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Sample records for dynamo generation due

  1. Tsunami: ocean dynamo generator.

    PubMed

    Sugioka, Hiroko; Hamano, Yozo; Baba, Kiyoshi; Kasaya, Takafumi; Tada, Noriko; Suetsugu, Daisuke

    2014-01-01

    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. PMID:24399356

  2. Tsunami: Ocean dynamo generator

    PubMed Central

    Sugioka, Hiroko; Hamano, Yozo; Baba, Kiyoshi; Kasaya, Takafumi; Tada, Noriko; Suetsugu, Daisuke

    2014-01-01

    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. PMID:24399356

  3. 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. PMID:17183319

  4. Generation and Properties of Large-Scale Non-axisymmetric Magnetic Fields by Solar Dynamo

    NASA Astrophysics Data System (ADS)

    Pipin, Valery; Kosovichev, Alexander

    2015-08-01

    Large-scale non-axisymmetric magnetic fields generated by the solar dynamo, and presumably responsible for the phenomenon of "active longitudes", play an important role in the distribution of solar activity and flares. By calculating 3D mean-field dynamo models, we show that nonlinear coupling between axisymmetric and non-axisymmetric modes, e.g. due to the magnetic feedback on the alpha-effect (see, e.g., [1]), can maintain a large-scale non-axisymmetric dynamo process. Non-axisymmetric random fluctuations of dynamo parameters can be another source for the non-axisymmetric magnetic fields on the Sun. Such fluctuations can provide a mechanism of the magnetic energy transfer from the global field to the non-axisymmetric modes. It is shown that the rotational periods of the non-axisymmetric field correspond to the dynamo process operating in the subsurface shear layer which is located in the range of depths 0.85-0.95R. We find that the magnetic helicity conservation quenches generation of the non-axisymmetric dynamo modes as well as it does for the axisymmetric dynamo. It is concluded that the 3D mean-field non-axisymmetric dynamo models can potentially explain the observed distribution of the solar magnetic activity.1. Moss, D.,Non-axisymmetric solar magnetic fields, 1999, MNRAS, 306, 300On 3/18/2015 2:29 PM, Valery Pipin wrote:

  5. 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.

  6. The Effect of Material Properties on Dynamo Generation in Planets

    NASA Astrophysics Data System (ADS)

    Vilim, Ryan

    2015-10-01

    In this thesis I use a three dimensional numerical dynamo model to explore the effect of novel material properties and core states on magnetic field generation in the planet Mercury, and in rocky extra-solar planets. In the first part of this work I focus on the recent evidence of pressure induced metallisation in materials which commonly comprise planetary mantles. In this scenario the materials which make up the lower mantle of a planet conduct electricity with a conductivity similar to that of iron. I show that a metallised mantle changes the way in which magnetic field is generated by providing a new source of magnetic shear between the fluid outer core and the solid mantle. I then show that this has the effect of making planetary magnetic fields more difficult to observe from Earth. The second and third parts of this work focus on the planet Mercury. First, I incorporate recent evidence of buoyancy sources mid-way through Mercury's liquid core (known as "snow zones") to show that they can explain the weak observed magnetic field of Mercury. In a second project on Mercury I test whether recent evidence of a dense solid layer at the top of Mercury's core, attributed to a solid, electrically conducting layer of FeS, could help explain Mercury's weak magnetic field. I find that the addition of this layer causes the dynamo to generate a strong, dipolar magnetic field, which does not match the observations made by the MESSENGER spacecraft.

  7. The Effect of an Electrically Conducting Lower Mantle on Dynamo Generated Planetary Magnetic Fields

    NASA Astrophysics Data System (ADS)

    Vilim, R.; Stanley, S.

    2012-12-01

    Recent studies have shown that the lower mantles of Earth[1], Mercury[2], and large terrestrial exoplanets[3, 4] may be good conductors of electricity. This raises questions about the effect of an electrically conducting lower mantle on magnetic field generation in these planets. A core dynamo generated magnetic field can interact with an electrically conducting mantle in two ways. First, magnetic fields lines can be be frozen into the solid mantle. The flows in the core can then stretch the magnetic field lines at the core mantle boundary increasing their strength. Second, any field observed at the surface will be attenuated due to the screening effect, which preferentially attenuates the components of the magnetic field that vary quickest in time. We use a numerical dynamo model to investigate the effect of a conducting mantle on dynamo generated planetary magnetic fields. [1] Ohta, K., Cohen, R. E., Hirose, K., Haule, K., Shimizu, K., and Ohishi, Y. (2012). Experimental and Theoretical Evidence for Pressure-Induced Metallization in FeO with Rocksalt-Type Structure. PRL, 108, 026403 [2] Smith, D. E., Zuber, M. T., Phillips, R. J., Solomon, S. C., Hauck, S. A. II, Lemoine, F. G., Mazarico, E., Neumann, G.A., Peale, S.J., Margot, J.L., Johnson C.L., Torrence, M.H., Perry, M.E., Rowlands D.D., Goossens, S., Head, J.W., Taylor, A.H. (2012). Gravity Field and Internal Structure of Mercury from MESSENGER. Science [3] Nellis, W. J. (2011). Metallic liquid hydrogen and likely Al2O3 metallic glass. The European Physical Journal Special Topics, 196, 121-130 [4] Tsuchiya, T. (2011). Prediction of a hexagonal SiO2 phase affecting stabilities of MgSiO3 and CaSiO3 at multimegabar pressures. PNAS, 108, 1252-1255

  8. 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.

  9. 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. PMID:26551120

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

    DOE PAGESBeta

    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

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

    SciTech Connect

    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 nature of velocity shear, the suggested mechanism may help explain the generation of large-scale magnetic fields across a wide range of astrophysical objects.

  12. Buoyant Magnetic Loops Generated by Global Convective Dynamo Action

    NASA Astrophysics Data System (ADS)

    Nelson, Nicholas J.; Brown, Benjamin P.; Sacha Brun, A.; Miesch, Mark S.; Toomre, Juri

    2014-02-01

    Our global 3D simulations of convection and dynamo action in a Sun-like star reveal that persistent wreaths of strong magnetism can be built within the bulk of the convention zone. Here we examine the characteristics of buoyant magnetic structures that are self-consistently created by dynamo action and turbulent convective motions in a simulation with solar stratification but rotating at three times the current solar rate. These buoyant loops originate within sections of the magnetic wreaths in which turbulent flows amplify the fields to much higher values than is possible through laminar processes. These amplified portions can rise through the convective layer by a combination of magnetic buoyancy and advection by convective giant cells, forming buoyant loops. We measure statistical trends in the polarity, twist, and tilt of these loops. Loops are shown to preferentially arise in longitudinal patches somewhat reminiscent of active longitudes in the Sun, although broader in extent. We show that the strength of the axisymmetric toroidal field is not a good predictor of the production rate for buoyant loops or the amount of magnetic flux in the loops that are produced.

  13. 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. PMID:17358418

  14. An alpha-omega-dynamo with an alpha-effect due to magnetostrophic waves

    NASA Astrophysics Data System (ADS)

    Schmitt, D.

