fluxes Observed During LASP/DYNAMO 2011

DTIC Science & Technology

2014-12-01

20  Figure 8.  FFM maneuver used in the LASP/DYNAMO experiment (from Wang et al. 2013...Atmosphere Response Experiment DYNAMO Dynamics of Madden-Julian Oscillation EM electro-magnetic EO electro-optical FFM flight-level flux mapping FVS...level flux mapping ( FFM ) modules. Convection modules consisted of dropsonde cloud survey or radar convective element maneuver. Dropsonde modules

A high magnetic Reynolds number dynamo

NASA Technical Reports Server (NTRS)

Perkins, F. W.; Zweibel, E. G.

1987-01-01

A boundary-layer solution to a high magnetic Reynolds number R periodic dynamo model shows that: (1) flux expulsion forces the magnetic field into flux sheets; (2) the principal contribution to the alpha effect arises from regions of flow stagnation along a flux sheet; and (3) the alpha effect scales as R exp-1/2. Arguments for these effects persisting in turbulent dynamos are given.

Chaotic flows and fast magnetic dynamos

NASA Technical Reports Server (NTRS)

Finn, John M.; Ott, Edward

1988-01-01

The kinematic dynamo problem is considered in the R(m) approaching infinity limit. It is shown that the magnetic field tends to concentrate on a zero volume fractal set; moreover, it displays arbitrarily fine-scaled oscillations between parallel and antiparallel directions. Consideration is given to the relationship between the dynamo growth rate and quantitative measures of chaos, such as the Liapunov element and topological entropy.

Experimental observation of spatially localized dynamo magnetic fields.

PubMed

Gallet, B; Aumaître, S; Boisson, J; Daviaud, F; Dubrulle, B; Bonnefoy, N; Bourgoin, M; Odier, Ph; Pinton, J-F; Plihon, N; Verhille, G; Fauve, S; Pétrélis, F

2012-04-06

We report the first experimental observation of a spatially localized dynamo magnetic field, a common feature of astrophysical dynamos and convective dynamo simulations. When the two propellers of the von Kármán sodium experiment are driven at frequencies that differ by 15%, the mean magnetic field's energy measured close to the slower disk is nearly 10 times larger than the one close to the faster one. This strong localization of the magnetic field when a symmetry of the forcing is broken is in good agreement with a prediction based on the interaction between a dipolar and a quadrupolar magnetic mode. © 2012 American Physical Society

Role of soft-iron impellers on the mode selection in the von kármán-sodium dynamo experiment.

PubMed

Giesecke, André; Stefani, Frank; Gerbeth, Gunter

2010-01-29

A crucial point for the understanding of the von Kármán-sodium (VKS) dynamo experiment is the influence of soft-iron impellers. We present numerical simulations of a VKS-like dynamo with a localized permeability distribution that resembles the shape of the flow driving impellers. It is shown that the presence of soft-iron material essentially determines the dynamo process in the VKS experiment. An axisymmetric magnetic field mode can be explained by the combined action of the soft-iron disk and a rather small alpha effect parametrizing the induction effects of unresolved small scale flow fluctuations.

Exploring the North-South asymmetry in a Babcock-Leighton dynamo

NASA Astrophysics Data System (ADS)

Belucz , B.; Forgács-Dajka, E.; Dikpati, M.

2013-11-01

We present here a Babcock-Leighton, kinematic flux-transport solar dynamo model, based on an earlier model (Dikpati & Charbonneau 1999), operating in a full spherical shell of the convection zone, to investigate the properties of North-South (N-S) asymmetry. We develop a C language code for this model in order to examine the N-S asymmetry. The main components of the model are a solar-like internal differential rotation profile, a depth-dependent diffusivity, and a Babcock-Leighton type poloidal source. Our purpose here is to study what kind of North-South asymmetry is produced in solar cycle patterns when the Babcock-Leighton poloidal source is asymmetric between North and South. We present our solutions in the form of model butterfly diagrams in which we plot the tachocline toroidal field and surface radial field, and compare them with observations. We find that the dynamos in the northern and southern hemispheres operate nearly independently - if the Babcock-Leighton source is much smaller in the southern hemisphere with respect to that in the northern hemisphere, the dynamo in the southern hemisphere gets weaker and weaker, but the dynamo in the northern hemisphere runs without being affected by the dynamo in the southern hemisphere.

Linear and nonlinear dynamo properties of time-dependent ABC flows

NASA Astrophysics Data System (ADS)

Brummell, N. H.; Cattaneo, F.; Tobias, S. M.

2001-04-01

The linear and nonlinear dynamo properties of a class of periodically forced flows is considered. The forcing functions are chosen to drive, in the absence of magnetic effects (kinematic regime), a time-dependent version of the ABC flow with A= B= C=1. The time-dependence consists of a harmonic displacement of the origin along the line x= y= z=1 with amplitude ɛ and frequency Ω. The finite-time Lyapunov exponents are computed for several values of ɛ and Ω. It is found that for values of these parameters near unity chaotic streamlines occupy most of the volume. In this parameter range, and for moderate kinetic and magnetic Reynolds numbers, the basic flow is both hydrodynamically and hydromagnetically unstable. However, the dynamo instability has a higher growth rate than the hydrodynamic one, so that the nonlinear regime can be reached with negligible departures from the basic ABC flow. In the nonlinear regime, two distinct classes of behaviour are observed. In one, the exponential growth of the magnetic field saturates and the dynamo settles to a stationary state whereby the magnetic energy is maintained indefinitely. In the other the velocity field evolves to a nondynamo state and the magnetic field, following an initial amplification, decays to zero. The transition from the dynamo to the nondynamo state can be mediated by the hydrodynamic instability or by magnetic perturbations. The properties of the ensuing nonlinear dynamo states are investigated for different parameter values. The implications for a general theory of nonlinear dynamos are discussed.

Equatorial ionospheric response to the 2015 St. Patrick's Day magnetic storm

NASA Astrophysics Data System (ADS)

Huang, C.; Wilson, G. R.; Hairston, M. R.; Zhang, Y.; Wang, W.; Liu, J.

2016-12-01

The geomagnetic storm on 17 March 2015 was the strongest storm during solar cycle 24 and caused significant disturbances in the global ionosphere. We present measurements of the Defense Meteorological Satellite Program satellites and identify the dynamic response of the equatorial ionosphere to the storm. Large penetration and disturbance dynamo electric fields are detected in both the dusk and the dawn sectors, and the characteristics of the electric fields are dramatically different in the two local time sectors. Penetration electric field is strong in the evening sector, but disturbance dynamo electric field is dominant in the dawn sector. The dynamo process is first observed in the post-midnight sector 4 hours after the beginning of the storm main phase and lasts for 31 hours, covering the major part of the storm main phase and the initial 20 hours of the recovery phase. The dynamo vertical ion drift is upward (up to 200 m/s) in the post-midnight sector and downward (up to 80 m/s) in the early morning sector. The dynamo zonal ion drift is westward at these locations and reaches 100 m/s. The dynamo process causes large enhancements of the oxygen ion concentration, and the variations of the oxygen ion concentration are well correlated with the vertical ion drift. The observations suggest that disturbance dynamo becomes dominant in the post-midnight equatorial ionosphere even during the storm main phase when disturbance neutral winds arrive there. The results provide new insight into storm-time equatorial ionospheric dynamics.

Constructing a Merged Cloud-Precipitation Radar Dataset for Tropical Convective Clouds during the DYNAMO/AMIE Experiment at Addu Atoll

DOE Office of Scientific and Technical Information (OSTI.GOV)

Feng, Zhe; McFarlane, Sally A.; Schumacher, Courtney

2014-05-16

To improve understanding of the convective processes key to the Madden-Julian-Oscillation (MJO) initiation, the Dynamics of the MJO (DYNAMO) and Atmospheric Radiation Measurement MJO Investigation Experiment (AMIE) collected four months of observations from three radars, the S-band Polarization Radar (S-Pol), the C-band Shared Mobile Atmospheric Research & Teaching Radar (SMART-R), and Ka-band Zenith Radar (KAZR) on Addu Atoll in the tropical Indian Ocean. This study compares the measurements from the S-Pol and SMART-R to those from the more sensitive KAZR in order to characterize the hydrometeor detection capabilities of the two scanning precipitation radars. Frequency comparisons for precipitating convective cloudsmore » and non-precipitating high clouds agree much better than non-precipitating low clouds for both scanning radars due to issues in ground clutter. On average, SMART-R underestimates convective and high cloud tops by 0.3 to 1.1 km, while S-Pol underestimates cloud tops by less than 0.4 km for these cloud types. S-Pol shows excellent dynamic range in detecting various types of clouds and therefore its data are well suited for characterizing the evolution of the 3D cloud structures, complementing the profiling KAZR measurements. For detecting non-precipitating low clouds and thin cirrus clouds, KAZR remains the most reliable instrument. However, KAZR is attenuated in heavy precipitation and underestimates cloud top height due to rainfall attenuation 4.3% of the time during DYNAMO/AMIE. An empirical method to correct the KAZR cloud top heights is described, and a merged radar dataset is produced to provide improved cloud boundary estimates, microphysics and radiative heating retrievals.« less

Dynamo action and magnetic buoyancy in convection simulations with vertical shear

NASA Astrophysics Data System (ADS)

Guerrero, G.; Käpylä, P. J.

2011-09-01

Context. A hypothesis for sunspot formation is the buoyant emergence of magnetic flux tubes created by the strong radial shear at the tachocline. In this scenario, the magnetic field has to exceed a threshold value before it becomes buoyant and emerges through the whole convection zone. Aims: We follow the evolution of a random seed magnetic field with the aim of study under what conditions it is possible to excite the dynamo instability and whether the dynamo generated magnetic field becomes buoyantly unstable and emerges to the surface as expected in the flux-tube context. Methods: We perform numerical simulations of compressible turbulent convection that include a vertical shear layer. Like the solar tachocline, the shear is located at the interface between convective and stable layers. Results: We find that shear and convection are able to amplify the initial magnetic field and form large-scale elongated magnetic structures. The magnetic field strength depends on several parameters such as the shear amplitude, the thickness and location of the shear layer, and the magnetic Reynolds number (Rm). Models with deeper and thicker tachoclines allow longer storage and are more favorable for generating a mean magnetic field. Models with higher Rm grow faster but saturate at slightly lower levels. Whenever the toroidal magnetic field reaches amplitudes greater a threshold value which is close to the equipartition value, it becomes buoyant and rises into the convection zone where it expands and forms mushroom shape structures. Some events of emergence, i.e. those with the largest amplitudes of the initial field, are able to reach the very uppermost layers of the domain. These episodes are able to modify the convective pattern forming either broader convection cells or convective eddies elongated in the direction of the field. However, in none of these events the field preserves its initial structure. The back-reaction of the magnetic field on the fluid is also observed in lower values of the turbulent velocity and in perturbations of approximately three per cent on the shear profile. Conclusions: The results indicate that buoyancy is a common phenomena when the magnetic field is amplified through dynamo action in a narrow layer. It is, however, very hard for the field to rise up to the surface without losing its initial coherence.

The generation and amplification of intergalactic magnetic fields in analogue laboratory experiments with high power lasers

NASA Astrophysics Data System (ADS)

Gregori, G.; Reville, B.; Miniati, F.

2015-11-01

The advent of high-power laser facilities has, in the past two decades, opened a new field of research where astrophysical environments can be scaled down to laboratory dimensions, while preserving the essential physics. This is due to the invariance of the equations of magneto-hydrodynamics to a class of similarity transformations. Here we review the relevant scaling relations and their application in laboratory astrophysics experiments with a focus on the generation and amplification of magnetic fields in cosmic environment. The standard model for the origin of magnetic fields is a multi stage process whereby a vanishing magnetic seed is first generated by a rotational electric field and is then amplified by turbulent dynamo action to the characteristic values observed in astronomical bodies. We thus discuss the relevant seed generation mechanisms in cosmic environment including resistive mechanism, collision-less and fluid instabilities, as well as novel laboratory experiments using high power laser systems aimed at investigating the amplification of magnetic energy by magneto-hydrodynamic (MHD) turbulence. Future directions, including efforts to model in the laboratory the process of diffusive shock acceleration are also discussed, with an emphasis on the potential of laboratory experiments to further our understanding of plasma physics on cosmic scales.

Topographic-driven instabilities in terrestrial bodies

NASA Astrophysics Data System (ADS)

Vantieghem, S.; Cebron, D.; Herreman, W.; Lacaze, L.

2013-12-01

Models of internal planetary fluid layers (core flows, subsurface oceans) commonly assume that these fluid envelopes have a spherical shape. This approximation however entails a serious restriction from the fluid dynamics point of view. Indeed, in the presence of mechanical forcings (precession, libration, nutation or tides) due to gravitational interaction with orbiting partners, boundary topography (e.g. of the core-mantle boundary) may excite flow instabilities and space-filling turbulence. These phenomena may affect heat transport and dissipation at the main order. Here, we focus on instabilities driven by longitudinal libration. Using a suite of theoretical tools and numerical simulations, we are able to discern a parameter range for which instability may be excited. We thereby consider deformations of different azimuthal order. This study gives the first numerical evidence of the tripolar instability. Furthermore, we explore the non-linear regime and investigate the amplitude as well as the dissipation of the saturated instability. Indeed, these two quantities control the torques on the solid layers and the thermal transport. Furthermore, based on this results, we address the issue of magnetic field generation associated with these flows (by induction or by dynamo process). This instability mechanism applies to both synchronized as non-synchronized bodies. As such, our results show that a tripolar instability might be present in various terrestrial bodies (Early Moon, Gallilean moons, asteroids, etc.), where it could participate in dynamo action. Simulation of a libration-driven tripolar instability in a deformed spherical fluid layer: snapshot of the velocity magnitude, where a complex 3D flow pattern is established.