    1987-03-01

    The effects of the latitude dependence of the dynamic alpha-effect on the solution of equations of alpha-omega-dynamos are investigated. The equations of kinematic rotationally symmetric alpha-omega-dynamos are evaluated using the spherical solar dynamo model of Deinzer and Stix (1971), in which the induction effects, differential rotation, and alpha-effect act in two separate infinitesimal thin shells. Butterfly diagrams are derived and analyzed. It is observed that the diagram has two branches: the ordinary sunspot branch, migrating from midlatitudes toward the equator during the cycle, and the polar branch, which migrates from the midlatitudes toward the pole. It is also found that, in order to obtain the correct propagation direction of the two dynamos, the alpha of the magnetostrophic waves requires a rotation decreasing with depth. The influence of various locations of the induction layers of alpha- and omega-effect are examined.

  15. The breakdown of dipolar magnetic field generation in planetary dynamo models (Invited)

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Magnetic field measurements show that each dynamo in our solar system is distinct with field strengths that differ by many orders of magnitude and morphologies that range from titled dipoles to nearly axisymmetric dipole-quadrupoles to non-axisymmetric multipoles. The characteristics of stellar and astrophysical magnetic fields are similarly wide-ranging. Towards understanding the diversity of this dynamo zoo, we investigate the breakdown of dipole-dominated magnetic fields in a suite of planetary dynamo and otherwise identical non-magnetic simulations where the convective vigor and rotation rate are varied systematically. This survey considers models with Prandtl number Pr=1, magnetic Prandtl numbers up to Pm = 5, Ekman numbers in the range 1e-3 <= E <= 1e-5, and Rayleigh numbers from near onset to more than 1000 times critical. A sharp transition from dipolar to multipolar dynamos is observed in models with moderate to high Ekman numbers. This breakdown of the dipole happens when inertial and viscous forces become comparable and coincides with a degradation of helicity in the flow, which also occurs when no magnetic field is present. Large-scale poloidal magnetic fields in these models then appear to be generated by a viscously controlled, macro-scale alpha-effect. Our lowest Ekman number cases, however, suggest that the dynamics may be changing as the viscous force decreases. Since viscosity is expected to be negligible in planetary and stellar interiors, an extrapolation of our results implies that moderate Ekman number models may not simulate the physical mechanisms of magnetic field generation in these bodies correctly.

  16. Compositionally Driven Dynamos

    NASA Astrophysics Data System (ADS)

    Soderlund, K. M.; Schubert, G.

    2014-12-01

    It is generally believed that compositional convection driven by inner core solidification is the main driver of the geodynamo. Thermal evolution considerations make it likely that compositional convection is also behind the present dynamos of Mercury and Ganymede as well as the early dynamos in the Moon, Mars and smaller solar system bodies. Compositional buoyancy can arise in several different ways, for example, through inner core solidification and FeS flotation with upward mixing and through freezing out and sinking of iron snow near the core-mantle boundary or deeper within the core. The mode of core cooling and freezing depends on conditions of temperature and pressure in the core and the concentration of light elements such as sulfur. Different distributions of compositional buoyancy will give rise to different patterns of core convection and dynamo magnetic fields. We report here the first results of a systematic study of the distribution of compositional buoyancy on the dynamo-generated magnetic fields, with an emphasis on Mars' core evolution due to iron rain.

  17. Dynamo generated magnetic configurations in accretion discs and the nature of quasi-periodic oscillations in accreting binary systems

    NASA Astrophysics Data System (ADS)

    Moss, D.; Sokoloff, D.; Suleimanov, V.

    2016-04-01

    Context. Magnetic fields are important for accretion disc structure. Magnetic fields in a disc system may be transported with the accreted matter. They can be associated with either the central body and/or jet, and be fossil or dynamo excited in situ. Aims: We consider dynamo excitation of magnetic fields in accretion discs of accreting binary systems in an attempt to clarify possible configurations of dynamo generated magnetic fields. We first model the entire disc with realistic radial extent and thickness using an alpha-quenching non-linearity. We then study the simultaneous effect of feedback from the Lorentz force from the dynamo-generated field. Methods: We perform numerical simulations in the framework of a relatively simple mean-field model which allows the generation of global magnetic configurations. Results: We explore a range of possibilities for the dynamo number, and find quadrupolar-type solutions with irregular temporal oscillations that might be compared to observed rapid luminosity fluctuations. The dipolar symmetry models with Rα< 0 have lobes of strong toroidal field adjacent to the rotation axis that could be relevant to jet launching phenomena. Conclusions: We have explored and extended the solutions known for thin accretion discs.

  18. 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. PMID:26727160

  19. Effect of small scale motions on dynamo actions generated by the Beltrami-like flows

    NASA Astrophysics Data System (ADS)

    Xu, Mingtian

    2016-08-01

    The geodynamo and solar dynamo are driven by the turbulent flows which involve motions of various scales. Of particular interest is what role is played by the small scale motions in these dynamos. In this paper, the integral equation approach is employed to investigate the effect of the small scale motions on dynamo actions driven by multiscale Beltrami-like flows in a cylindrical vessel. The result shows that some small scale motions can trigger a transition of a dynamo from a steady to an unsteady state. Our results also show that when the poloidal components of the small and large scale flows share the same direction in the equatorial plane, the small scale flows have more positive or less detrimental effect on the onsets of the dynamo actions in comparison with the case that the poloidal components have different directions. These findings shed light on the effect of the small scale turbulence on dynamo actions.

  20. Moffatt-drift-driven large-scale dynamo due to fluctuations with non-zero correlation times

    NASA Astrophysics Data System (ADS)

    Singh, Nishant K.

    2016-07-01

    We present a theory of large-scale dynamo action in a turbulent flow that has stochastic, zero-mean fluctuations of the $\\alpha$ parameter. Particularly interesting is the possibility of the growth of the mean magnetic field due to Moffatt drift, which is expected to be finite in a statistically anisotropic turbulence. We extend the Kraichnan-Moffatt model to explore effects of finite memory of $\\alpha$ fluctuations, in a spirit similar to that of Sridhar & Singh (2014), hereafter SS14. Using the first-order smoothing approximation, we derive a linear integro-differential equation governing the dynamics of the large-scale magnetic field, which is non-perturbative in the $\\alpha$-correlation time $\\tau_{\\alpha}$. We recover earlier results in the exactly solvable white-noise (WN) limit where the Moffatt drift does not contribute to the dynamo growth/decay. To study finite memory effects, we reduce the integro-differential equation to a partial differential equation by assuming that the $\\tau_{\\alpha}$ be small but nonzero and the large-scale magnetic field is slowly varying. We derive the dispersion relation and provide explicit expression for the growth rate as a function of four independent parameters. When $\\tau_{\\alpha}\

  1. The role of rotation in the evolution of dynamo-generated magnetic fields in Super Earths

    NASA Astrophysics Data System (ADS)

    Zuluaga, Jorge I.; Cuartas, Pablo A.