A new simple dynamo model for solar activity cycle

NASA Astrophysics Data System (ADS)

Yokoi, Nobumitsu; Schmitt, Dieter

2015-04-01

The solar magnetic activity cycle has been investigated in an elaborated manner with several types of dynamo models [1]. In most of the current mean-field approaches, the inhomogeneity of the large-scale flow is treated as an essential ingredient in the mean magnetic field equation whereas it is completely neglected in the turbulence equation. In this work, a new simple model for the solar activity cycle is proposed. The present model differs from the previous ones mainly in two points. First, in addition to the helicity coefficient α, we consider a term related to the cross helicity, which represents the effect of the inhomogeneous mean flow, in the turbulent electromotive force [2, 3]. Second, this transport coefficient (γ) is not treated as an adjustable parameter, but the evolution equation for γ is simultaneously solved. The basic scenario for the solar activity cycle in this approach is as follows: The toroidal field is induced by the toroidal rotation in mediation by the turbulent cross helicity. Then due to the α or helicity effect, the poloidal field is generated from the toroidal field. The poloidal field induced by the α effect produces a turbulent cross helicity whose sign is opposite to the original one (negative cross-helicity production). The cross helicity with this opposite sign induces a reversed toroidal field. Results of the eigenvalue analysis of the model equations are shown, which confirm the above scenario. References [1] Charbonneau, Living Rev. Solar Phys. 7, 3 (2010). [2] Yoshizawa, A. Phys. Fluids B 2, 1589 (1990). [3] Yokoi, N. Geophys. Astrophys. Fluid Dyn. 107, 114 (2013).

Effects of density gradient caused by multi-pulsing CHI on two-fluid flowing equilibria of spherical torus plasmas

NASA Astrophysics Data System (ADS)

Kanki, T.; Nagata, M.

2014-10-01

Two-fluid dynamo relaxation is examined to understand sustainment mechanism of spherical torus (ST) plasmas by multi-pulsing CHI (M-CHI) in the HIST device. The steeper density gradient between the central open flux column (OFC) and closed flux regions by applying the second CHI pulse is observed to cause not only the E × B drift but also the ion diamagnetic drift, leading the two-fluid dynamo. The purpose of this study is to investigate the effects of the steep change in the density gradient on the ST equilibria by using the two-fluid equilibrium calculations. The toroidal magnetic field becomes from a diamagnetic to a paramagnetic profile in the closed flux region while it remains a diamagnetic profile in the OFC region. The toroidal ion flow velocity is increased from negative to positive values in the closed flux region. Here, the negative ion flow velocity is the opposite direction to the toroidal current. The poloidal ion flow velocity between the OFC and closed flux regions is increased, because the ion diamagnetic drift velocity is changed in the same direction as the E × B drift velocity through the steeper ion pressure gradient. As a result, the strong shear flow and the paramagnetic toroidal field are generated in the closed flux region. Here, the ion flow velocity is the same direction as the poloidal current. The radial electric field shear between the OFC and closed flux regions is enhanced due to the strong dependence on the magnetic force through the interaction of toroidal ion flow velocity and axial magnetic field. The two-fluid effect is significant there due to the ion diamagnetic effect.

Coupled thermo-chemical boundary conditions in double-diffusive geodynamo models at arbitrary Lewis numbers.

NASA Astrophysics Data System (ADS)

Bouffard, M.

2016-12-01

Convection in the Earth's outer core is driven by the combination of two buoyancy sources: a thermal source directly related to the Earth's secular cooling, the release of latent heat and possibly the heat generated by radioactive decay, and a compositional source due to the crystallization of the growing inner core which releases light elements into the liquid outer core. The dynamics of fusion/crystallization being dependent on the heat flux distribution, the thermochemical boundary conditions are coupled at the inner core boundary which may affect the dynamo in various ways, particularly if heterogeneous conditions are imposed at one boundary. In addition, the thermal and compositional molecular diffusivities differ by three orders of magnitude. This can produce significant differences in the convective dynamics compared to pure thermal or compositional convection due to the potential occurence of double-diffusive phenomena. Traditionally, temperature and composition have been combined into one single variable called codensity under the assumption that turbulence mixes all physical properties at an "eddy-diffusion" rate. This description does not allow for a proper treatment of the thermochemical coupling and is certainly incorrect within stratified layers in which double-diffusive phenomena can be expected. For a more general and rigorous approach, two distinct transport equations should therefore be solved for temperature and composition. However, the weak compositional diffusivity is technically difficult to handle in current geodynamo codes and requires the use of a semi-Lagrangian description to minimize numerical diffusion. We implemented a "particle-in-cell" method into a geodynamo code to properly describe the compositional field. The code is suitable for High Parallel Computing architectures and was successfully tested on two benchmarks. Following the work by Aubert et al. (2008) we use this new tool to perform dynamo simulations including thermochemical coupling at the inner core boundary as well as exploration of the infinite Lewis number limit to study the effect of a heterogeneous core mantle boundary heat flow on the inner core growth.

Magnetohydrodynamic Turbulence and the Geodynamo

NASA Technical Reports Server (NTRS)

Shebalin, John V.

2014-01-01

The ARES Directorate at JSC has researched the physical processes that create planetary magnetic fields through dynamo action since 2007. The "dynamo problem" has existed since 1600, when William Gilbert, physician to Queen Elizabeth I, recognized that the Earth was a giant magnet. In 1919, Joseph Larmor proposed that solar (and by implication, planetary) magnetism was due to magnetohydrodynamics (MHD), but full acceptance did not occur until Glatzmaier and Roberts solved the MHD equations numerically and simulated a geomagnetic reversal in 1995. JSC research produced a unique theoretical model in 2012 that provided a novel explanation of these physical observations and computational results as an essential manifestation of broken ergodicity in MHD turbulence. Research is ongoing, and future work is aimed at understanding quantitative details of magnetic dipole alignment in the Earth as well as in Mercury, Jupiter and its moon Ganymede, Saturn, Uranus, Neptune, and the Sun and other stars.

A Single Mode Study of a Quasi-Geostrophic Convection-Driven Dynamo Model

NASA Astrophysics Data System (ADS)

Plumley, M.; Calkins, M. A.; Julien, K. A.; Tobias, S.

2017-12-01

Planetary magnetic fields are thought to be the product of hydromagnetic dynamo action. For Earth, this process occurs within the convecting, turbulent and rapidly rotating outer core, where the dynamics are characterized by low Rossby, low magnetic Prandtl and high Rayleigh numbers. Progress in studying dynamos has been limited by current computing capabilities and the difficulties in replicating the extreme values that define this setting. Asymptotic models that embrace these extreme parameter values and enforce the dominant balance of geostrophy provide an option for the study of convective flows with actual relevance to geophysics. The quasi-geostrophic dynamo model (QGDM) is a multiscale, fully-nonlinear Cartesian dynamo model that is valid in the asymptotic limit of low Rossby number. We investigate the QGDM using a simplified class of solutions that consist of a single horizontal wavenumber which enforces a horizontal structure on the solutions. This single mode study is used to explore multiscale time stepping techniques and analyze the influence of the magnetic field on convection.

Helioseismology Observations of Solar Cycles and Dynamo Modeling

NASA Astrophysics Data System (ADS)

Kosovichev, A. G.; Guerrero, G.; Pipin, V.

2017-12-01

Helioseismology observations from the SOHO and SDO, obtained in 1996-2017, provide unique insight into the dynamics of the Sun's deep interior for two solar cycles. The data allow us to investigate variations of the solar interior structure and dynamics, and compare these variations with dynamo models and simulations. We use results of the local and global helioseismology data processing pipelines at the SDO Joint Science Operations Center (Stanford University) to study solar-cycle variations of the differential rotation, meridional circulation, large-scale flows and global asphericity. By comparing the helioseismology results with the evolution of surface magnetic fields we identify characteristic changes associated the initiation and development of Solar Cycles 23 and 24. For the physical interpretation of observed variations, the results are compared with the current mean-field dynamo models and 3D MHD dynamo simulations. It is shown that the helioseismology inferences provide important constraints on the solar dynamo mechanism, may explain the fundamental difference between the two solar cycles, and also give information about the next solar cycle.

Fluctuation dynamo and turbulent induction at small Prandtl number.

PubMed

Eyink, Gregory L

2010-10-01

We study the Lagrangian mechanism of the fluctuation dynamo at zero Prandtl number and infinite magnetic Reynolds number, in the Kazantsev-Kraichnan model of white-noise advection. With a rough velocity field corresponding to a turbulent inertial range, flux freezing holds only in a stochastic sense. We show that field lines arriving to the same point which were initially separated by many resistive lengths are important to the dynamo. Magnetic vectors of the seed field that point parallel to the initial separation vector arrive anticorrelated and produce an "antidynamo" effect. We also study the problem of "magnetic induction" of a spatially uniform seed field. We find no essential distinction between this process and fluctuation dynamo, both producing the same growth rates and small-scale magnetic correlations. In the regime of very rough velocity fields where fluctuation dynamo fails, we obtain the induced magnetic energy spectra. We use these results to evaluate theories proposed for magnetic spectra in laboratory experiments of turbulent induction.

Mean-field model of the von Kármán sodium dynamo experiment using soft iron impellers.

PubMed

Nore, C; Léorat, J; Guermond, J-L; Giesecke, A

2015-01-01

It has been observed that dynamo action occurs in the von-Kármán-Sodium (VKS) experiment only when the rotating disks and the blades are made of soft iron. The purpose of this paper is to numerically investigate the role of soft iron in the VKS dynamo scenario. This is done by using a mean-field model based on an axisymmetric mean flow, a localized permeability distribution, and a localized α effect modeling the action of the small velocity scales between the blades. The action of the rotating blades is modeled by an axisymmetric effective permeability field. Key properties of the flow giving to the numerical magnetic field a geometric structure similar to that observed experimentally are identified. Depending on the permeability of the disks and the effective permeability of the blades, the dynamo that is obtained is either oscillatory or stationary. Our numerical results confirm the leading role played by the ferromagnetic impellers. A scenario for the VKS dynamo is proposed.

Upper Ocean Response to the Atmospheric Cold Pools Associated With the Madden-Julian Oscillation

NASA Astrophysics Data System (ADS)

Pei, Suyang; Shinoda, Toshiaki; Soloviev, Alexander; Lien, Ren-Chieh

2018-05-01

Atmospheric cold pools are frequently observed during the Madden-Julian Oscillation events and play an important role in the development and organization of large-scale convection. They are generally associated with heavy precipitation and strong winds, inducing large air-sea fluxes and significant sea surface temperature (SST) fluctuations. This study provides a first detailed investigation of the upper ocean response to the strong cold pools associated with the Madden-Julian Oscillation, based on the analysis of in situ data collected during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign and one-dimensional ocean model simulations validated by the data. During strong cold pools, SST drops rapidly due to the atmospheric cooling in a shoaled mixed layer caused by the enhanced near-surface salinity stratification generated by heavy precipitation. Significant contribution also comes from the component of surface heat flux produced by the cold rain temperature. After the period of heavy rain, while net surface cooling remains, SST gradually recovers due to the enhanced entrainment of warmer waters below the mixed layer.

Investigating repeatable ionospheric features during large space storms and superstorms

DTIC Science & Technology

2014-08-25

operated under continuous disturbance dynamo electric field effects. Thus, this storm offered the opportunity to study the impact of eastward PPEF on the...daytime Equatorial Ionization Anomaly (EIA) while the disturbance dynamo continued. The 25 September 1998 great storm (Dst = -220 nT) was a unique...as the prompt penetration electric field (PPEF) developed and operated under continuous disturbance dynamo electric field (DDEF) effects. Thus, this

Dynamo Scaling Laws for Uranus and Neptune: The Role of Convective Shell Thickness on Dipolarity

NASA Astrophysics Data System (ADS)

Stanley, Sabine; Yunsheng Tian, Bob

2017-10-01

Previous dynamo scaling law studies (Christensen and Aubert, 2006) have demonstrated that the morphology of a planet’s magnetic field is determined by the local Rossby number (Ro_l): a non-dimensional diagnostic variable that quantifies the ratio of inertial forces to Coriolis forces on the average length scale of the flow. Dynamos with Ro_l <~ 0.1 produce dipolar dominated magnetic fields whereas dynamos with Ro_l >~ 0.1 produce multipolar magnetic fields. Scaling studies have also determined the dependence of the local Rossby number on non-dimensional parameters governing the system - specifically the Ekman, Prandtl, magnetic Prandtl and flux-based Rayleigh numbers (Olson and Christensen, 2006). When these scaling laws are applied to the planets, it appears that Uranus and Neptune should have dipole-dominated fields, contrary to observations. However, those scaling laws were derived using the specific convective shell thickness of the Earth’s core. Here we investigate the role of convective shell thickness on dynamo scaling laws. We find that the local Rossby number depends exponentially on the convective shell thickness. Including this new dependence on convective shell thickness, we find that the dynamo scaling laws now predict that Uranus and Neptune reside deeply in the multipolar regime, thereby resolving the previous contradiction with observations.

THE TURBULENT DYNAMO IN HIGHLY COMPRESSIBLE SUPERSONIC PLASMAS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Federrath, Christoph; Schober, Jennifer; Bovino, Stefano

The turbulent dynamo may explain the origin of cosmic magnetism. While the exponential amplification of magnetic fields has been studied for incompressible gases, little is known about dynamo action in highly compressible, supersonic plasmas, such as the interstellar medium of galaxies and the early universe. Here we perform the first quantitative comparison of theoretical models of the dynamo growth rate and saturation level with three-dimensional magnetohydrodynamical simulations of supersonic turbulence with grid resolutions of up to 1024{sup 3} cells. We obtain numerical convergence and find that dynamo action occurs for both low and high magnetic Prandtl numbers Pm = ν/ηmore » = 0.1-10 (the ratio of viscous to magnetic dissipation), which had so far only been seen for Pm ≥ 1 in supersonic turbulence. We measure the critical magnetic Reynolds number, Rm{sub crit}=129{sub −31}{sup +43}, showing that the compressible dynamo is almost as efficient as in incompressible gas. Considering the physical conditions of the present and early universe, we conclude that magnetic fields need to be taken into account during structure formation from the early to the present cosmic ages, because they suppress gas fragmentation and drive powerful jets and outflows, both greatly affecting the initial mass function of stars.« less

A wet, heterogeneous lunar interior: Lower mantle and core dynamo evolution

NASA Astrophysics Data System (ADS)

Evans, A. J.; Zuber, M. T.; Weiss, B. P.; Tikoo, S. M.