    2012-01-01

    Planetary magnetic fields could impact the evolution of planetary atmospheres and have a role in the determination of the required conditions for the emergence and evolution of life (planetary habitability). We study here the role of rotation in the evolution of dynamo-generated magnetic fields in massive Earth-like planets, Super Earths (1-10 M⊕). Using the most recent thermal evolution models of Super Earths (Gaidos, E., Conrad, C.P., Manga, M., Hernlund, J. [2010]. Astrophys. J. 718, 596-609; Tachinami, C., Senshu, H., Ida, S. [2011]. Astrophys. J. 726, 70) and updated scaling laws for convection-driven dynamos, we predict the evolution of the local Rossby number. This quantity is one of the proxies for core magnetic field regime, i.e. non-reversing dipolar, reversing dipolar and multipolar. We study the dependence of the local Rossby number and hence the core magnetic field regime on planetary mass and rotation rate. Previous works have focused only on the evolution of core magnetic fields assuming rapidly rotating planets, i.e. planets in the dipolar regime. In this work we go further, including the effects of rotation in the evolution of planetary magnetic field regime and obtaining global constraints to the existence of intense protective magnetic fields in rapidly and slowly rotating Super Earths. We find that the emergence and continued existence of a protective planetary magnetic field is not only a function of planetary mass but also depend on rotation rate. Low-mass Super Earths ( M ≲ 2 M⊕) develop intense surface magnetic fields but their lifetimes will be limited to 2-4 Gyrs for rotational periods larger than 1-4 days. On the other hand and also in the case of slowly rotating planets, more massive Super Earths ( M ≳ 2 M⊕) have weak magnetic fields but their dipoles will last longer. Finally we analyze tidally locked Super Earths inside and outside the habitable zone of GKM stars. Using the results obtained here we develop a classification of

  2. An Experimental MHD Dynamo

    SciTech Connect

    Forest, C. B.

    2002-11-15

    The project is designed to understand current and magnetic field generation in plasmas and other magnetohydrodynamic systems. The experiments will investigate the generation of a dynamo using liquid Na.

  3. HELICITY CONSERVATION IN NONLINEAR MEAN-FIELD SOLAR DYNAMO

    SciTech Connect

    Pipin, V. V.; Sokoloff, D. D.; Zhang, H.; Kuzanyan, K. M.

    2013-05-01

    It is believed that magnetic helicity conservation is an important constraint on large-scale astrophysical dynamos. In this paper, we study a mean-field solar dynamo model that employs two different formulations of the magnetic helicity conservation. In the first approach, the evolution of the averaged small-scale magnetic helicity is largely determined by the local induction effects due to the large-scale magnetic field, turbulent motions, and the turbulent diffusive loss of helicity. In this case, the dynamo model shows that the typical strength of the large-scale magnetic field generated by the dynamo is much smaller than the equipartition value for the magnetic Reynolds number 10{sup 6}. This is the so-called catastrophic quenching (CQ) phenomenon. In the literature, this is considered to be typical for various kinds of solar dynamo models, including the distributed-type and the Babcock-Leighton-type dynamos. The problem can be resolved by the second formulation, which is derived from the integral conservation of the total magnetic helicity. In this case, the dynamo model shows that magnetic helicity propagates with the dynamo wave from the bottom of the convection zone to the surface. This prevents CQ because of the local balance between the large-scale and small-scale magnetic helicities. Thus, the solar dynamo can operate in a wide range of magnetic Reynolds numbers up to 10{sup 6}.

  4. The lunar dynamo.

    PubMed

    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. PMID:25477467

  5. 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.

  6. The energy coupling function and the power generated by the solar wind-magnetosphere dynamo

    NASA Technical Reports Server (NTRS)

    Kan, J. R.; Lee, L. C.; Akasofu, S.-I.

    1980-01-01

    A solar wind parameter epsilon, known as the energy coupling function, has been shown to correlate with the power consumption in the magnetosphere. It is shown in the present paper that the parameter epsilon can be identified semi-quantitatively as the dynamo power delivered from the solar wind to an open magnetosphere. This identification not only provides a theoretical basis for the energy coupling function, but also constitutes an observational verification of the solar wind-magnetosphere dynamo along the magnetotail. Moreover, one can now conclude that a substorm results when the dynamo power exceeds 10 to the 18th erg/s.

  7. 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

  8. Dynamo Transition

    SciTech Connect

    Verma, M. K.; Yadav, R.; Chandra, M.; Paul, S.; Wahi, P.

    2010-11-23

    In this article we review the experimental and numerical results related to the dynamo transitions. Recent experiments of Von Karman Sodium (VKS) exhibit various dynamo states including constant, time-periodic, and chaotic magnetic fields. Similarly pseudospectral simulations of dynamo show constant, time-periodic, quasiperiodic, and chaotic magnetic field configurations. One of the windows for the magnetic Prandtl number of unity shows period doubling route to chaos. Quasiperiodic route to chaos has been reported for the Prandtl number of 0.5. The dynamo simulations also reveal coexisting multiple attractors that were obtained for different initial conditions.

  9. Galactic dynamos

    NASA Astrophysics Data System (ADS)

    Moss, David

    There is a broad agreement between the predictions of galactic dynamo theory and observations; although there are still some unresolved difficulties, the theory appears to be robust. Now attention is turning from generic models to studies of particular features of the large-scale magnetic fields, and also to models for specific galaxies. The effects of noncircular flows, for example driven by the interaction of spiral arms and galactic bars with the dynamo, are of current interest.

  10. F-Region Dynamo Simulations at Low and Mid-Latitude

    NASA Astrophysics Data System (ADS)

    Maute, Astrid; Richmond, Arthur D.

    2016-07-01

    The " F-layer dynamo" or " F-region dynamo" concept was introduced by Rishbeth (Planet. Space Sci. 19(2):263-267, 1971a; 19(3):357-369, 1971b). F-region winds blow the plasma across magnetic field lines setting up transverse drifts and polarization electric fields leading to equatorial downward current during the daytime and upward current at dusk which were confirmed by satellite observations. In the daytime the F-region current can close through the highly conducting E-region. At night when the E-region conductivity is small the F-region dynamo generates polarization electric fields and is mainly responsible for the nighttime drift variations. In the evening the F-region dynamo is instrumental in generating an enhanced vertical drift, the pre-reversal enhancement. The current due to the F-region dynamo is larger at day than at night, but the F-region dynamo contributes approximately 10-15 % to the total current at day versus approximately 50 % at night (Rishbeth in J. Atmos. Sol.-Terr. Phys. 43(56):387-392, 1981). The F-region dynamo effects strongly depend on the Pedersen conductivity and therefore on the solar cycle. We will review the influence of the F-region dynamo on the ionosphere in general and particularly focus on the role it plays in generating ionospheric currents and magnetic perturbations at low-earth orbiting (LEO) satellite altitudes.

  11. 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.

  12. Stellar Dynamos

    NASA Astrophysics Data System (ADS)

    Charbonneau, Paul

    This chapter steps finally away from the sun and towards the stars, the idea being to apply the physical insight gained so far to see how much of stellar magnetism can be understood in terms of dynamo action. Dynamo action in the convective core of massive main-sequence stars is first considered and shown viable. For intermediate-mass main-sequence stars the fossil field hypothesis will carry the day, although possible dynamo alternatives are also briefly discussed. The extension of the solar dynamo models investigated in Chap. 3 (10.1007/978-3-642-32093-4_3) to other solar-type stars will first take us through an important detour in first having to understand rotational evolution in response to angular momentum loss in a magnetized wind. Dynamo action in fully convective stars comes next, and the chapter closes with an overview of the situation for pre- and post-main-sequence stars and compact objects, leading finally to the magnetic fields of galaxies and beyond.

  13. Secular variation of a metallic asteroid dynamo

    NASA Astrophysics Data System (ADS)

    Bryson, J. F. J.; Harrison, R. J.; Neufeld, J. A.; Nimmo, F.; Herrero-Albillos, J.; Kronast, F.; Weiss, B. P.