2014-05-01

While recent analyses of lunar samples indicate the Moon had a core dynamo from at least 4.2-3.56 Ga, mantle convection models of the Moon yield inadequate heat flux at the core-mantle boundary to sustain thermal core convection for such a long time. Past investigations of lunar dynamos have focused on a generally homogeneous, relatively dry Moon, while an initial compositionally stratified mantle is the expected consequence of a postaccretionary lunar magma ocean. Furthermore, recent re-examination of Apollo samples and geophysical data suggests that the Moon contains at least some regions with high water content. Using a finite element model, we investigate the possible consequences of a heterogeneously wet, compositionally stratified interior for the evolution of the Moon. We find that a postoverturn model of mantle cumulates could result in a core heat flux sufficiently high to sustain a dynamo through 2.5 Ga and a maximum surface, dipolar magnetic field strength of less than 1 μT for a 350-km core and near ˜2 μT for a 450-km core. We find that if water was transported or retained preferentially in the deep interior, it would have played a significant role in transporting heat out of the deep interior and reducing the lower mantle temperature. Thus, water, if enriched in the lower mantle, could have influenced core dynamo timing by over 1.0 Gyr and enhanced the vigor of a lunar core dynamo. Our results demonstrate the plausibility of a convective lunar core dynamo even beyond the period currently indicated by the Apollo samples.

Special issue on current research in astrophysical magnetism

NASA Astrophysics Data System (ADS)

Kosovichev, Alexander; Lundstedt, Henrik; Brandenburg, Axel

2012-06-01

Much of what Hannes Alfvén envisaged some 70 years ago has now penetrated virtually all branches of astrophysical research. Indeed, magnetic fields can display similar properties over a large range of scales. We have therefore been able to take advantage of the transparency of galaxies and the interstellar medium to obtain measurements inside them. On the other hand, the Sun is much closer, allowing us to obtain a detailed picture of the interaction of flows and magnetic fields at the surface, and more recently in the interior by helioseismology. Moreover, the solar timescales are generally much shorter, making studies of dynamical processes more direct. This special issue on current research in astrophysical magnetism is based on work discussed during a one month Nordita program Dynamo, Dynamical Systems and Topology and comprises papers that fall into four different categories (A)-(D). (A) Papers on small-scale magnetic fields and flows in astrophysics 1. E M de Gouveia Dal Pino, M R M Leão, R Santos-Lima, G Guerrero, G Kowal and A Lazarian Magnetic flux transport by turbulent reconnection in astrophysical flows 2. Philip R Goode, Valentyna Abramenko and Vasyl Yurchyshyn New solar telescope in Big Bear: evidence for super-diffusivity and small-scale solar dynamos? 3. I N Kitiashvili, A G Kosovichev, N N Mansour, S K Lele and A A Wray Vortex tubes of turbulent solar convection The above collection of papers begins with a review of astrophysical reconnection and introduces the concept of dynamos necessary to explain the existence of contemporary magnetic fields both on galactic and solar scales (paper 1). This is complemented by observations with the new Big Bear Solar Observatory telescope, allowing us to see magnetic field amplification on small scales (paper 2). This in turn is complemented by realistic simulations of subsurface and surface flow patterns (paper 3). (B) Papers on theoretical approaches to turbulent fluctuations 4. Nathan Kleeorin and Igor Rogachevskii Growth rate of small-scale dynamo at low magnetic Prandtl numbers 5. Erico L Rempel, Abraham C-L Chian and Axel Brandenburg Lagrangian chaos in an ABC-forced nonlinear dynamo 6. J E Snellman, M Rheinhardt, P J Käpylä, M J Mantere and A Brandenburg Mean-field closure parameters for passive scalar turbulence Research in dynamo theory has been actively pursued for over half a century. It started by trying to understand the large-scale magnetic fields of the Sun and the Earth, and subsequently also in galaxies. Such large-scale fields can nowadays be understood in terms of mean-field dynamo theory that explains the possibility of large-scale field generation under anisotropic conditions lacking mirror symmetry. However, even when none of this is the case, dynamos can still work, and they are called small-scale dynamos that were referred to in paper 2. This was studied originally under the assumption that the flow is smooth compared with the magnetic field, but in the Sun the opposite is the case. This is because viscosity is much smaller than magnetic diffusivity, i.e., their ratio, which is the magnetic Prandtl number, is small. In that case the physics of small-scale dynamos changes, but dynamos still exist even then (paper 4). Tracing the flow lines in nonlinear small-scale dynamos is important for understanding their mixing properties (paper 5). Turbulent mixing is a generic concept that applies not only to magnetic field, but also to passive scalars which are often used as a prototype for studying this. Turbulence simulations have helped tremendously in quantifying the ability of turbulent flows to mix, but the more we know, the more complicated it becomes. It turns out that spatial and temporal coupling is an important consideration for allowing accurate comparison between numerical simulations and mean-field theory (paper 6). (C) The large-scale solar cycle 7. V V Pipin and D D Sokoloff The fluctuating α-effect and Waldmeier relations in the nonlinear dynamo models1 8. Radostin D Simitev and Friedrich H Busse Solar cycle properties described by simple convection-driven dynamos The mean-field concept has helped us constructing detailed models of the solar cycle and to make comparison with observed features of the solar 11-year cycle. One such feature is the Waldmeier relation between growth time and amplitude of the cycle, and there is another relation for the declining part of the cycle. These relations reflect nonlinear aspects of the model and therefore constitute an important test of the model (paper 7). While mean-field theory is a useful concept for modeling solar activity, it must eventually be tested against fully three-dimensional simulations. At present, such simulations are often quite idealized, because only the large scales of the turbulent convection of stars can be resolved. Nevertheless, numerical simulations begin to show many properties that are also seen in the Sun (paper 8). (D) Flow and dynamo properties in spherical shells 9. Maxim Reshetnyak and Pavel Hejda Kinetic energy cascades in quasi-geostrophic convection in a spherical shell 10. Radostin D Simitev and Friedrich H Busse Bistable attractors in a model of convection-driven spherical dynamos As the rotation speed is increased, the flow becomes more strongly constrained by the Coriolis force. In a spherical shell, such a flow is additionally constrained by gravity, or at least by the geometry of the domain. Such flows are called geostrophic. Only now are we beginning to learn about the subtle properties of the kinetic energy cascade in such flows (paper 9). Turbulent systems are highly nonlinear and it is in principle possible to find multiple solutions of the equations even for the same boundary and initial conditions. For turbulent systems, we can only ask about the statistical properties of the solutions, and the question of multiple solutions is then less obvious. However, in turbulent dynamos in convective shells, a nice example has been found where this is possible. A detailed account of this is given in paper 10. Most of the participants of the Nordita program were able to stay for the full month of the program, allowing them to think about new ideas that will be reflected not only in papers on the short term, but also in new projects and collaborations on a larger scale in the years to come. We therefore thank Nordita for providing a stimulating atmosphere and acknowledge the generous support. 1This paper has been published as V V Pipin and D D Sokoloff 2011 Phys. Scr. 84 065903.

Molecular DYNAmics of Soil Organic carbon (DYNAMOS *): a project focusing on soils and carbon through data and modeling

NASA Astrophysics Data System (ADS)

Hatté, C.; Balesdent, J.; Derenne, S.; Derrien, D.; Dignac, M.; Egasse, C.; Ezat, U.; Gauthier, C.; Mendez-Millan, M.; Nguyen Tu, T.; Rumpel, C.; Sicre, M.; Zeller, B.

2009-12-01

Here we present the first results of the DynaMOS project whose main issue is the build-up of a new generation of soil carbon model. The modeling will describe together soil organic geochemistry and soil carbon dynamics in a generalized, quantitative representation. The carbon dynamics time scale envisaged here will cover the 1 to 1000 yr range and described molecules will be carbohydrate, peptide, amino acid, lignin, lipids, their products of biodegradation and uncharacterized carbonaceous species of biological origin. Three main characteristics define DYNAMOS model originalities: it will consider organic matter at the molecular scale, integrate back to global scale and account for component vertical movements. In a first step, specific data acquisition will concern the production, fate and age of carbon of individual organic compounds. Dynamic parameters will be acquired by compound-specific carbon isotope analysis of both 13C and 14C, by GC/C/IR-MS and AMS. Sites for data acquisition, model calibration and model validation will be chosen on the base of their isotopic history and environmental constraints: 13C natural labeling (with and without C3/C4 vegetation changes), 13C/15N-labelled litter application in both forest and cropland. They include some long-term experiments owned by the partners themselves plus a worldwide panel of sites. In a second step the depth distribution of organic species, isotopes and ages in soils (1D representation) will be modeled by coupling carbon dynamics and vertical movement. Besides the main objective of providing a robust soil carbon dynamics model, DYNAMOS will assess and model the alteration of the isotopic signature of molecules throughout decay and create a shared database of both already published and new data of compound specific information. Issues of the project will concern different scientific fields: global geochemical cycles by refining the description of the terrestrial carbon cycle and entering the chemical composition of organic matter in carbon models; forestry or agriculture by offering a chemical frame for the management of crop residues or organic wastes; geochronology, paleoecology and paleo climatology by modeling the alteration of isotope signature and the preservation of terrestrial biomarkers. (*) funded by the French National Agency of Research (ANR): ANR-07-Blan-0222-01, http://dynamos.lsce.ipsl.fr

Iron snow in the Martian core?

NASA Astrophysics Data System (ADS)

Davies, Christopher J.; Pommier, Anne

2018-01-01

The decline of Mars' global magnetic field some 3.8-4.1 billion years ago is thought to reflect the demise of the dynamo that operated in its liquid core. The dynamo was probably powered by planetary cooling and so its termination is intimately tied to the thermochemical evolution and present-day physical state of the Martian core. Bottom-up growth of a solid inner core, the crystallization regime for Earth's core, has been found to produce a long-lived dynamo leading to the suggestion that the Martian core remains entirely liquid to this day. Motivated by the experimentally-determined increase in the Fe-S liquidus temperature with decreasing pressure at Martian core conditions, we investigate whether Mars' core could crystallize from the top down. We focus on the "iron snow" regime, where newly-formed solid consists of pure Fe and is therefore heavier than the liquid. We derive global energy and entropy equations that describe the long-timescale thermal and magnetic history of the core from a general theory for two-phase, two-component liquid mixtures, assuming that the snow zone is in phase equilibrium and that all solid falls out of the layer and remelts at each timestep. Formation of snow zones occurs for a wide range of interior and thermal properties and depends critically on the initial sulfur concentration, ξ0. Release of gravitational energy and latent heat during growth of the snow zone do not generate sufficient entropy to restart the dynamo unless the snow zone occupies at least 400 km of the core. Snow zones can be 1.5-2 Gyrs old, though thermal stratification of the uppermost core, not included in our model, likely delays onset. Models that match the available magnetic and geodetic constraints have ξ0 ≈ 10% and snow zones that occupy approximately the top 100 km of the present-day Martian core.

Solar Dynamo Driven by Periodic Flow Oscillation

NASA Technical Reports Server (NTRS)

Mayr, Hans G.; Hartle, Richard E.; Einaudi, Franco (Technical Monitor)

2001-01-01

We have proposed that the periodicity of the solar magnetic cycle is determined by wave mean flow interactions analogous to those driving the Quasi Biennial Oscillation in the Earth's atmosphere. Upward propagating gravity waves would produce oscillating flows near the top of the radiation zone that in turn would drive a kinematic dynamo to generate the 22-year solar magnetic cycle. The dynamo we propose is built on a given time independent magnetic field B, which allows us to estimate the time dependent, oscillating components of the magnetic field, (Delta)B. The toroidal magnetic field (Delta)B(sub phi) is directly driven by zonal flow and is relatively large in the source region, (Delta)(sub phi)/B(sub Theta) much greater than 1. Consistent with observations, this field peaks at low latitudes and has opposite polarities in both hemispheres. The oscillating poloidal magnetic field component, (Delta)B(sub Theta), is driven by the meridional circulation, which is difficult to assess without a numerical model that properly accounts for the solar atmosphere dynamics. Scale-analysis suggests that (Delta)B(sub Theta) is small compared to B(sub Theta) in the dynamo region. Relative to B(sub Theta), however, the oscillating magnetic field perturbations are expected to be transported more rapidly upwards in the convection zone to the solar surface. As a result, (Delta)B(sub Theta) (and (Delta)B(sub phi)) should grow relative to B(sub Theta), so that the magnetic fields reverse at the surface as observed. Since the meridional and zonai flow oscillations are out of phase, the poloidal magnetic field peaks during times when the toroidal field reverses direction, which is observed. With the proposed wave driven flow oscillation, the magnitude of the oscillating poloidal magnetic field increases with the mean rotation rate of the fluid. This is consistent with the Bode-Blackett empirical scaling law, which reveals that in massive astrophysical bodies the magnetic moment tends to increase with the angular momentum of the fluid.

Models of magnetic field generation in partly stable planetary cores: Applications to Mercury and Saturn

NASA Astrophysics Data System (ADS)

Christensen, Ulrich R.; Wicht, Johannes

2008-07-01

A substantial part of Mercury's iron core may be stably stratified because the temperature gradient is subadiabatic. A dynamo would operate only in a deep sublayer. We show that such a situation arises for a wide range of values for the heat flow and the sulfur content in the core. In Saturn the upper part of the metallic hydrogen core could be stably stratified because of helium depletion. The magnetic field is unusually weak in the case of Mercury and unusually axisymmetric at Saturn. We study numerical dynamo models in rotating spherical shells with a stable outer region. The control parameters are chosen such that the magnetic Reynolds number is in the range of expected Mercury values. Because of its slow rotation, Mercury may be in a regime where the dipole contribution to the internal magnetic field is weak. Most of our models are in this regime, where the dynamo field consists mainly of rapidly varying higher multipole components. They can hardly pass the stable conducting layer because of the skin effect. The weak low-degree components vary more slowly and control the structure of the field outside the core, whose strength matches the observed field strength at Mercury. In some models the axial dipole dominates at the planet's surface and in others the axial quadrupole is dominant. Differential rotation in the stable layer, representing a thermal wind, is important for attenuating non-axisymmetric components in the exterior field. In some models that we relate to Saturn the axial dipole is intrinsically strong inside the dynamo. The surface field strength is much larger than in the other cases, but the stable layer eliminates non-axisymmetric modes. The Messenger and Bepi Colombo space missions can test our predictions that Mercury's field is large-scaled, fairly axisymmetric, and shows no secular variations on the decadal time scale.

An integrated model for Jupiter's dynamo action and mean jet dynamics

NASA Astrophysics Data System (ADS)

Gastine, Thomas; Wicht, Johannes; Duarte, Lucia; Heimpel, Moritz

2014-05-01

Data from various space crafts revealed that Jupiter's large scale interior magnetic field is very Earth-like. This is surprising since numerical simulations have demonstrated that, for example, the radial dependence of density, electrical conductivity and other physical properties, which is only mild in the iron cores of terrestrial planets but very drastic in gas planets, can significantly affect the interior dynamics. Jupiter's dynamo action is thought to take place in the deeper envelope where hydrogen, the main constituent of Jupiter's atmosphere, assumes metallic properties. The potential interaction between the observed zonal jets and the deeper dynamo region is an unresolved problem with important consequences for the magnetic field generation. Here we present the first numerical simulation that is based on recent interior models and covers 99% of the planetary radius (below the 1 bar level). A steep decease in the electrical conductivity over the outer 10% in radius allowed us to model both the deeper metallic region and the outer molecular layer in an integrated approach. The magnetic field very closely reproduces Jupiter's known large scale field. A strong equatorial zonal jet remains constrained to the molecular layer while higher latitude jets are suppressed by Lorentz forces. This suggests that Jupiter's higher latitude jets remain shallow and are driven by an additional effect not captured in our deep convection model. The dynamo action of the equatorial jet produces a band of magnetic field located around the equator. The unprecedented magnetic field resolution expected from the Juno mission will allow to resolve this feature allowing a direct detection of the equatorial jet dynamics at depth. Typical secular variation times scales amount to around 750 yr for the dipole contribution but decrease to about 5 yr at the expected Juno resolution (spherical harmonic degree 20). At a nominal mission duration of one year Juno should therefore be able to directly detect secular variation effects in the higher field harmonics.

Aircraft Measurements for Understanding Air-Sea Coupling and Improving Coupled Model Predictions

DTIC Science & Technology

2014-09-30

layer thermodynamic properties across the DYNAMO domain during the suppressed and active phase of MJO; and 3) variability and distribution of upper ocean...structure during suppressed, active and restoring phase of MJO. One of the unique aspects of LASP/ DYNAMO WP-3D project was to supplement the point...observations by probing the atmospheric and oceanic variability across the DYNAMO domain. Adhering to this aspect, vertical cross section of lower

The Influence of Atmosphere-Ocean Interaction on MJO Development and Propagation

DTIC Science & Technology

2014-09-30

evaluate modeling results and process studies. The field phase of this project is associated with DYNAMO , which is the US contribution to the...influence on ocean temperature 4. Extended run for DYNAMO with high vertical resolution NCOM RESULTS Summary of project results The work funded...model experiments of the November 2011 MJO – the strongest MJO episode observed during the DYNAMO . The previous conceptual model that was based on TOGA

An Investigation of Turbulent Heat Exchange in the Subtropics

DTIC Science & Technology

2014-09-30

meteorological sensors aboard the research vessel the R/V Revelle during the DYNAMO field program. In situ meteorology and high-rate flux sensors operated...continuously while in the sampling period for DYNAMO Leg 3. This included all sensors operating during Leg 2 with the addition of a closed-path LI...stress; wave data; surface and near surface sea temperatures, salinity and currents; and other key variables specifically requested by DYNAMO /LASP PIs

Towards a turbulent magnetic dysnamo platform

NASA Astrophysics Data System (ADS)

Flippo, Kirk; Rasmus, Alexander; Li, Hui; Li, Shengtai; Kuranz, Carolyn; Levesque, Joseph; Klein, Sallee; Tzeferacos, Petros

2017-10-01

It is known through astronomical observations that most of the Universe is ionized, magnetized, and often turbulent and filled with jets. One theorized process to create strong magnetic fields and jets is the turbulent magnetic dynamo. The magnetic dynamo is a fundamental process in plasma physics, taking kinetic energy and converting it to magnetic energy and is very important to planetary physics and astrophysics. We report on recent Omega EP experiments to produce platform with a turbulent plume of magnetized material with which to study the turbulent magnetic dynamo process. The laser interaction with the target can seed magnetic fields that can be advected into the plume and amplified to saturation by the turbulent magnetic dynamo process. The experimentally measured plume characteristics are compared to hydro code calculations.

Fast dynamos with finite resistivity in steady flows with stagnation points

NASA Technical Reports Server (NTRS)

Lau, Yun-Tung; Finn, John M.

1993-01-01

Results are presented of a kinematic fast dynamo problem for two classes of steady incompressible flows: the ABC flow and the spatially aperiodic flow of Lau and Finn (1992). The numerical method used to find the solutions is described, together with convergence studies with respect to the time step and the number of points N of the spatial grid. It is shown that the growth rate and frequency can be extrapolated to N = infinity. Results are presented indicating that fast kinematic dynamos can exist in both these flows and that chaotic flow is a necessary condition. It was found that, for the ABC flow with A = B = C, there are two dynamo modes: an oscillating mode and a purely growing mode.

Solar Cycle #24 and the Solar Dynamo

NASA Technical Reports Server (NTRS)

Schatten, Kenneth; Pesnell, W. Dean

2007-01-01

We focus on two solar aspects related to flight dynamics. These are the solar dynamo and long-term solar activity predictions. The nature of the solar dynamo is central to solar activity predictions, and these predictions are important for orbital planning of satellites in low earth orbit (LEO). The reason is that the solar ultraviolet (UV) and extreme ultraviolet (EUV) spectral irradiances inflate the upper atmospheric layers of the Earth, forming the thermosphere and exosphere through which these satellites orbit. Concerning the dynamo, we discuss some recent novel approaches towards its understanding. For solar predictions we concentrate on a solar precursor method, in which the Sun's polar field plays a major role in forecasting the next cycle s activity based upon the Babcock-Leighton dynamo. With a current low value for the Sun s polar field, this method predicts that solar cycle #24 will be one of the lowest in recent times, with smoothed F10.7 radio flux values peaking near 130 plus or minus 30 (2 sigma), in the 2013 timeframe. One may have to consider solar activity as far back as the early 20th century to find a cycle of comparable magnitude. Concomitant effects of low solar activity upon satellites in LEO will need to be considered, such as enhancements in orbital debris. Support for our prediction of a low solar cycle #24 is borne out by the lack of new cycle sunspots at least through the first half of 2007. Usually at the present epoch in the solar cycle (approx. 7+ years after the last solar maximum), for a normal size following cycle, new cycle sunspots would be seen. The lack of their appearance at this time is only consistent with a low cycle #24. Polar field observations of a weak magnitude are consistent with unusual structures seen in the Sun s corona. Polar coronal holes are the hallmarks of the Sun's open field structures. At present, it appears that the polar coronal holes are relatively weak, and there have been many equatorial coronal holes. This appears consistent with a weakening polar field, but coronal hole data must be scrutinized carefully as observing techniques have changed. We also discuss new solar dynamo ideas, and the SODA (SOlar Dynamo Amplitude) index, which provides the user with the ability to track the Sun's hidden, dynamo magnetic fields throughout the various stages of the Sun's cycle. Our solar dynamo ideas are a modernization and rejuvenation of the Babcock-Leighton original idea of a shallow solar dynamo, using modern observations that appear to support their shallow dynamo viewpoint. We are in awe of being able to see an object the size of the Sun undergoing as dramatic a change as our model provides in a few short years. The Sun, however, has undergone changes as rapid as this before! The weather on the Sun is at least as fickle as the weather on the Earth.

Solar Cycle #24 and the Solar Dynamo

NASA Technical Reports Server (NTRS)

Pesnell, W. Dean; Schatten, Kenneth

2007-01-01

We focus on two solar aspects related to flight dynamics. These are the solar dynamo and long-term solar activity predictions. The nature of the solar dynamo is central to solar activity predictions, and these predictions are important for orbital planning of satellites in low earth orbit (LEO). The reason is that the solar ultraviolet (UV) and extreme ultraviolet (EUV) spectral irradiances inflate the upper atmospheric layers of the Earth, forming the thermosphere and exosphere through which these satellites orbit. Concerning the dynamo, we discuss some recent novel approaches towards its understanding. For solar predictions we concentrate on a solar precursor method, in which the Sun s polar field plays a major role in forecasting the next cycle s activity based upon the Babcock- Leighton dynamo. With a current low value for the Sun s polar field, this method predicts that solar cycle #24 will be one of the lowest in recent times, with smoothed F10.7 radio flux values peaking near 130+ 30 (2 4, in the 2013 timeframe. One may have to consider solar activity as far back as the early 20th century to find a cycle of comparable magnitude. Concomitant effects of low solar activity upon satellites in LEO will need to be considered, such as enhancements in orbital debris. Support for our prediction of a low solar cycle #24 is borne out by the lack of new cycle sunspots at least through the first half of 2007. Usually at the present epoch in the solar cycle (-7+ years after the last solar maximum), for a normal size following cycle, new cycle sunspots would be seen. The lack of their appearance at this time is only consistent with a low cycle #24. Polar field observations of a weak magnitude are consistent with unusual structures seen in the Sun s corona. Polar coronal holes are the hallmarks of the Sun s open field structures. At present, it appears that the polar coronal holes are relatively weak, and there have been many equatorial coronal holes. This appears consistent with a weakening polar field, but coronal hole data must be scrutinized carefully as observing techniques have changed. We also discuss new solar dynamo ideas, and the SODA (Solar Dynamo Amplitude) index, which provides the user with the ability to track the Sun s hidden, dynamo magnetic fields throughout the various stages of the Sun s cycle. Our solar dynamo ideas are a modernization and rejuvenation of the Babcock-Leighton original idea of a shallow solar dynamo, using modem observations that appear to support their shallow dynamo viewpoint. We are in awe of being able to see an object the size of the Sun undergoing as dramatic a change as our model provides in a few short years. The Sun, however, has undergone changes as rapid as this before! The weather on the Sun is at least as fickle as the weather on the Earth.

Non-axisymmetric α2Ω-dynamo waves in thin stellar shells

NASA Astrophysics Data System (ADS)

Bassom, Andrew P.; Kuzanyan, Kirill M.; Sokoloff, Dmitry; Soward, Andrew M.

2005-04-01

Linear α2Ω-dynamo waves are investigated in a thin turbulent, differentially rotating convective stellar shell. A simplified one-dimensional model is considered and an asymptotic solution constructed based on the small aspect ratio of the shell. In a previous paper Griffiths et al. (Griffiths, G.L., Bassom, A.P., Soward, A.M. and Kuzanyan, K.M., Nonlinear α2Ω-dynamo waves in stellar shells, Geophys. Astrophys. Fluid Dynam., 2001, 94, 85-133) considered the modulation of dynamo waves, linked to a latitudinal-dependent local α-effect and radial gradient of the zonal shear flow. These effects are measured at latitude θ by the magnetic Reynolds numbers Rαf(θ) and RΩg(θ). The modulated Parker wave, which propagates towards the equator, is localised at some mid-latitude θp under a Gaussian envelope. In this article, we include the influence of a latitudinal-dependent zonal flow possessing angular velocity Ω*(θ) and consider the possibility of non-axisymmetric dynamo waves with azimuthal wave number m. We find that the critical dynamo number Dc = RαRΩ is minimised by axisymmetric modes in the αΩ-limit (Rα→0). On the other hand, when Rα ≠ 0 there may exist a band of wave numbers 0 < m < m† for which the non-axisymmetric modes have a smaller Dc than in the axisymmetric case. Here m† is regarded as a continuous function of Rα with the property m†→0 as Rα→0 and the band is only non-empty when m† >1, which happens for sufficiently large Rα. The preference for non-axisymmetric modes is possible because the wind-up of the non-axisymmetric structures can be compensated by phase mixing inherent to the α2Ω-dynamo. For parameter values resembling solar conditions, the Parker wave of maximum dynamo activity at latitude θp not only propagates equatorwards but also westwards relative to the local angular velocity Ω*(θp). Since the critical dynamo number Dc = RαRΩ is O (1) for small Rα, the condition m† > 1 for non-axisymmetric mode preference imposes an upper limit on the size of |dΩ*/dθ|.

The Dynamo package for tomography and subtomogram averaging: components for MATLAB, GPU computing and EC2 Amazon Web Services

PubMed Central

Castaño-Díez, Daniel

2017-01-01

Dynamo is a package for the processing of tomographic data. As a tool for subtomogram averaging, it includes different alignment and classification strategies. Furthermore, its data-management module allows experiments to be organized in groups of tomograms, while offering specialized three-dimensional tomographic browsers that facilitate visualization, location of regions of interest, modelling and particle extraction in complex geometries. Here, a technical description of the package is presented, focusing on its diverse strategies for optimizing computing performance. Dynamo is built upon mbtools (middle layer toolbox), a general-purpose MATLAB library for object-oriented scientific programming specifically developed to underpin Dynamo but usable as an independent tool. Its structure intertwines a flexible MATLAB codebase with precompiled C++ functions that carry the burden of numerically intensive operations. The package can be delivered as a precompiled standalone ready for execution without a MATLAB license. Multicore parallelization on a single node is directly inherited from the high-level parallelization engine provided for MATLAB, automatically imparting a balanced workload among the threads in computationally intense tasks such as alignment and classification, but also in logistic-oriented tasks such as tomogram binning and particle extraction. Dynamo supports the use of graphical processing units (GPUs), yielding considerable speedup factors both for native Dynamo procedures (such as the numerically intensive subtomogram alignment) and procedures defined by the user through its MATLAB-based GPU library for three-dimensional operations. Cloud-based virtual computing environments supplied with a pre-installed version of Dynamo can be publicly accessed through the Amazon Elastic Compute Cloud (EC2), enabling users to rent GPU computing time on a pay-as-you-go basis, thus avoiding upfront investments in hardware and longterm software maintenance. PMID:28580909

The cross-over to magnetostrophic convection in planetary dynamo systems

PubMed Central

King, E. M.

2017-01-01

Global scale magnetostrophic balance, in which Lorentz and Coriolis forces comprise the leading-order force balance, has long been thought to describe the natural state of planetary dynamo systems. This argument arises from consideration of the linear theory of rotating magnetoconvection. Here we test this long-held tenet by directly comparing linear predictions against dynamo modelling results. This comparison shows that dynamo modelling results are not typically in the global magnetostrophic state predicted by linear theory. Then, in order to estimate at what scale (if any) magnetostrophic balance will arise in nonlinear dynamo systems, we carry out a simple scaling analysis of the Elsasser number Λ, yielding an improved estimate of the ratio of Lorentz and Coriolis forces. From this, we deduce that there is a magnetostrophic cross-over length scale, LX≈(Λo2/Rmo)D, where Λo is the linear (or traditional) Elsasser number, Rmo is the system scale magnetic Reynolds number and D is the length scale of the system. On scales well above LX, magnetostrophic convection dynamics should not be possible. Only on scales smaller than LX should it be possible for the convective behaviours to follow the predictions for the magnetostrophic branch of convection. Because LX is significantly smaller than the system scale in most dynamo models, their large-scale flows should be quasi-geostrophic, as is confirmed in many dynamo simulations. Estimating Λo≃1 and Rmo≃103 in Earth’s core, the cross-over scale is approximately 1/1000 that of the system scale, suggesting that magnetostrophic convection dynamics exists in the core only on small scales below those that can be characterized by geomagnetic observations. PMID:28413338

The Dynamo package for tomography and subtomogram averaging: components for MATLAB, GPU computing and EC2 Amazon Web Services.