    2015-12-01

    The mechanisms by which inward core solidification may drive dynamo activity, and the properties of any fields that may result from this process, are highly uncertain. The fast cooling rates of the IVA iron meteorites suggest that their parent core had its silicate mantle removed by planetary collisions during the early solar system. Due to the resulting rapid radiative surface cooling, the IVA parent core solidified from the top-down, permitting a cold metallic crust that feasibly experienced fields generated by the hot interior liquid as it inwardly solidified. The IVA meteorites therefore potentially contain unique paleomagnetic information regarding top-down solidification. Through x-ray microscopy of the cloudy zone in the Steinbach and Chinautla meteorites and traditional paleomagnetic measurements on silicates extracted from the Steinbach, Bishop Canyon and São João Nepomuceno meteorites, we argue that the IVA parent core generated an intense (>100 μT) and secularly varying (time-scale <100 kyr) field during top-down solidification. These results show that certain iron meteorites are capable of having experienced dynamo fields, and that asteroids can generate directionally varying magnetic activity, strengthening claims that the fundamentals of dynamo activity are consistent across small and large bodies. Models of the thermochemical evolution and solidification of an unmantled core suggest that this field resulted from liquid motion induced by the repeated delamination and sinking of material from the base of the inwardly solidifying crust. This efficient dynamo generation mechanism was likely capable of readily creating magnetic activity at the slow cooling rates expected within mantled, inwardly solidifying cores (e.g., Mercury, Ganymede, many asteroids). Combining this observation with that of efficient solidification-driven dynamos during bottom-up asteroid core solidification, it is likely that magnetic activity was widespread in the early solar

  14. Formation of sunspots and active regions through the emergence of magnetic flux generated in a solar convective dynamo

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

    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 near surface layer radiation MHD simulations of magneto-convection and flux emergence with the MURaM code. The latter code simulates 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 come most probably from the intrinsic asymmetries in the emerging fields imposed at the bottom boundary, where the horizontal fields are already tilted and the leading sides of the emerging flux tubes are usually up against the downdraft lanes of the giant cells. It is also found that penumbrae with numerous filamentary structures form in regions of strong horizontal magnetic fields that naturally comes from the ongoing flux emergence. In contrast to previous models, the penumbrae and umbrae are divided by very sharp boarders, which is highly consistent with observations.

  15. Numerical models of galactic dynamos

    NASA Astrophysics Data System (ADS)

    Elstner, Detlef

    The state of the art for dynamo models in spiral galaxies is reviewed. The comparison of numerical models with special properties of observed magnetic fields yields constraints for the turbulent diffusivity and the α-effect. The derivation of the turbulence parameters from the vertical structure of the interstellar medium gives quite reasonable values for modelling the regular magnetic fields in galaxies with an α2Ω-dynamo. Considering the differences of the turbulence between spiral arms and interarm regions, the observed interarm magnetic fields are recovered in the numerical models due to the special properties of the α2Ω-dynamo.

  16. 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.

  17. Invisible dynamo in mean-field models

    NASA Astrophysics Data System (ADS)

    Reshetnyak, M. Yu.

    2016-07-01

    The inverse problem in a spherical shell to find the two-dimensional spatial distributions of the α-effect and differential rotation in a mean-field dynamo model has been solved. The derived distributions lead to the generation of a magnetic field concentrated inside the convection zone. The magnetic field is shown to have no time to rise from the region of maximum generation located in the lower layers to the surface in the polarity reversal time due to magnetic diffusion. The ratio of the maximum magnetic energy in the convection zone to its value at the outer boundary reaches two orders of magnitude or more. This result is important in interpreting the observed stellar and planetary magnetic fields. The proposed method of solving the inverse nonlinear dynamo problem is easily adapted for a wide class of mathematical-physics problems.

  18. Characterization of experimental dynamos

    NASA Astrophysics Data System (ADS)

    Peffley, Nicholas L.; Goumilevski, Alexei G.; Cawthrone, A. B.; Lathrop, Daniel P.

    2000-07-01

    Laboratory models of geophysical magnetic field production require new experi-mental characterization methods. Self-generating liquid metal magnetic dynamos are explored using two new experiments. Kinematic dynamo studies lead us to charac-terize the magnetic field dynamics in terms of eigenvalues and eigenfrequencies of the induction equation. Observing the decay of magnetic field pulses indicates the real part of the leading eigenvalue of the induction equation, while a chirp magnetic field diagnoses the imaginary part of the eigenvalue. Finally, a single-frequency applied magnetic field characterizes the structure of the velocity field. These measurements provide a new means to characterize and measure the approach to self-generation. We present data from numerical simulations and laboratory experiments using these techniques.

  19. 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.

  20. Dependence of Stellar Magnetic Activity Cycles on Rotational Period in a Nonlinear Solar-type Dynamo

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

    We study the turbulent generation of large-scale magnetic fields using nonlinear dynamo models for solar-type stars in the range of rotational periods from 14 to 30 days. Our models take into account nonlinear effects of dynamical quenching of magnetic helicity, and escape of magnetic field from the dynamo region due to magnetic buoyancy. The results show that the observed correlation between the period of rotation and the duration of activity cycles can be explained in the framework of a distributed dynamo model with a dynamical magnetic feedback acting on the turbulent generation from either magnetic buoyancy or magnetic helicity. We discuss implications of our findings for the understanding of dynamo processes operating in solar-like stars.

  1. Planetary Dynamos

    NASA Technical Reports Server (NTRS)

    Busse, F. H.

    1985-01-01

    The MAGSAT-program has added significantly to our knowledge of planetary magnetism. The accuracy of observations has been improved such that a reliable extrapolation of the magnetic field to the core surface is now much more feasible than it has been before, and the prospect of further MAGSAT missions raises the expectation that the time dependence of the geomagnetic field will be known with similar accuracy in the future. In the research support it has been attempted to develop dynamo theory with these applications in mind.

  2. Dynamos in stars and planets

    NASA Astrophysics Data System (ADS)

    Jones, C. A.

    2001-12-01

    There has been significant progress in the development of numerical geodynamo models over the last eight years. Advances in computer technology have made it possible to perform three-dimensional simulations, with thermal or compositional convection as the driving mechanism. These numerical simulations give reasonable results for the morphology and strength of the field at the core-mantle boundary, and the models are also capable of giving reversals and excursions which can be compared with paleomagnetic observations. Some useful constraints are obtained by considering the entropy balance and the ohmic dissipation. However, recent studies of plane layer dynamos suggest that the current generation of dynamo models have not yet reached the correct dynamical regime. A rather severe test of how well we understand the geodynamo comes when we try to apply the theory to the magnetic fields of stars and other planets. It becomes clear that not all dynamos are in the same dynamical regime. Some, like the Earth, are in magnetostrophic balance; others like the Sun, are not. Some are in a strong field regime with Elsasser number of order one, others (including some planetary dynamos) are not. Even within late type stars, the rotation rate strongly affects the dynamical regime that the dynamo operates in. The prospects for classifying the various type of convection driven dynamo, by elucidating the possible dynamical regimes, will be reviewed.

  3. The Solar Dynamos

    NASA Astrophysics Data System (ADS)

    Cattaneo, F.