PubMed

Castaño-Díez, Daniel

2017-06-01

Dynamo is a package for the processing of tomographic data. As a tool for subtomogram averaging, it includes different alignment and classification strategies. Furthermore, its data-management module allows experiments to be organized in groups of tomograms, while offering specialized three-dimensional tomographic browsers that facilitate visualization, location of regions of interest, modelling and particle extraction in complex geometries. Here, a technical description of the package is presented, focusing on its diverse strategies for optimizing computing performance. Dynamo is built upon mbtools (middle layer toolbox), a general-purpose MATLAB library for object-oriented scientific programming specifically developed to underpin Dynamo but usable as an independent tool. Its structure intertwines a flexible MATLAB codebase with precompiled C++ functions that carry the burden of numerically intensive operations. The package can be delivered as a precompiled standalone ready for execution without a MATLAB license. Multicore parallelization on a single node is directly inherited from the high-level parallelization engine provided for MATLAB, automatically imparting a balanced workload among the threads in computationally intense tasks such as alignment and classification, but also in logistic-oriented tasks such as tomogram binning and particle extraction. Dynamo supports the use of graphical processing units (GPUs), yielding considerable speedup factors both for native Dynamo procedures (such as the numerically intensive subtomogram alignment) and procedures defined by the user through its MATLAB-based GPU library for three-dimensional operations. Cloud-based virtual computing environments supplied with a pre-installed version of Dynamo can be publicly accessed through the Amazon Elastic Compute Cloud (EC2), enabling users to rent GPU computing time on a pay-as-you-go basis, thus avoiding upfront investments in hardware and longterm software maintenance.

DYNAMO-HIA--a Dynamic Modeling tool for generic Health Impact Assessments.

PubMed

Lhachimi, Stefan K; Nusselder, Wilma J; Smit, Henriette A; van Baal, Pieter; Baili, Paolo; Bennett, Kathleen; Fernández, Esteve; Kulik, Margarete C; Lobstein, Tim; Pomerleau, Joceline; Mackenbach, Johan P; Boshuizen, Hendriek C

2012-01-01

Currently, no standard tool is publicly available that allows researchers or policy-makers to quantify the impact of policies using epidemiological evidence within the causal framework of Health Impact Assessment (HIA). A standard tool should comply with three technical criteria (real-life population, dynamic projection, explicit risk-factor states) and three usability criteria (modest data requirements, rich model output, generally accessible) to be useful in the applied setting of HIA. With DYNAMO-HIA (Dynamic Modeling for Health Impact Assessment), we introduce such a generic software tool specifically designed to facilitate quantification in the assessment of the health impacts of policies. DYNAMO-HIA quantifies the impact of user-specified risk-factor changes on multiple diseases and in turn on overall population health, comparing one reference scenario with one or more intervention scenarios. The Markov-based modeling approach allows for explicit risk-factor states and simulation of a real-life population. A built-in parameter estimation module ensures that only standard population-level epidemiological evidence is required, i.e. data on incidence, prevalence, relative risks, and mortality. DYNAMO-HIA provides a rich output of summary measures--e.g. life expectancy and disease-free life expectancy--and detailed data--e.g. prevalences and mortality/survival rates--by age, sex, and risk-factor status over time. DYNAMO-HIA is controlled via a graphical user interface and is publicly available from the internet, ensuring general accessibility. We illustrate the use of DYNAMO-HIA with two example applications: a policy causing an overall increase in alcohol consumption and quantifying the disease-burden of smoking. By combining modest data needs with general accessibility and user friendliness within the causal framework of HIA, DYNAMO-HIA is a potential standard tool for health impact assessment based on epidemiologic evidence.

The cross-over to magnetostrophic convection in planetary dynamo systems.

PubMed

Aurnou, J M; King, E M

2017-03-01

Global scale magnetostrophic balance, in which Lorentz and Coriolis forces comprise the leading-order force balance, has long been thought to describe the natural state of planetary dynamo systems. This argument arises from consideration of the linear theory of rotating magnetoconvection. Here we test this long-held tenet by directly comparing linear predictions against dynamo modelling results. This comparison shows that dynamo modelling results are not typically in the global magnetostrophic state predicted by linear theory. Then, in order to estimate at what scale (if any) magnetostrophic balance will arise in nonlinear dynamo systems, we carry out a simple scaling analysis of the Elsasser number Λ , yielding an improved estimate of the ratio of Lorentz and Coriolis forces. From this, we deduce that there is a magnetostrophic cross-over length scale, [Formula: see text], where Λ o is the linear (or traditional) Elsasser number, Rm o is the system scale magnetic Reynolds number and D is the length scale of the system. On scales well above [Formula: see text], magnetostrophic convection dynamics should not be possible. Only on scales smaller than [Formula: see text] should it be possible for the convective behaviours to follow the predictions for the magnetostrophic branch of convection. Because [Formula: see text] is significantly smaller than the system scale in most dynamo models, their large-scale flows should be quasi-geostrophic, as is confirmed in many dynamo simulations. Estimating Λ o ≃1 and Rm o ≃10 3 in Earth's core, the cross-over scale is approximately 1/1000 that of the system scale, suggesting that magnetostrophic convection dynamics exists in the core only on small scales below those that can be characterized by geomagnetic observations.

Subsurface Fluxes Beneath Large-Scale Convective Centers in the Indian Ocean: Coupled Air-Wave-Sea Processes in the Subtropics

DTIC Science & Technology

2013-09-30

Figure 1 – Measurement systems installed on R/V Roger Revelle for DYNAMO /LASP. Inset map shows locations of land-based sounding stations...oceanographic moorings and the research vessels Mirai and Revelle during the intensive observation period of DYNAMO . The black line outlines the flight...under which each dominates. Transmission profile plus near-surface mixing measurements from LASP/ DYNAMO are being used to assess bounds on the

Dynamical systems for modeling the evolution of the magnetic field of stars and Earth

NASA Astrophysics Data System (ADS)

Popova, H.

2016-02-01

The cycles of solar magnetic activity are connected with a solar dynamo that operates in the convective zone. Solar dynamo mechanism is based on the combined action of the differential rotation and the alpha-effect. Application of these concepts allows us to get an oscillating solution as a wave of the toroidal field propagating from middle latitudes to the equator. We investigated the dynamo model with the meridional circulation by the low-mode approach. This approach is based on an assumption that the solar magnetic field can be described by non-linear dynamical systems with a relatively small number of parameters. Such non-linear dynamical systems are based on the equations of dynamo models. With this method dynamical systems have been built for media which contains the meridional flow and thickness of the convection zone of the star. It was shown the possibility of coexistence of quiasi-biennial and 22-year cycle. We obtained the different regimes (oscillations, vacillations, dynamo-bursts) depending on the value of the dynamo-number, the meridional circulation, and thickness of the convection zone. We discuss the features of these regimes and compare them with the observed features of evolution of the solar and geo magnetic fields. We built theoretical paleomagnetic time scale and butterfly-diagrams for the helicity and toroidal magnetic field for different regimes.

Dynamical systems for modeling evolution of the magnetic field of the Sun, stars and planets

NASA Astrophysics Data System (ADS)

Popova, E.

2016-12-01

The magnetic activity of the Sun, stars and planets are connected with a dynamo process based on the combined action of the differential rotation and the alpha-effect. Application of this concept allows us to get different types of solutions which can describe the magnetic activity of celestial bodies. We investigated the dynamo model with the meridional circulation by the low-mode approach. This approach is based on an assumption that the magnetic field can be described by non-linear dynamical systems with a relatively small number of parameters. Such non-linear dynamical systems are based on the equations of dynamo models. With this method dynamical systems have been built for media which contains the meridional flow and thickness of the spherical shell where dynamo process operates. It was shown the possibility of coexistence of quiasi-biennial oscillations, 22-year cycle, and grand minima of magnetic activity which is consistent with the observational data for the solar activity. We obtained different regimes (oscillations, vacillations, dynamo-bursts) depending on a value of the dynamo-number, the meridional circulation, and thickness of the spherical shell. We discuss features of these regimes and compare them with the observed features of the magnetic fields of the Sun, stars and Earth. We built theoretical paleomagnetic time scale and butterfly-diagrams for the helicity and toroidal magnetic field for different regimes.

Ribbons characterize magnetohydrodynamic magnetic fields better than lines: a lesson from dynamo theory

NASA Astrophysics Data System (ADS)

Blackman, Eric G.; Hubbard, Alexander

2014-08-01

Blackman and Brandenburg argued that magnetic helicity conservation in dynamo theory can in principle be captured by diagrams of mean field dynamos when the magnetic fields are represented by ribbons or tubes, but not by lines. Here, we present such a schematic ribbon diagram for the α2 dynamo that tracks magnetic helicity and provides distinct scales of large-scale magnetic helicity, small-scale magnetic helicity, and kinetic helicity involved in the process. This also motivates our construction of a new `2.5 scale' minimalist generalization of the helicity-evolving equations for the α2 dynamo that separately allows for these three distinct length-scales while keeping only two dynamical equations. We solve these equations and, as in previous studies, find that the large-scale field first grows at a rate independent of the magnetic Reynolds number RM before quenching to an RM-dependent regime. But we also show that the larger the ratio of the wavenumber where the small-scale current helicity resides to that of the forcing scale, the earlier the non-linear dynamo quenching occurs, and the weaker the large-scale field is at the turnoff from linear growth. The harmony between the theory and the schematic diagram exemplifies a general lesson that magnetic fields in magnetohydrodynamic are better visualized as two-dimensional ribbons (or pairs of lines) rather than single lines.

Large scale magnetic fields from torsion modes and massive photon inflation

NASA Astrophysics Data System (ADS)

Garcia de Andrade, L. C.

2017-10-01

Previously, Barrow and Tsagas (2008 Phys. Rev. D 77 107302) showed that a slower decay of magnetic fields are present in open Friedmann universes, with traditional Maxwell equations. In their paper magnetic fields of the order of B˜ 10-33~G , which are far below the value required to seed galactic dynamos, were obtained. In this paper, galactic dynamo seeds of the order of B˜ 10-23~G are obtained from massive electrodynamics in an Einstein-Cartan-Proca expanding universe of de Sitter type. Slow decay of magnetic fields in photon-torsion coupling in quantum electrodynamics (Garcia de Andrade 2011 Phys. Lett. B 468 28) have been recently shown by the author Garcia de Andrade (2012 Phys. Lett. B 711 143) to also not be able to seed galactic dynamos. Torsion modes are constrained by the field equations. Spacetime torsion is shown to be explicitly responsible for the slow decay of a cosmic magnetic field. In the absence of massive photon torsion coupling the magnetic field decay is of the order B˜ t-\\frac{3{2}} , when torsion turns on B˜ t-1.2 . The pure massive-photon-torsion contribution amplifies the magnetic field by B_torsion˜ t0.1 which characterizes an extremely slow magnetic dynamo action due to purely torsion gravitational effects. Recently Barrow, Tsagas and Yamamoto (2012 Phys. Rev. D 86 023535) have obtained superadiabatic amplification of B-fields in Friedmann open cosmology which lies within {10-20~G} and 10-12~G which falls very comfortably within the limits to seed galactic dynamos. The are other simple solutions where a B-field decays as B˜ a-1 , a relatively weak photon-torsion coupling approximation. These solutions are obtained for de Sitter and Friedmann metrics. Numerical values as displayed in this new version of the paper specifically for GUT phases of inflation with and without massive photons; without photons we obtain the well known value of GR which is B_GUT˜ 1048~G while for the values with massive photons one obtains B_GUT/γ˜ 10-3.5Gauss . At the present time one obtains B_today≤slant{10-21.6~G} and with massive photons one obtains B_today/γ˜ 10-1.8~G , which is unfortunately much stronger than the value obtained by astronomical observations of 10-9~G .

Gravitational dynamos and the low-frequency geomagnetic secular variation.

PubMed

Olson, P

2007-12-18

Self-sustaining numerical dynamos are used to infer the sources of low-frequency secular variation of the geomagnetic field. Gravitational dynamo models powered by compositional convection in an electrically conducting, rotating fluid shell exhibit several regimes of magnetic field behavior with an increasing Rayleigh number of the convection, including nearly steady dipoles, chaotic nonreversing dipoles, and chaotic reversing dipoles. The time average dipole strength and dipolarity of the magnetic field decrease, whereas the dipole variability, average dipole tilt angle, and frequency of polarity reversals increase with Rayleigh number. Chaotic gravitational dynamos have large-amplitude dipole secular variation with maximum power at frequencies corresponding to a few cycles per million years on Earth. Their external magnetic field structure, dipole statistics, low-frequency power spectra, and polarity reversal frequency are comparable to the geomagnetic field. The magnetic variability is driven by the Lorentz force and is characterized by an inverse correlation between dynamo magnetic and kinetic energy fluctuations. A constant energy dissipation theory accounts for this inverse energy correlation, which is shown to produce conditions favorable for dipole drift, polarity reversals, and excursions.

Gravitational dynamos and the low-frequency geomagnetic secular variation

PubMed Central

Olson, P.

2007-01-01

Self-sustaining numerical dynamos are used to infer the sources of low-frequency secular variation of the geomagnetic field. Gravitational dynamo models powered by compositional convection in an electrically conducting, rotating fluid shell exhibit several regimes of magnetic field behavior with an increasing Rayleigh number of the convection, including nearly steady dipoles, chaotic nonreversing dipoles, and chaotic reversing dipoles. The time average dipole strength and dipolarity of the magnetic field decrease, whereas the dipole variability, average dipole tilt angle, and frequency of polarity reversals increase with Rayleigh number. Chaotic gravitational dynamos have large-amplitude dipole secular variation with maximum power at frequencies corresponding to a few cycles per million years on Earth. Their external magnetic field structure, dipole statistics, low-frequency power spectra, and polarity reversal frequency are comparable to the geomagnetic field. The magnetic variability is driven by the Lorentz force and is characterized by an inverse correlation between dynamo magnetic and kinetic energy fluctuations. A constant energy dissipation theory accounts for this inverse energy correlation, which is shown to produce conditions favorable for dipole drift, polarity reversals, and excursions. PMID:18048345

Paleomagnetic Records of Ancient Core Dynamos

NASA Astrophysics Data System (ADS)

Tikoo, S. M.