    2000-05-01

    Magnetic activity on the Sun presents us with an interesting dichotomy. On large spatial and temporal scales the solar magnetic field displays a remarkable degree of organization. The 11 years cadence of the solar cycle, Hales' polarity law, and the systematic drift of the regions of emergence of active regions towards the equator throughout the solar cycle are all indicative of a powerful organizing process. On small spatial and temporal scales, the Solar magnetic field appears random and chaotic. It is interesting that recent advances in dynamo theory provide us with a unified approach to solar magnetic activity whereby both large and small scales emerge naturally as dynamo processes associated with rotationally constrained and unconstrained scales of motions in the convection zone (or directly below it). According to this view all coherent scales of motions produce magnetic structures of comparable coherence length. Those that are further endowed with lack of reflectional symmetry by virtue of being rotationally constrained are further associated with inverse cascades that can generate magnetic structures on larger scales still. The picture that emerges is one in which dynamo action proceeds on different time scales all over the convection zone. But only in very special regions, like for instance the solar tachocline, is the magnetic field organized on large scales. This idea provides a natural explanation for the origin of active regions, ephemeral regions, and intra--network fields.

  4. Persistence of the lunar dynamo: The role of compositional convection

    NASA Astrophysics Data System (ADS)

    Soderlund, K. M.; Schubert, G.; Scheinberg, A. L.

    2013-12-01

    Although the Moon does not currently have an active magnetic field, it does have magnetic anomalies associated with magnetized materials in the lunar crust. The crustal magnetic anomalies, originally detected during the Apollo era, have been mapped in detail by instruments on the Lunar Prospector and Kaguya (SELENE) spacecraft. Laboratory analyses of the magnetization of some lunar basalts returned from Apollo suggest that a field of approximately 10 microTesla persisted until 3.56 Gyr. Seismic measurements further imply that the Moon has a metallic core with both solid iron and liquid iron alloy components at present day. Thus, it is generally agreed that the early lunar magnetic field was generated by a dynamo. However, the mechanism driving the dynamo is a subject of current debate. Thermal convection alone is likely not sufficient to explain the duration of the dynamo because thermal evolution models predict lunar heat flow through the core to become sub-adiabatic within a few hundred million years. Alternatively, power for the dynamo may be derived from precession of the lunar mantle, impact-induced changes in the Moon's rotation rate, and/or compositional convection due to the formation of a solid inner core. Here, we will present results from a numerical dynamo model designed to simulate magnetic field generation at a number of different times during the Moon's history as predicted by thermal evolution models. These simulations will test the hypothesis that thermo-compositional convection can explain the persistence of the lunar dynamo and advance our understanding of how terrestrial bodies evolve through geologic time.

  5. 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.

  6. The solar dynamo wave in Parker's migratory dynamo

    NASA Astrophysics Data System (ADS)

    Kuzanyan, K. M.; Sokoloff, D.

    A kinematic -dynamo model of magnetic field generation in a thin convection shell with a non-uniform but sign-constant helicity profile for large dynamo numbers is considered in the framework of Parker's migratory dynamo. We obtained an asymptotic solution of equations governing the magnetic field with the form of an anharmonic travelling dynamo wave. This wave propagates over most latitudes of the solar hemisphere from high latitudes to the equator, and the amplitude of the magnetic field first increases and then decreases with propagation. However, over subpolar latitude the dynamo wave reverses; there the wave propagates polewards and decays with latitude. Butterfly diagrams are plotted and analysed; and these show that even a simple model may reveal some properties of the solar magnetic fields. There is an attractive opportunity to develop a more quantitatively precise model taking into account helioseismological data on the differential rotation and fitting the solar observational data on the magnetic field and turbulence, i.e. helicity.

  7. Dynamo and anomalous transport in the reversed field pinch

    SciTech Connect

    Prager, S.C.

    1998-08-01

    The reversed field pinch is an effective tool to study the macroscopic consequences of magnetic fluctuations, such as the dynamo effect and anomalous transport. Several explanations exist for the dynamo (the self-generation of plasma current)--the MHD dynamo, the kinetic dynamo, and the diamagnetic dynamo. There is some experimental evidence for each, particularly from measurements of ion velocity and electron pressure fluctuations. Magnetic fluctuations are known to produce energy and particle flux in the RFP core. Current profile control is able to decrease fluctuation-induced transport by a factor of five. Improved confinement regimes are also obtained at deep reversal and, possibly, with flow shear.

  8. Numerical Simulations of Boundary-Driven Dynamos

    NASA Astrophysics Data System (ADS)

    White, K.; Brummell, N.; Glatzmaier, G. A.

    2012-12-01

    An important topic of physics research is how magnetic fields are generated and maintained in the many astrophysical bodies where they are ubiquitously observed. Of particular interest, are reversals of magnetic fields of planets and stars, especially those of the Earth and the Sun. In an attempt to provide intuition on this problem, numerous physical dynamo experiments have been performed in different configurations. Recently, a tremendous breakthrough was made in the Von Karman sodium (VKS) experiments in France when the most realistic laboratory fluid dynamo to date was produced by driving an unconstrained flow in a cylinder of liquid sodium (Monchaux et al, 2007, PRL). One of the curiosities of the VKS experiment however is the effect of the composition of the impellers that drive the flow. Steel blades failed to produce a dynamo, but soft iron impellers, which have much higher magnetic permeability, succeeded. The role of the magnetic properties of the boundaries in boundary-driven dynamos is therefore clearly of interest. Kinematic and laminar numerical dynamo simulations (Giesecke et al, 2010, PRL & Gissinger et al, 2008 EPL) have shed some light but turbulent, nonlinear simulations are necessary. Roberts, Glatzmaier & Clune 2010 created a simplified model of the VKS setup by using three-dimensional numerical simulations in a spherical geometry with differential zonal motions of the boundary replacing the driving impellers of the VKS experiment. We have extended these numerical simulations further towards a more complete understanding of such boundary-forced dynamos. In particular, we have examined the effect of the magnetic boundary conditions - changes in the wall thickness, the magnetic permeability, and the electrical conductivity - on the mechanisms responsible for dynamo generation. Enhanced permeability, conductivity and wall thickness all help dynamo action to different degrees. We are further extending our investigations to asymmetric forcing to

  9. A Model of the Turbulent Electric Dynamo in Multi-Phase Media

    NASA Astrophysics Data System (ADS)

    Dementyeva, Svetlana; Mareev, Evgeny

    2016-04-01

    Many terrestrial and astrophysical phenomena witness the conversion of kinetic energy into electric energy (the energy of the quasi-stationary electric field) in conducting media, which is natural to treat as manifestations of electric dynamo by analogy with well-known theory of magnetic dynamo. Such phenomena include thunderstorms and lightning in the Earth's atmosphere and atmospheres of other planets, electric activity caused by dust storms in terrestrial and Martian atmospheres, snow storms, electrical discharges occurring in technological setups, connected with intense mixing of aerosol particles like in the milling industry. We have developed a model of the large-scale turbulent electric dynamo in a weakly conducting medium, containing two heavy-particle components. We have distinguished two main classes of charging mechanisms (inductive and non-inductive) in accordance with the dependence or independence of the electric charge, transferred during a particle collision, on the electric field intensity and considered the simplified models which demonstrate the possibility of dynamo realization and its specific peculiarities for these mechanisms. Dynamo (the large-scale electric field growth) appears due to the charge separation between the colliding and rebounding particles. This process is may be greatly intensified by the turbulent mixing of particles with different masses and, consequently, different inertia. The particle charge fluctuations themselves (small-scale dynamo), however, do not automatically mean growth of the large-scale electric field without a large-scale asymmetry. Such an asymmetry arises due to the dependence of the transferred charge magnitude on the electric field intensity in the case of the inductive mechanism of charge separation, or due to the gravity and convection for non-inductive mechanisms. We have found that in the case of the inductive mechanism the large-scale dynamo occurs if the medium conductivity is small enough while the