2018-05-01

We review paleomagnetic results that constrain the field intensities and longevities of ancient core dynamos operating within the Moon as well as within the parent bodies of several meteorite classes.

The postsunset vertical plasma drift during geomagnetic storms and its effects on the generation of equatorial spread F

NASA Astrophysics Data System (ADS)

Huang, C.

2017-12-01

We will present two distinct phenomena related to the postsunset vertical plasma drift and equatorial spread F (ESF) observed by the Communication/Navigation Outage Forecasting System satellite over six years. The first phenomenon is the behavior of the prereversal enhancement (PRE) of the vertical plasma drift during geomagnetic storms. Statistically, storm-time disturbance dynamo electric fields cause the PRE to decrease from 30 to 0 m/s when Dst changes from -60 to -100 nT, but the PRE does not show obvious variations when Dst varies from 0 to -60 nT. The observations show that the storm activities affect the evening equatorial ionosphere only for Dst < -60 nT and that the dynamo electric field becomes dominant during the storm recovery phase. The second phenomenon is the relationship between the PRE and the generation of ESF. It is found that the occurrence of large-amplitude ESF irregularities is well correlated with the PRE and that the occurrence of small-amplitude ESF irregularities does not show a clear pattern at low solar activity but is anti-correlated with large-amplitude irregularities and the PRE at moderate solar activity. That is, the months and longitudes with high occurrence probability of large-amplitude irregularities are exactly those with low occurrence probability of small-amplitude irregularities, and vice versa. The generation of large-amplitude ESF irregularities is controlled by the PRE, and the generation of small-amplitude ESF irregularities may be caused by gravity waves and other disturbances, rather than by the PRE.

The Global Solar Dynamo

NASA Astrophysics Data System (ADS)

Cameron, R. H.; Dikpati, M.; Brandenburg, A.

2017-09-01

A brief summary of the various observations and constraints that underlie solar dynamo research are presented. The arguments that indicate that the solar dynamo is an alpha-omega dynamo of the Babcock-Leighton type are then shortly reviewed. The main open questions that remain are concerned with the subsurface dynamics, including why sunspots emerge at preferred latitudes as seen in the familiar butterfly wings, why the cycle is about 11 years long, and why the sunspot groups emerge tilted with respect to the equator (Joy's law). Next, we turn to magnetic helicity, whose conservation property has been identified with the decline of large-scale magnetic fields found in direct numerical simulations at large magnetic Reynolds numbers. However, magnetic helicity fluxes through the solar surface can alleviate this problem and connect theory with observations, as will be discussed.

Simulations of the kinematic dynamo onset of spherical Couette flows with smooth and rough boundaries.

PubMed

Finke, K; Tilgner, A

2012-07-01

We study numerically the dynamo transition of an incompressible electrically conducting fluid filling the gap between two concentric spheres. In a first series of simulations, the fluid is driven by the rotation of a smooth inner sphere through no-slip boundary conditions, whereas the outer sphere is stationary. In a second series a volume force intended to simulate a rough surface drives the fluid next to the inner sphere within a layer of thickness one-tenth of the gap width. We investigate the effect of the boundary layer thickness on the dynamo threshold in the turbulent regime. The simulations show that the boundary forcing simulating the rough surface lowers the necessary rotation rate, which may help to improve spherical dynamo experiments.

Dynamo Action in a Quasi-Keplerian Taylor-Couette Flow.

PubMed

Guseva, Anna; Hollerbach, Rainer; Willis, Ashley P; Avila, Marc

2017-10-20

We numerically compute the flow of an electrically conducting fluid in a Taylor-Couette geometry where the rotation rates of the inner and outer cylinders satisfy Ω_{o}/Ω_{i}=(r_{o}/r_{i})^{-3/2}. In this quasi-Keplerian regime, a nonmagnetic system would be Rayleigh stable for all Reynolds numbers Re, and the resulting purely azimuthal flow incapable of kinematic dynamo action for all magnetic Reynolds numbers Rm. For Re = 10^{4} and Rm=10^{5}, we demonstrate the existence of a finite-amplitude dynamo, whereby a suitable initial condition yields mutually sustaining turbulence and magnetic fields, even though neither could exist without the other. This dynamo solution results in significantly increased outward angular momentum transport, with the bulk of the transport being by Maxwell rather than Reynolds stresses.

Turbulent convection in liquid metal with and without rotation

PubMed Central

King, Eric M.; Aurnou, Jonathan M.

2013-01-01

The magnetic fields of Earth and other planets are generated by turbulent, rotating convection in liquid metal. Liquid metals are peculiar in that they diffuse heat more readily than momentum, quantified by their small Prandtl numbers, . Most analog models of planetary dynamos, however, use moderate fluids, and the systematic influence of reducing is not well understood. We perform rotating Rayleigh–Bénard convection experiments in the liquid metal gallium over a range of nondimensional buoyancy forcing and rotation periods (E). Our primary diagnostic is the efficiency of convective heat transfer . In general, we find that the convective behavior of liquid metal differs substantially from that of moderate fluids, such as water. In particular, a transition between rotationally constrained and weakly rotating turbulent states is identified, and this transition differs substantially from that observed in moderate fluids. This difference, we hypothesize, may explain the different classes of magnetic fields observed on the Gas and Ice Giant planets, whose dynamo regions consist of and fluids, respectively. PMID:23569262

CONVECTIVE BABCOCK-LEIGHTON DYNAMO MODELS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Miesch, Mark S.; Brown, Benjamin P., E-mail: miesch@ucar.edu

We present the first global, three-dimensional simulations of solar/stellar convection that take into account the influence of magnetic flux emergence by means of the Babcock-Leighton (BL) mechanism. We have shown that the inclusion of a BL poloidal source term in a convection simulation can promote cyclic activity in an otherwise steady dynamo. Some cycle properties are reminiscent of solar observations, such as the equatorward propagation of toroidal flux near the base of the convection zone. However, the cycle period in this young sun (rotating three times faster than the solar rate) is very short ({approx}6 months) and it is unclearmore » whether much longer cycles may be achieved within this modeling framework, given the high efficiency of field generation and transport by the convection. Even so, the incorporation of mean-field parameterizations in three-dimensional convection simulations to account for elusive processes such as flux emergence may well prove useful in the future modeling of solar and stellar activity cycles.« less

Role of boundary conditions in helicoidal flow collimation: Consequences for the von Kármán sodium dynamo experiment.

PubMed

Varela, J; Brun, S; Dubrulle, B; Nore, C

2015-12-01

We present hydrodynamic and magnetohydrodynamic (MHD) simulations of liquid sodium flow with the PLUTO compressible MHD code to investigate influence of magnetic boundary conditions on the collimation of helicoidal motions. We use a simplified cartesian geometry to represent the flow dynamics in the vicinity of one cavity of a multiblades impeller inspired by those used in the Von-Kármán-sodium (VKS) experiment. We show that the impinging of the large-scale flow upon the impeller generates a coherent helicoidal vortex inside the blades, located at a distance from the upstream blade piloted by the incident angle of the flow. This vortex collimates any existing magnetic field lines leading to an enhancement of the radial magnetic field that is stronger for ferromagnetic than for conducting blades. The induced magnetic field modifies locally the velocity fluctuations, resulting in an enhanced helicity. This process possibly explains why dynamo action is more easily triggered in the VKS experiment when using soft iron impellers.

23. DETAIL VIEW OF THE C. 1905 DAYTON GLOBE TURBINE ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

23. DETAIL VIEW OF THE C. 1905 DAYTON GLOBE TURBINE THAT IS STILL EXTANT IN THE SECOND FLUME (COUNTING FROM THE DOWNSTREAM FLUME TOWARDS THE UPSTREAM FLUMES) BENEATH THE GENERATOR FLOOR. THE DYNAMO THIS TURBINE WAS ATTACHED TO IS NO LONGER EXTANT. - Potomac Power Plant, On West Virginia Shore of Potomac River, about 1 mile upriver from confluence with Shenandoah River, Harpers Ferry, Jefferson County, WV

Differential Rotation within the Earth's Outer Core

NASA Technical Reports Server (NTRS)

Hide, R.; Boggs, D. H.; Dickey, J. O.

1998-01-01

Non-steady differential rotation drive by bouyancy forces within the Earth's liquid outer core (OC) plays a key role not only in the generation of the main geomagnetic field by the magnetohydrodynamic (MHD) dynamo process but also in the excitation of irregular fluctuations in the angular speed of rotation of the overlying solid mantle, as evidenced by changes in the length of the day (LOD) on decadal and longer timescales (1-8).

First Spectroscopic Detection of Surface Structures on a Normal A-Type Star - The Case of Vega

NASA Astrophysics Data System (ADS)

Böhm, Torsten

2018-04-01

For the first time the existence of spots on the surface of the intermediate mass star Vega has been shown. This unexpected result sets new important constraints on the stellar evolution of intermediate mass stars and in particular on the magnetic field generation mechanisms. Vega (α Lyrae) is an intermediate mass star (spectral class A0) in rapid rotation (Prot = 0.68 d). Since more than 150 years it is a stability reference for photometry. Despite the fact that very small sporadic light variations had been announced in the past, no periodicity had been detected in its light curve. In 2009 a very faint magnetic field has been detected on Vega (Lignières et al., 2009, A&A, 500L, 41) and subsequently also on other stars of the same spectral class (A). While the solar magnetic field is generated by a dynamo mechanism in its convective envelope, the origin of magnetic field in stars exempt of convective envelopes, such as Vega, remains mysterious. One of the characteristics of the solar dynamo is its temporal variability revealed by the appearance or disappearance of solar spots. Are there similar structures on the surface of Vega? 2015 A&A, 577, 64. & Nature Research Highlights

Comments on the photospheric dynamo model of Henoux and Somov

NASA Technical Reports Server (NTRS)

Melrose, D. B.; Khan, J. I.

1989-01-01

A detailed model for a photospheric dynamo has been presented by Henoux and Somov (1987), who used the three-fluid model to treat the properties of the weakly ionized plasma. Only the equations for the two ionized components were solved. The equation for the neutral component is considered, and it is argued that the model is unacceptable becaused of an implied impossibly large unbalanced stress on the neutral gas. It is argued more generally that all existing photospheric dynamo models are untenable.

Fast-dynamo action in unsteady flows and maps in three dimensions

NASA Technical Reports Server (NTRS)

Bayly, B. J.; Childress, S.

1987-01-01

Unsteady fast-dynamo action is obtained in a family of stretch-fold-shear maps applied to a spatially periodic magnetic field in three dimensions. Exponential growth of a mean field in the limit of vanishing diffusivity is demonstrated by a numerical method which alternates instantaneous deformations with molecular diffusion over a finite time interval. Analysis indicates that the dynamo is a coherent feature of the large scales, essentially independent of the cascade of structure to small scales.

Aircraft Measurements for Understanding Air-Sea Coupling and Improving Coupled Model Predictions

DTIC Science & Technology

2013-09-30

physical parameterizations of the coupled model in various large-scale forcing conditions. OBJECTIVES The NOAA WP-3D efforts of DYNAMO /LASP intend...various phases of the MJO; 3) to extend point measurements on island and ships to a broader area near the DYNAMO region; and 4) To obtain a suite of...upper ocean characteristics from a large number of AXBT/AXCTD data. In addition, as one of the unique measurement strategy of LASP/ DYNAMO WP-3D project

Earth's Paleomagnetosphere and Planetary Habitability

NASA Astrophysics Data System (ADS)

Tarduno, J. A.; Blackman, E. G.; Oda, H.; Bono, R. K.; Carroll-Nellenback, J.; Cottrell, R. D.; Nimmo, F.

2017-12-01

The geodynamo is thought to play an important role in protecting Earth's hydrosphere, vital for life as we know it, from loss due to the erosive potential of the solar wind. Here we consider the mechanisms and history of this shielding. A larger core dynamo magnetic field strength provides more pressure to abate the solar wind dynamic pressure, increasing the magnetopause radius. However, the larger magnetopause also implies a larger collecting area for solar wind flux during phases of magnetic reconnection. The important variable is not mass capture but energy transfer, which does not scale linearly with magnetosphere size. Moreover, the ordered field provides the magnetic topology for recapturing atmospheric components in the opposite hemisphere such that the net global loss might not be greatly affected. While a net protection role for magnetospheres is suggested, forcing by the solar wind will change with stellar age. Paleomagnetism utilizing the single silicate crystal approach, defines a relatively strong field some 3.45 billion years ago (the Paleoarchean), but with a reduced magnetopause of 5 Earth radii, implying the potential for some atmospheric loss. Terrestrial zircons from the Jack Hills (Western Australia) and other localities host magnetic inclusions, whose magnetization has now been recorded by a new generation of ultra-sensitive 3-component SQUID magnetometer (U. Rochester) and SQUID microscope (GSJ/AIST). Paleointensity data suggest the presence of a terrestrial dynamo and magnetic shielding for Eoarchean to Hadean times, at ages as old as 4.2 billion years ago. However, the magnetic data suggest that for intervals >100,000 years long, magnetopause standoff distances may have reached 3 to 4 Earth radii or less. The early inception of the geodynamo, which probably occurred shortly after the lunar-forming impact, its continuity, and an early robust hydrosphere, appear to be key ingredients for Earth's long-term habitability.

Dynamics of ionospheric disturbances during the 17-19 March 2015 geomagnetic storm over East Asia

NASA Astrophysics Data System (ADS)

Polekh, N.; Zolotukhina, N.; Kurkin, V.; Zherebtsov, G.; Shi, J.; Wang, G.; Wang, Z.