  10. STELLAR WIND INFLUENCE ON PLANETARY DYNAMOS

    SciTech Connect

    Heyner, Daniel; Glassmeier, Karl-Heinz; Schmitt, Dieter

    2012-05-10

    We examine the possible influence of early stellar wind conditions on the evolution of planetary dynamo action. In our model, the dynamo operates within a significant ambient magnetospheric magnetic field generated by the interaction between the stellar wind and the planetary magnetic field. This provides a negative feedback mechanism which quenches the dynamo growth. The external magnetic field magnitude which the dynamo experiences, and thus the strength of the quenching, depends on the stellar wind dynamic pressure. As this pressure significantly changes during stellar evolution, we argue that under early stellar system conditions the coupling between the stellar wind and the interior dynamics of a planet is much more important than has been thought up to now. We demonstrate the effects of the feedback coupling in the course of stellar evolution with a planet at a similar distance to the central star as Mercury is to the Sun.

  11. Pallasite paleomagnetism: Quiescence of a core dynamo

    NASA Astrophysics Data System (ADS)

    Nichols, Claire I. O.; Bryson, James F. J.; Herrero-Albillos, Julia; Kronast, Florian; Nimmo, Francis; Harrison, Richard J.

    2016-05-01

    Recent paleomagnetic studies of two Main Group pallasites, the Imilac and Esquel, have found evidence for a strong, late-stage magnetic field on the parent body. It has been hypothesized that this magnetic field was generated by a core dynamo, driven by compositional convection during core solidification. Cooling models suggest that the onset of core solidification occurred ∼200 Ma after planetary accretion. Prior to core solidification, a core dynamo may have been generated by thermal convection; however a thermal dynamo is predicted to be short-lived, with a duration of ∼10 Ma to ∼40 Ma after planetary accretion. These models predict, therefore, a period of quiescence between the thermally driven dynamo and the compositionally driven dynamo, when no core dynamo should be active. To test this hypothesis, we have measured the magnetic remanence recorded by the Marjalahti and Brenham pallasites, which based on cooling-rate data locked in any magnetic field signals present ∼95 Ma to ∼135 Ma after planetary accretion, before core solidification began. The cloudy zone, a region of nanoscale tetrataenite islands within a Fe-rich matrix was imaged using X-ray photoemission electron microscopy. The recovered distribution of magnetisation within the cloudy zone suggests that the Marjalahti and Brenham experienced a very weak magnetic field, which may have been induced by a crustal remanence, consistent with the predicted lack of an active core dynamo at this time. We show that the transition from a quiescent period to an active, compositionally driven dynamo has a distinctive paleomagnetic signature, which may be a crucial tool for constraining the time of core solidification on differentiated bodies, including Earth.

  12. Solar dynamo theory : Solar dynamo theory: a new look at the origin of small-scale magnetic fields

    NASA Astrophysics Data System (ADS)

    Cattaneo, Fausto; Hughes, David W.

    2001-06-01

    Fausto Cattaneo and David W Hughes delve beneath the surface of the Sun with numerical models of turbulent convection. Although magnetic dynamo action is traditionally associated with rotation, fast dynamo theory shows that chaotic flows, even without rotation, can act as efficient small-scale dynamos. Indeed, numerical simulations suggest that granular and supergranular convection may generate locally a substantial part of the field in the quiet photosphere.

  13. 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.

  14. 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.

  15. Planetary Dynamos: Magnetic Constraints on the Interior Structure and Evolution of a Planet

    NASA Astrophysics Data System (ADS)

    Tian, Bob Yunsheng

    Planetary magnetism is a phenomenon that not only protects humanity from the destructive forces of nature, but also provides us with a natural probe into our planet's deep interior. In this dissertation, I will explore some of the insights concerning planetary interiors that can be gained by combining the techniques of interior structure modelling with constraints provided by planetary dynamo theory. Applications to the dynamical history of the Moon, the interior evolution of Jovian planets, and predicted magnetic fields of planets in our solar system and beyond are considered under this framework. The inferred intensity and longevity of the lunar dynamo from paleomagnetic studies has led to the proposition of mechanical stirring, caused by differential rotation of the inner core and the mantle relative to the fluid outer core, as an energy source alternative to convection. Using fully three-dimensional magnetohydrodynamics (MHD) modelling techniques, I simulated the purported mechanism, and found it to reproduce not only the strength and longevity of the inferred lunar dynamo, but also its precipitous decline later in its history. For the Jovian planets, due to the lack of constraints, there are a wide range of acceptable interior models in the literature. By combining 1-D interior modelling techniques with constraints imposed by theories of the planet's dynamo, I was able to construct improved models of these planets' interior structure. The discrepancy between the pictures of the Neptunian interior suggested by dynamo models and by thermal evolution models motivated improvements on our current theories about multipolar magnetic field generation. Therefore, I determined some predictive scaling laws for the magnetic field morphologies of planets (and exoplanets) using parameter studies of interior structure and dynamo models. These results will aid in our understandings of the link between interior properties and observed magnetic field characteristics for planets

  16. The Solar Dynamo

    NASA Astrophysics Data System (ADS)

    Brandenburg, Axel; Tuominen, Ilkka

    The traditional -dynamo as a model for the solar cycle has been successful in explaining the butterfly diagram, phase relations between poloidal and toroidal field, and polar branch migration features. Observational and theoretical achievements in recent years have however shaken this picture. The current trend is towards dynamos operating in the overshoot region of the convection zone. Nevertheless, there are many open questions and a consistent picture has not been established. In this paper we compare recent approaches and discuss remaining problems.

  17. Final Technical Report for DOE DE-FG02-05ER54831 "Laboratory Studies of Dynamos."

    SciTech Connect

    Forest, Cary B.

    2014-11-06

    Laboratory Studies of Dynamos: Executive Summary. The self-generation of magnetic fields by astrophysical bodies like planets, stars, accretion disks, galaxies, and even galaxy clusters arises due to a mechanism referred to as a homogeneous dynamo. It is quite simple to demonstrate the generation of a magnetic fi eld from a rotating copper disk coupled with a coil of wire, a device known as the homopolar dynamo. The device works like a magnetic fi eld ampli er with a feedback circuit: the differential rotation of a metal disk past an infinitesimally small seed magnetic field induces currents in the disk which, when coupled to a coil winding, can amplify the field until it becomes strong enough to slow the rotation of the disk. What is remarkable is that the same type of circuit may be achieved in a flowing conducting fluid such as a liquid metal in the case of planetary dynamos or a plasma in the case of astrophysical dynamos. The complexity of describing planetary and stellar dynamos despite their ubiquity and the plethora of observational data from the Earth and the Sun motivates the demonstration of a laboratory homogenous dynamo. To create a homogenous dynamo, one first needs a su fficiently large, fast flow of a highly conducting fluid that the velocity shear in the fluid can bend magnetic field lines. With a high Rm-flow, the magnetic fi eld can be ampli ed by the stretching action provided by di fferential rotation. The other critical ingredient is a flow geometry that provides feedback so that the ampli ed eld reinforces the initial in nitesimal seed field - a mechanism that recreates the feedback provided by the coil of wire in the homopolar dynamo. In the Madison Dynamo Experiment, this combination of magnetic ampli cation and feedback is feasible in the simple geometry of two counter-rotating helical vortices in a 1 meter-diameter spherical vessel lled with liquid sodium. For an optimal helical pitch of the flow the threshold for exciting a dynamo is

  18. On large-scale dynamo action at high magnetic Reynolds number

    SciTech Connect

    Cattaneo, F.; Tobias, S. M.