2017-12-01

Based on vertical sounding data from nine ionosondes located at 19-66°N, 100-130°E we investigated the latitude-temporal dynamics of ionospheric disturbances during the 17-19 March 2015 severe two-step geomagnetic storm, and compared it with temporal dynamics of total electron content (TEC) profiles along 120°E. The phenomena that accompanied the main and early recovery storm phases were in particular focused on in this study. The distinct storm-related ionospheric disturbances began 2.5, 4 and 5 h after onset of the storm main phase at subauroral, middle and low latitudes, respectively. To clarify the main mechanisms causing the disturbances at different latitudes we compared the changes in ionospheric parameters and TEC profiles with changes in the northern polar cap index and geomagnetic field in the vicinity of 120°E. The equatorward shift of the main ionospheric trough (MIT) and diffuse precipitations zone accompanied by an increase in precipitating particle flux was found to have a substantial influence on the subauroral ionosphere during the main and early recovery phases. The thermosphere Joule heating due to westward and polarized jets led to an increase in neutral wind velocity and generation of disturbed dynamo electric field. The strengthened wind was the main reason of the positive ionospheric disturbance observed at middle latitudes in the evening on 17 March. The further enhancement of magnetospheric convection caused the displacement of MIT and its associated negative ionospheric disturbance to middle latitudes. At low latitudes superposition of prompt penetration and disturbed dynamo electric fields play the decisive role in the ionosphere behavior till the end of the early recovery phase.

10 CFR 1002.11 - Description of official seal.

Code of Federal Regulations, 2013 CFR

2013-01-01

... is emblazoned a gold-colored symbolic sun, atom, oil derrick, windmill, and dynamo. It is crested by... increasing demands for energy. The sun, atom, oil derrick, windmill, and dynamo serve as representative...

10 CFR 1002.11 - Description of official seal.

Code of Federal Regulations, 2010 CFR

2010-01-01

... is emblazoned a gold-colored symbolic sun, atom, oil derrick, windmill, and dynamo. It is crested by... increasing demands for energy. The sun, atom, oil derrick, windmill, and dynamo serve as representative...

10 CFR 1002.11 - Description of official seal.

Code of Federal Regulations, 2011 CFR

2011-01-01

... is emblazoned a gold-colored symbolic sun, atom, oil derrick, windmill, and dynamo. It is crested by... increasing demands for energy. The sun, atom, oil derrick, windmill, and dynamo serve as representative...

10 CFR 1002.11 - Description of official seal.

Code of Federal Regulations, 2014 CFR

2014-01-01

... is emblazoned a gold-colored symbolic sun, atom, oil derrick, windmill, and dynamo. It is crested by... increasing demands for energy. The sun, atom, oil derrick, windmill, and dynamo serve as representative...

10 CFR 1002.11 - Description of official seal.

Code of Federal Regulations, 2012 CFR

2012-01-01

... is emblazoned a gold-colored symbolic sun, atom, oil derrick, windmill, and dynamo. It is crested by... increasing demands for energy. The sun, atom, oil derrick, windmill, and dynamo serve as representative...

Core solidification and dynamo evolution in a mantle-stripped planetesimal

NASA Astrophysics Data System (ADS)

Scheinberg, A.; Elkins-Tanton, L. T.; Schubert, G.; Bercovici, D.

2016-01-01

The physical processes active during the crystallization of a low-pressure, low-gravity planetesimal core are poorly understood but have implications for asteroidal magnetic fields and large-scale asteroidal structure. We consider a core with only a thin silicate shell, which could be analogous to some M-type asteroids including Psyche, and use a parameterized thermal model to predict a solidification timeline and the resulting chemical profile upon complete solidification. We then explore the potential strength and longevity of a dynamo in the planetesimal's early history. We find that cumulate inner core solidification would be capable of sustaining a dynamo during solidification, but less power would be available for a dynamo in an inward dendritic solidification scenario. We also model and suggest limits on crystal settling and compaction of a possible cumulate inner core.

Dynamos driven by weak thermal convection and heterogeneous outer boundary heat flux

NASA Astrophysics Data System (ADS)

Sahoo, Swarandeep; Sreenivasan, Binod; Amit, Hagay

2016-01-01

We use numerical dynamo models with heterogeneous core-mantle boundary (CMB) heat flux to show that lower mantle lateral thermal variability may help support a dynamo under weak thermal convection. In our reference models with homogeneous CMB heat flux, convection is either marginally supercritical or absent, always below the threshold for dynamo onset. We find that lateral CMB heat flux variations organize the flow in the core into patterns that favour the growth of an early magnetic field. Heat flux patterns symmetric about the equator produce non-reversing magnetic fields, whereas anti-symmetric patterns produce polarity reversals. Our results may explain the existence of the geodynamo prior to inner core nucleation under a tight energy budget. Furthermore, in order to sustain a strong geomagnetic field, the lower mantle thermal distribution was likely dominantly symmetric about the equator.

REVIEWS OF TOPICAL PROBLEMS: The hydromagnetic dynamo as the source of planetary, solar, and galactic magnetism

NASA Astrophysics Data System (ADS)

Zeldovich, Ya B.; Ruzmaĭkin, A. A.

1987-06-01

The magnetism of most celestial bodies, i.e., planets, stars, and galaxies, is of hydromagnetic origin. The turbulent hydromagnetic dynamo is the principal mechanism whereby the magnetic field is amplified and maintained, and the theory of this phenomenon has advanced significantly in recent years. This review discusses applications of the theory of the turbulent dynamo to real objects, taking the Sun, the Earth, and the Galaxy as examples. Most of the discussion is concentrated on the large-scale magnetic field averaged over turbulent fluctuations. The average field is amplified and maintained by the average helicity of turbulent motion and large-scale shear flows such as differential rotation. The dynamo theory explains striking phenomena such as geomagnetic field reversal, the solar cycle, and the ring and bisymmetric structure of spiral galaxies.

Reconnecting flux-rope dynamo.

PubMed

Baggaley, Andrew W; Barenghi, Carlo F; Shukurov, Anvar; Subramanian, Kandaswamy

2009-11-01

We develop a model of the fluctuation dynamo in which the magnetic field is confined to thin flux ropes advected by a multiscale model of turbulence. Magnetic dissipation occurs only via reconnection of the flux ropes. This model can be viewed as an implementation of the asymptotic limit R_{m}-->infinity for a continuous magnetic field, where magnetic dissipation is strongly localized to small regions of strong-field gradients. We investigate the kinetic-energy release into heat mediated by the dynamo action, both in our model and by solving the induction equation with the same flow. We find that a flux-rope dynamo is an order of magnitude more efficient at converting mechanical energy into heat. The probability density of the magnetic energy release in reconnections has a power-law form with the slope -3 , consistent with the solar corona heating by nanoflares.

Approaching a realistic force balance in geodynamo simulations

PubMed Central

Yadav, Rakesh K.; Gastine, Thomas; Christensen, Ulrich R.; Wolk, Scott J.; Poppenhaeger, Katja

2016-01-01

Earth sustains its magnetic field by a dynamo process driven by convection in the liquid outer core. Geodynamo simulations have been successful in reproducing many observed properties of the geomagnetic field. However, although theoretical considerations suggest that flow in the core is governed by a balance between Lorentz force, rotational force, and buoyancy (called MAC balance for Magnetic, Archimedean, Coriolis) with only minute roles for viscous and inertial forces, dynamo simulations must use viscosity values that are many orders of magnitude larger than in the core, due to computational constraints. In typical geodynamo models, viscous and inertial forces are not much smaller than the Coriolis force, and the Lorentz force plays a subdominant role; this has led to conclusions that these simulations are viscously controlled and do not represent the physics of the geodynamo. Here we show, by a direct analysis of the relevant forces, that a MAC balance can be achieved when the viscosity is reduced to values close to the current practical limit. Lorentz force, buoyancy, and the uncompensated (by pressure) part of the Coriolis force are of very similar strength, whereas viscous and inertial forces are smaller by a factor of at least 20 in the bulk of the fluid volume. Compared with nonmagnetic convection at otherwise identical parameters, the dynamo flow is of larger scale and is less invariant parallel to the rotation axis (less geostrophic), and convection transports twice as much heat, all of which is expected when the Lorentz force strongly influences the convection properties. PMID:27790991

SHEAR-DRIVEN DYNAMO WAVES IN THE FULLY NONLINEAR REGIME

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pongkitiwanichakul, P.; Nigro, G.; Cattaneo, F.

2016-07-01

Large-scale dynamo action is well understood when the magnetic Reynolds number ( Rm ) is small, but becomes problematic in the astrophysically relevant large Rm limit since the fluctuations may control the operation of the dynamo, obscuring the large-scale behavior. Recent works by Tobias and Cattaneo demonstrated numerically the existence of large-scale dynamo action in the form of dynamo waves driven by strongly helical turbulence and shear. Their calculations were carried out in the kinematic regime in which the back-reaction of the Lorentz force on the flow is neglected. Here, we have undertaken a systematic extension of their work tomore » the fully nonlinear regime. Helical turbulence and large-scale shear are produced self-consistently by prescribing body forces that, in the kinematic regime, drive flows that resemble the original velocity used by Tobias and Cattaneo. We have found four different solution types in the nonlinear regime for various ratios of the fluctuating velocity to the shear and Reynolds numbers. Some of the solutions are in the form of propagating waves. Some solutions show large-scale helical magnetic structure. Both waves and structures are permanent only when the kinetic helicity is non-zero on average.« less

Mean-field dynamo action in renovating shearing flows.

PubMed

Kolekar, Sanved; Subramanian, Kandaswamy; Sridhar, S

2012-08-01

We study mean-field dynamo action in renovating flows with finite and nonzero correlation time (τ) in the presence of shear. Previous results obtained when shear was absent are generalized to the case with shear. The question of whether the mean magnetic field can grow in the presence of shear and nonhelical turbulence, as seen in numerical simulations, is examined. We show in a general manner that, if the motions are strictly nonhelical, then such mean-field dynamo action is not possible. This result is not limited to low (fluid or magnetic) Reynolds numbers nor does it use any closure approximation; it only assumes that the flow renovates itself after each time interval τ. Specifying to a particular form of the renovating flow with helicity, we recover the standard dispersion relation of the α(2)Ω dynamo, in the small τ or large wavelength limit. Thus mean fields grow even in the presence of rapidly growing fluctuations, surprisingly, in a manner predicted by the standard quasilinear closure, even though such a closure is not strictly justified. Our work also suggests the possibility of obtaining mean-field dynamo growth in the presence of helicity fluctuations, although having a coherent helicity will be more efficient.

Relating Stellar Cycle Periods to Dynamo Calculations

NASA Technical Reports Server (NTRS)

Tobias, S. M.

1998-01-01

Stellar magnetic activity in slowly rotating stars is often cyclic, with the period of the magnetic cycle depending critically on the rotation rate and the convective turnover time of the star. Here we show that the interpretation of this law from dynamo models is not a simple task. It is demonstrated that the period is (unsurprisingly) sensitive to the precise type of non-linearity employed. Moreover the calculation of the wave-speed of plane-wave solutions does not (as was previously supposed) give an indication of the magnetic period in a more realistic dynamo model, as the changes in length-scale of solutions are not easily captured by this approach. Progress can be made, however, by considering a realistic two-dimensional model, in which the radial length-scale of waves is included. We show that it is possible in this case to derive a more robust relation between cycle period and dynamo number. For all the non-linearities considered in the most realistic model, the magnetic cycle period is a decreasing function of IDI (the amplitude of the dynamo number). However, discriminating between different non-linearities is difficult in this case and care must therefore be taken before advancing explanations for the magnetic periods of stars.

CORLISS ENGINE WITH DYNAMO AND ROPE DRIVE. LOCATION UNIDENTIFIED. PHOTOCOPY ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

CORLISS ENGINE WITH DYNAMO AND ROPE DRIVE. LOCATION UNIDENTIFIED. PHOTOCOPY OF c. 1900 VIEW. From the collection of the Manchester Historic Association, Manchester, N. H. - Amoskeag Millyard, Canal Street, Manchester, Hillsborough County, NH

Dynamic Monte Carlo simulations of radiatively accelerated GRB fireballs

NASA Astrophysics Data System (ADS)

Chhotray, Atul; Lazzati, Davide

2018-05-01

We present a novel Dynamic Monte Carlo code (DynaMo code) that self-consistently simulates the Compton-scattering-driven dynamic evolution of a plasma. We use the DynaMo code to investigate the time-dependent expansion and acceleration of dissipationless gamma-ray burst fireballs by varying their initial opacities and baryonic content. We study the opacity and energy density evolution of an initially optically thick, radiation-dominated fireball across its entire phase space - in particular during the Rph < Rsat regime. Our results reveal new phases of fireball evolution: a transition phase with a radial extent of several orders of magnitude - the fireball transitions from Γ ∝ R to Γ ∝ R0, a post-photospheric acceleration phase - where fireballs accelerate beyond the photosphere and a Thomson-dominated acceleration phase - characterized by slow acceleration of optically thick, matter-dominated fireballs due to Thomson scattering. We quantify the new phases by providing analytical expressions of Lorentz factor evolution, which will be useful for deriving jet parameters.

Current drive by helicon waves

DOE Office of Scientific and Technical Information (OSTI.GOV)

Paul, Manash Kumar; Bora, Dhiraj; ITER Organization, Cadarache Centre-building 519, 131008 St. Paul-Lez-Durance

2009-01-01

Helicity in the dynamo field components of helicon wave is examined during the novel study of wave induced helicity current drive. Strong poloidal asymmetry in the wave magnetic field components is observed during helicon discharges formed in a toroidal vacuum chamber of small aspect ratio. High frequency regime is chosen to increase the phase velocity of helicon waves which in turn minimizes the resonant wave-particle interactions and enhances the contribution of the nonresonant current drive mechanisms. Owing to the strong poloidal asymmetry in the wave magnetic field structures, plasma current is driven mostly by the dynamo-electric-field, which arise due tomore » the wave helicity injection by helicon waves. Small, yet finite contribution from the suppressed wave-particle resonance cannot be ruled out in the operational regime examined. A brief discussion on the parametric dependence of plasma current along with numerical estimations of nonresonant components is presented. A close agreement between the numerical estimation and measured plasma current magnitude is obtained during the present investigation.« less

The Atmospheric Dynamics of Alpha Tau (K5 III) -- Clues to Understanding the Magnetic Dynamo

NASA Technical Reports Server (NTRS)

Carpenter Kenneth G.

2008-01-01

Using HST/GHRS, HST/STIS and FUSE archival data for (alpha) Tau and the CHIANTI spectroscopic code, we have derived line shifts, volumetric emission measures, and plasma density estimates, and calculated filling factors for a number of UV lines forming between 10,000 K and 300,000 K in the outer atmosphere of this red giant star. The data suggest the presence of low-temperature extended regions and high-temperature compact regions, associated with magnetically open and closed structures in the stellar atmosphere, respectively. The signatures of UV lines from Alpha Tau can be consistently understood via a model of upward-traveling Alfven waves in a gravitationally stratified atmosphere. These wakes cause non-thermal broadening in UV lines due to unresolved wave motions and downward plasma motions in compact magnetic loops heated by resonant .4lf\\en wave heating. We discuss implications of this interpretation for understanding the nature of magnetic dynamos operating in late-type giants.

Long-lived magnetism on chondrite parent bodies

NASA Astrophysics Data System (ADS)

Shah, Jay; Bates, Helena C.; Muxworthy, Adrian R.; Hezel, Dominik C.; Russell, Sara S.; Genge, Matthew J.

2017-10-01

We present evidence for both early- and late-stage magnetic activity on the CV and L/LL parent bodies respectively from chondrules in Vigarano and Bjurböle. Using micro-CT scans to re-orientate chondrules to their in-situ positions, we present a new micron-scale protocol for the paleomagnetic conglomerate test. The paleomagnetic conglomerate test determines at 95% confidence, whether clasts within a conglomerate were magnetized before or after agglomeration, i.e., for a chondritic meteorite whether the chondrules carry a pre- or post-accretionary remanent magnetization. We found both meteorites passed the conglomerate test, i.e., the chondrules had randomly orientated magnetizations. Vigarano's heterogeneous magnetization is likely of shock origin, due to the 10 to 20 GPa impacts that brecciated its precursor material on the parent body and transported it to re-accrete as the Vigarano breccia. The magnetization was likely acquired during the break-up of the original body, indicating a CV parent body dynamo was active ∼9 Ma after Solar System formation. Bjurböle's magnetization is due to tetrataenite, which transformed from taenite as the parent body cooled to below 320 °C, when an ambient magnetic field imparted a remanence. We argue either the high intrinsic anisotropy of tetrataenite or brecciation on the parent body manifests as a randomly orientated distribution, and a L/LL parent body dynamo must have been active at least 80 to 140 Ma after peak metamorphism. Primitive chondrites did not originate from entirely primitive, never molten and/or differentiated parent bodies. Primitive chondrite parent bodies consisted of a differentiated interior sustaining a long-lived magnetic dynamo, encrusted by a layer of incrementally accreted primitive meteoritic material. The different ages of carbonaceous and ordinary chondrite parent bodies might indicate a general difference between carbonaceous and ordinary chondrite parent bodies, and/or formation location in the protoplanetary disk.

Understanding The Behavior Of The Sun'S Large Scale Magnetic Field And Its Relation With The Meridional Flow

NASA Astrophysics Data System (ADS)

Hazra, Gopal

2018-02-01

In this thesis, various studies leading to better understanding of the 11-year solar cycle and its theoretical modeling with the flux transport dynamo model are performed. Although this is primarily a theoretical thesis, there is a part dealing with the analysis of observational data. The various proxies of solar activity (e.g., sunspot number, sunspot area and 10.7 cm radio flux) from various observatory including the sunspot area records of Kodaikanal Observatory have been analyzed to study the irregular aspects of solar cycles and an analysis has been carried out on the correlation between the decay rate and the next cycle amplitude. The theoretical analysis starts with explaining how the magnetic buoyancy has been treated in the flux transport dynamo models, and advantages and disadvantages of different treatments. It is found that some of the irregular properties of the solar cycle in the decaying phase can only be well explained using a particular treatment of the magnetic buoyancy. Next, the behavior of the dynamo with the different spatial structures of the meridional flow based on recent helioseismology results has been studied. A theoretical model is constructed considering the back reaction due to the Lorentz force on the meridional flows which explains the observed variation of the meridional flow with the solar cycle. Finally, some results with 3D FTD models are presented. This 3D model is developed to handle the Babcock-Leighton mechanism and magnetic buoyancy more realistically than previous 2D models and can capture some important effects connected with the subduction of the magnetic field in polar regions, which are missed in 2D surface flux transport models. This 3D model is further used to study the evolution of the magnetic fields due to a turbulent non-axisymmetric velocity field and to compare the results with the results obtained by using a simple turbulent diffusivity coefficient.

Constructing a Plastic Bottle Wind Turbine as a Practical Aid for Learning about Using Wind Energy to Generate Electricity

ERIC Educational Resources Information Center

Appleyard, S. J.

2009-01-01

A simple horizontal axis wind turbine can be easily constructed using a 1.5 l PET plastic bottle, a compact disc and a small dynamo. The turbine operates effectively at low wind speeds and has a rotational speed of 500 rpm at a wind speed of about 14 km h[superscript -1]. The wind turbine can be used to demonstrate the relationship between open…

IUE observations of the M dwarfs CM Draconis and Rossiter 137 B - Magnetic activity at saturated levels

NASA Technical Reports Server (NTRS)

Vilhu, O.; Brandenburg, A.; Ambruster, C. W.; Neff, J. E.; Linsky, J. L.

1989-01-01

IUE observations of two active M dwarfs with known rotation rate or age and presumed to be almost totally convective are presented. The first of these stars, CM Draconis (Gl 630.1), is an old Population II binary with its components in tidally induced rapid rotation (P = 1.27 d, dM 4 + dM 4). The other one, Rossiter 137 B, forms with HD 36705 (AB Dor) a visual pair of young active stars. The activity of CM Dra is due to the forced rotation in a close binary, while Rst 137 B is assumed to rotate fast enough to generate its magnetica activity. These results are compared with those for M dwarfs, particularly AU Mic and YZ CMi, that have known rotational periods and measured ultraviolet emission line fluxes. The chromospheric-coronal saturation levels of cool dwarfs between 0.3 less than B-V less than 1.6 is determined. It is found that saturated F stars have stronger chromospheres than saturated M stars, but the opposite is true for the corona. Results of these observations seem to favor a rotation-dependent distributed dynamo generating magnetic flux in totally convective stars.

DYNAMO-HIA–A Dynamic Modeling Tool for Generic Health Impact Assessments

PubMed Central

Lhachimi, Stefan K.; Nusselder, Wilma J.; Smit, Henriette A.; van Baal, Pieter; Baili, Paolo; Bennett, Kathleen; Fernández, Esteve; Kulik, Margarete C.; Lobstein, Tim; Pomerleau, Joceline; Mackenbach, Johan P.; Boshuizen, Hendriek C.

2012-01-01

Background Currently, no standard tool is publicly available that allows researchers or policy-makers to quantify the impact of policies using epidemiological evidence within the causal framework of Health Impact Assessment (HIA). A standard tool should comply with three technical criteria (real-life population, dynamic projection, explicit risk-factor states) and three usability criteria (modest data requirements, rich model output, generally accessible) to be useful in the applied setting of HIA. With DYNAMO-HIA (Dynamic Modeling for Health Impact Assessment), we introduce such a generic software tool specifically designed to facilitate quantification in the assessment of the health impacts of policies. Methods and Results DYNAMO-HIA quantifies the impact of user-specified risk-factor changes on multiple diseases and in turn on overall population health, comparing one reference scenario with one or more intervention scenarios. The Markov-based modeling approach allows for explicit risk-factor states and simulation of a real-life population. A built-in parameter estimation module ensures that only standard population-level epidemiological evidence is required, i.e. data on incidence, prevalence, relative risks, and mortality. DYNAMO-HIA provides a rich output of summary measures – e.g. life expectancy and disease-free life expectancy – and detailed data – e.g. prevalences and mortality/survival rates – by age, sex, and risk-factor status over time. DYNAMO-HIA is controlled via a graphical user interface and is publicly available from the internet, ensuring general accessibility. We illustrate the use of DYNAMO-HIA with two example applications: a policy causing an overall increase in alcohol consumption and quantifying the disease-burden of smoking. Conclusion By combining modest data needs with general accessibility and user friendliness within the causal framework of HIA, DYNAMO-HIA is a potential standard tool for health impact assessment based on epidemiologic evidence. PMID:22590491

Chiral dynamos and magnetogenesis induced by torsionful Maxwell-Chern Simons electrodynamics

NASA Astrophysics Data System (ADS)

de Andrade, L. C. Garcia

2018-03-01

Recently chiral anomalous currents have been investigated by Boyarsky et al. and Brandenburg et al. with respect to applications to the early universe. In this paper we show that these magnetic field anomalies, which can give rise to dynamo magnetic field amplification can also be linked to spacetime torsion through the use of a chemical potential and Maxwell electrodynamics with torsion firstly proposed by de Sabbata and Gasperini. When the axial torsion is constant this electrodynamics acquires the form of a Maxwell-Chern-Simmons (MCS) equations where the chiral current appears naturally and the zero component of torsion plays the role of a chemical potential, while the other components play the role of anisotropic conductivity. The chiral dynamo equation in torsionful spacetime is derived here from MSC electrodynamics. Here we have used a recently derived a torsion LV bound of T0˜ {10^{-26}} GeV and the constraint that this chiral magnetic field is a seed for galactic dynamo. This estimate is weaker than the one obtained from the chiral battery seed of ˜ {10^{30}} G without making use of Cartan torsion. The torsion obtained here was derived at 500 pc coherence scale. When a chiral MF is forced to seed a galactic dynamo one obtains a yet weaker MF, of the order of B˜ {10^{12}} G, which is the value of a MF at nucleosynthesis. By the use of chiral dynamo equations from parity-violating torsion one obtains a seed field of B˜ {10^{27}} G, which is a much stronger MF closer to the one obtained by making use of chiral batteries. Chiral vortical currents in non-Riemannian spacetimes derived in Riemannian spaces previously by Flaschi and Fukushima are extended to include minimal coupling with torsion. The present universe yields B˜ {10^{-24}} G, still sufficient to seed galactic dynamos.

Earth's dynamo limit of predictability controlled by magnetic dissipation

NASA Astrophysics Data System (ADS)

Lhuillier, Florian; Aubert, Julien; Hulot, Gauthier

2011-08-01

To constrain the forecast horizon of geomagnetic data assimilation, it is of interest to quantify the range of predictability of the geodynamo. Following earlier work in the field of dynamic meteorology, we investigate the sensitivity of numerical dynamos to various perturbations applied to the magnetic, velocity and temperature fields. These perturbations result in some errors, which affect all fields in the same relative way, and grow at the same exponential rate λ=τ-1e, independent of the type and the amplitude of perturbation. Errors produced by the limited resolution of numerical dynamos are also shown to produce a similar amplification, with the same exponential rate. Exploring various possible scaling laws, we demonstrate that the growth rate is mainly proportional to an advection timescale. To better understand the mechanism responsible for the error amplification, we next compare these growth rates with two other dynamo outputs which display a similar dependence on advection: the inverse τ-1SV of the secular-variation timescale, characterizing the secular variation of the observable field produced by these dynamos; and the inverse (τmagdiss)-1 of the magnetic dissipation time, characterizing the rate at which magnetic energy is produced to compensate for Ohmic dissipation in these dynamos. The possible role of viscous dissipation is also discussed via the inverse (τkindiss)-1 of the analogous viscous dissipation time, characterizing the rate at which kinetic energy is produced to compensate for viscous dissipation. We conclude that τe tends to equate τmagdiss for dynamos operating in a turbulent regime with low enough Ekman number, and such that τmagdiss < τkindiss. As these conditions are met in the Earth's outer core, we suggest that τe is controlled by magnetic dissipation, leading to a value τe=τmagdiss≈ 30 yr. We finally discuss the consequences of our results for the practical limit of predictability of the geodynamo.

Global modeling of storm-time thermospheric dynamics and electrodynamics

NASA Astrophysics Data System (ADS)

Fuller-Rowell, T. J.; Richmond, A. D.; Maruyama, N.

Understanding the neutral dynamic and electrodynamic response of the upper atmosphere to geomagnetic storms, and quantifying the balance between prompt penetration and disturbance dynamo effects, are two of the significant challenges facing us today. This paper reviews our understanding of the dynamical and electrodynamic response of the upper atmosphere to storms from a modeling perspective. After injection of momentum and energy at high latitude during a geomagnetic storm, the neutral winds begin to respond almost immediately. The high-latitude wind system evolves quickly by the action of ion drag and the injection of kinetic energy; however, Joule dissipation provides the bulk of the energy source to change the dynamics and electrodynamics globally. Impulsive energy injection at high latitudes drives large-scale gravity waves that propagate globally. The waves transmit pressure gradients initiating a change in the global circulation. Numerical simulations of the coupled thermosphere, ionosphere, plasmasphere, and electrodynamic response to storms indicate that although the wind and waves are dynamic, with significant apparent "sloshing" between the hemispheres, the net effect is for an increased equatorward wind. The dynamic changes during a storm provide the conduit for many of the physical processes that ensue in the upper atmosphere. For instance, the increased meridional winds at mid latitudes push plasma parallel to the magnetic field to regions of different composition. The global circulation carries molecular rich air from the lower thermosphere upward and equatorward, changing the ratio of atomic and molecular neutral species, and changing loss rates for the ionosphere. The storm wind system also drives the disturbance dynamo, which through plasma transport modifies the strength and location of the equatorial ionization anomaly peaks. On a global scale, the increased equatorward meridional winds, and the generation of zonal winds at mid latitudes via the Coriolis effects, produce a current system opposing the normal quiet-time Sq current system. At the equator, the storm-time zonal electric fields reduce or reverse the normal upward and downward plasma drift on the dayside and nightside, respectively. In the numerical simulations, on the dayside, the disturbance dynamo appears fairly uniform, whereas at night a stronger local time dependence is apparent with increased upward drift between midnight and dawn. The simulations also indicate the possibility for a rapid dynamo response at the equator, within 2 h of storm onset, before the arrival of the large-scale gravity waves. All these wind-driven processes can result in dramatic ionospheric changes during storms. The disturbance dynamo can combine and interact with the prompt penetration of magnetospheric electric fields to the equator.

37. VIEW OF TWO STEEL DYNAMOS IN STARBOARD CORNER OF ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

37. VIEW OF TWO STEEL DYNAMOS IN STARBOARD CORNER OF ENGINE ROOM, SHOWING VESSEL'S ELECTRICAL DISTRIBUTION PANEL TO THE RIGHT - Steam Schooner WAPAMA, Kaiser Shipyard No. 3 (Shoal Point), Richmond, Contra Costa County, CA