    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 of 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.

  19. Hydromagnetic dynamos in rotating spherical fluid shells in dependence on the Prandtl number, density stratification and electromagnetic boundary conditions

    NASA Astrophysics Data System (ADS)

    Šoltis, Tomáš; Šimkanin, Ján

    2014-12-01

    We present an investigation of dynamo in a simultaneous dependence on the non-uniform stratification, electrical conductivity of the inner core and the Prandtl number. Computations are performed using the MAG dynamo code. In all the investigated cases, the generated magnetic fields are dipolar. Our results show that the dynamos, especially magnetic field structures, are independent in our investigated cases on the electrical conductivity of the inner core. This is in agreement with results obtained in previous analyses. The influence of non-uniform stratification is for our parameters weak, which is understandable because most of the shell is unstably stratified, and the stably stratified region is only a thin layer near the CMB. The teleconvection is not observed in our study. However, the influence of the Prandtl number is strong. The generated magnetic fields do not become weak in the polar regions because the magnetic field inside the tangent cylinder is always regenerated due to the weak magnetic diffusion.

  20. Dynamo theory and liquid metal MHD experiments

    NASA Astrophysics Data System (ADS)

    Lielausis, O.

    1994-06-01

    High values of magnetic Reynolds number Rm are characteristic not only to astrophysics, but also to other interesting objects, including liquid metal (LM) flows. LM experiments have been performed illustrating important predictions of the dynamo theory, for example, about the existence and features of the alpha effect. Consideration of so called 'laminar' dynamos provides a theoretical base for direct experimental realization and examination of the dynamo process. First step results, gathered a subcritical conditions, confirm the statement that self-excitation in LM experiments can be achieved practically today. In such devices as LM (sodium) cooled fast breeders Rm can reach values of up to 50 and specific MHD phenomena have been observed in operating fast reactors. Cautions against crisis like processes have been expressed. It is important for the dynamo theory to understand what kind of perturbed motion is able to coexist with the generated magnetic field. Fundamentally new ideas here are issuing from the theory of 2D MHD turbulence. LM MHD served for the first direct proves, confirming, that the predicted surprising features of 2D turbulence can be observed in reality. It is worth incorporating these already not new ideas in the dynamo theory. In such a way a field for new solutions could be established.

  1. 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.

  2. 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. PMID:27035981

  3. 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

  4. Computational Fluid Dynamics (CFD) simulation of the Madison Dynamo Experiment.

    NASA Astrophysics Data System (ADS)

    Haehn, N. S.; Forest, C. B.; Weber, C. R.; Kendrick, R. D.; Taylor, N. Z.; Oakley, J. G.; Bonazza, R.; Spence, Erik

    2007-11-01

    The Madison Dynamo Experiment is designed to study a self-generated magnetic field called a dynamo. The flow characteristics of a water experiment that is dimensionally similar to the liquid sodium experiment has been modeled using the Computational Fluid Dynamics (CFD) software Fluent. Results from the CFD simulations are used to confirm flow characteristics measured experimentally by both Laser Doppler Velocimetry (LDV) and Particle Imaging Velocimetry (PIV). Simulations can also give insight into the flow characteristics in regions of the experiment which are not accessible via the LDV and PIV systems. The results from the simulations are also used as input for a MHD code to predict the threshold for Dynamo onset. The CFD simulations -- in conjunction with the MHD dynamo prediction code -- can be used to design modifications to the experiment to minimize costly changes. The CFD code has shown that the addition of an equatorial baffle along with several poloidal baffles can lower the threshold for Dynamo onset.

  5. 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. PMID:23650386

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

    DOE PAGESBeta

    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

  7. Magnetic Helicity in Solar Dynamo Simulations

    NASA Astrophysics Data System (ADS)

    Miesch, Mark; Augustson, Kyle C.; Zhang, Mei

    2015-08-01

    Magnetic helicity is a fundamental agent for magnetic self-organization in magnetohydrodynamic (MHD) dynamos. As a conserved quantity in ideal MHD, it establishes a strict topological coupling between large and small-scale magnetic fields. The generation of magnetic fields on scales larger than the velocity field is linked to an upscale transfer of magnetic helicity, either locally in spectral space as in the inverse cascade of magnetic helicity in MHD turbulence or non-locally, as in the turbulent alpha-effect of mean-field dynamo theory. Thus, understanding the generation, transport, and dissipation of magnetic helicity is an essential prerequisite to understanding manifestations of magnetic self-organization in the solar dynamo, including sunspots, the prominent dipole and quadrupole moments, and the 22-year magnetic activity cycle. Yet, despite its significance, magnetic helicity is often neglected in observational and theoretical studies of solar magnetism. This can be attributed to two factors; First, the calculation of the magnetic helicity is not unique; in general it depends on an electromagnetic guage through the magnetic vector potential. Second, unless it is explicitly calculated as part of the computational algorithm in numerical models, it is not always straightforward to obtain the magnetic vector potential. Here we consider gauge-invariant measures of the magnetic helicity and magnetic helicity flux and we describe how they can be computed from measurable quantities such as the magnetic field, the bulk plasma velocity, and the electrical current density. These measures can be applied to local Cartesian geometries as well as global spherical shells. Here we apply them to two global dynamo simulations, each exhibiting regular magnetic cycles. These include a convective MHD dynamo model and a 3-D Babcock-Leighton dynamo model. Both exhibit patterns of magnetic helicity evolution that reflect the global restructuring of the magnetic field over the

  8. Earthquake waves and the geomagnetic dynamo.

    PubMed

    Mullan, D J

    1973-08-10

    It is proposed that earthquake waves energize the geomagnetic dynamo. Fluid motions generated by earthquakes may have enough energy to be in equipartition with fields as large as 100 gauss. Seismic waves from meteoritic impacts with energies sufficient to reverse the field occur every 170,000 years. PMID:17777805

  9. 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

  10. Oppositely directed waves of stellar activity in simple dynamo models

    NASA Astrophysics Data System (ADS)

    Tarbeeva, S. M.; Sokoloff, D. D.

    2016-07-01

    Excitations of two oppositely directed waves of stellar activity generated by two dynamo-active layers located in a single stellar hemisphere are examined using simple dynamo models. The domains of model parameters corresponding to various types and directions of the activity waves are found. It is shown that oppositely directed waves of activity are generated if the dynamo numbers have the same order of magnitude, ~105-106, but opposite signs. How frequently this case can be observed among real stars remains open to question. The report of oppositely directed waves of stellar activity in the literature is especially valuable in this connection.

  11. The Global Solar Dynamo

    NASA Astrophysics Data System (ADS)

    Cameron, R. H.; Dikpati, M.; Brandenburg, A.

    2016-02-01

    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.

  12. Magnetic Helicity in a Cyclic Convective Dynamo

    NASA Astrophysics Data System (ADS)

    Miesch, Mark S.; Zhang, Mei; Augustson, Kyle C.

    2016-05-01

    Magnetic helicity is a fundamental agent for magnetic self-organization in magnetohydrodynamic (MHD) dynamos. As a conserved quantity in ideal MHD, it establishes a strict topological coupling between large and small-scale magnetic fields. The generation of magnetic fields on scales larger than the velocity field is linked to an upscale transfer of magnetic helicity, either locally in spectral space as in the inverse cascade of magnetic helicity in MHD turbulence or non-locally, as in the turbulent alpha-effect of mean-field dynamo theory. Thus, understanding the generation, transport, and dissipation of magnetic helicity is an essential prerequisite to understanding manifestations of magnetic self-organization in the solar dynamo, including sunspots, the prominent dipole and quadrupole moments, and the 22-year magnetic activity cycle. We investigate the role of magnetic helicity in a convective dynamo model that exhibits regular magnetic cycles. The cycle is marked by coherent bands of toroidal field that persist within the convection zone and that are antisymmetric about the equator. When these toriodal bands interact across the equator, it initiates a global restructuring of the magnetic topology that contributes to the reversal of the dipole moment. Thus, the polar field reversals are preceeded by a brief reversal of the subsurface magnetic helicity. There is some evidence that the Sun may exhibit a similar magnetic helicity reversal prior to its polar field reversals.

  13. A Precession-Driven Lunar Dynamo

    NASA Astrophysics Data System (ADS)

    Tian, B. Y.; Stanley, S.; Tikoo, S. M.; Weiss, B. P.

    2014-12-01

    Paleomagnetic studies of Apollo samples suggest that the Moon generated a magnetic field with surface field intensities of several tens of microteslas until at least 3.56 billion years ago (Ga). The field then declined by an order of magnitude from 3.56 - 3.19 Ga. Because of difficulties in reproducing such a long-lived and intense field with convection-driven dynamos, a dynamo driven by precession of the mantle relative to the core was proposed as an alternative. However, there have not been any detailed numerical models demonstrating the feasibility, lifetime, and intensity of such a lunar dynamo. Using fully 3D magnetohydrodynamic simulations, we determined the strength and duration of a mechanically-driven dynamo powered by mantle precession. We found that this mechanism was capable of not only generating the 10-100μT paleomagnetic intensities observed in Apollo samples aged between 4.25 and 3.56 Ga, but also reproducing the precipitous decline in paleointensity beyond 3.56 Ga as the obliquity of the Moon decreased below 15°.

  14. TIDALLY DRIVEN DYNAMOS IN A ROTATING SPHERE

    SciTech Connect

    Cébron, D.; Hollerbach, R. E-mail: r.hollerbach@leeds.ac.uk

    2014-07-01

    Large-scale planetary or stellar magnetic fields generated by a dynamo effect are mostly attributed to flows forced by buoyancy forces in electrically conducting fluid layers. However, these large-scale fields may also be controlled by tides, as previously suggested for the star τ-boo, Mars, or the early Moon. By simulating a small local patch of a rotating fluid, Barker and Lithwick have recently shown that tides can drive small-scale dynamos by exciting a hydrodynamic instability, the so-called elliptical (or tidal) instability. By performing global magnetohydrodynamic simulations of a rotating spherical fluid body, we investigate if this instability can also drive the observed large-scale magnetic fields. We are thus interested in the dynamo threshold and the generated magnetic field in order to test if such a mechanism is relevant for planets and stars. Rather than solving the problem in a geometry deformed by tides, we consider a spherical fluid body and add a body force to mimic the tidal deformation in the bulk of the fluid. This allows us to use an efficient spectral code to solve the magnetohydrodynamic problem. We first compare the hydrodynamic results with theoretical asymptotic results and numerical results obtained in a truly deformed ellipsoid, which confirms the presence of elliptical instability. We then perform magnetohydrodynamic simulations and investigate the dynamo capability of the flow. Kinematic and self-consistent dynamos are finally simulated, showing that the elliptical instability is capable of generating a dipole-dominated large-scale magnetic field in global simulations of a fluid rotating sphere.

  15. Tidally Driven Dynamos in a Rotating Sphere

    NASA Astrophysics Data System (ADS)

    Cébron, D.; Hollerbach, R.

    2014-07-01

    Large-scale planetary or stellar magnetic fields generated by a dynamo effect are mostly attributed to flows forced by buoyancy forces in electrically conducting fluid layers. However, these large-scale fields may also be controlled by tides, as previously suggested for the star τ-boo, Mars, or the early Moon. By simulating a small local patch of a rotating fluid, Barker & Lithwick have recently shown that tides can drive small-scale dynamos by exciting a hydrodynamic instability, the so-called elliptical (or tidal) instability. By performing global magnetohydrodynamic simulations of a rotating spherical fluid body, we investigate if this instability can also drive the observed large-scale magnetic fields. We are thus interested in the dynamo threshold and the generated magnetic field in order to test if such a mechanism is relevant for planets and stars. Rather than solving the problem in a geometry deformed by tides, we consider a spherical fluid body and add a body force to mimic the tidal deformation in the bulk of the fluid. This allows us to use an efficient spectral code to solve the magnetohydrodynamic problem. We first compare the hydrodynamic results with theoretical asymptotic results and numerical results obtained in a truly deformed ellipsoid, which confirms the presence of elliptical instability. We then perform magnetohydrodynamic simulations and investigate the dynamo capability of the flow. Kinematic and self-consistent dynamos are finally simulated, showing that the elliptical instability is capable of generating a dipole-dominated large-scale magnetic field in global simulations of a fluid rotating sphere.

  16. Stretch fast dynamo mechanism via conformal mapping in Riemannian manifolds

    SciTech Connect

    Garcia de Andrade, L. C.

    2007-10-15

    Two new analytical solutions of the self-induction equation in Riemannian manifolds are presented. The first represents a twisted magnetic flux tube or flux rope in plasma astrophysics, where the rotation of the flow implies that the poloidal field is amplified from toroidal field, in the spirit of dynamo theory. The value of the amplification depends on the Frenet torsion of the magnetic axis of the tube. Actually this result illustrates the Zeldovich stretch, twist, and fold method to generate dynamos from straight and untwisted ropes. Based on the fact that this problem was previously handled, using a Riemannian geometry of twisted magnetic flux ropes [Phys Plasmas 13, 022309 (2006)], investigation of a second dynamo solution, conformally related to the Arnold kinematic fast dynamo, is obtained. In this solution, it is shown that the conformal effect on the fast dynamo metric enhances the Zeldovich stretch, and therefore a new dynamo solution is obtained. When a conformal mapping is performed in an Arnold fast dynamo line element, a uniform stretch is obtained in the original line element.

  17. Convection in Rotating Spherical Fluid Shells and its Dynamo Action

    NASA Astrophysics Data System (ADS)

    Busse, Friedrich

    2015-04-01

    Convection in rotating spherical fluid shells is characterized by the relative orientation of gravity and rotation vector. Outside the virtual cylinder touching the inner boundary at its equator convection assumes the form of thermal Rossby waves which are particularly suitable for the generation of magnetic fields. Without magnetic field the shear generated by the thermal Rossby waves tends to destroy them. Only localized convection or intermittent convection may survive the shearing action. Dipolar, quadrupolar and hemispherical dynamos can be realized. Lorentz forces counteract the shearing action of the differential rotation and thus permit an efficient heat transport. Of particular interest are regimes of bistability where depending on initial conditions either dynamos with strong mean magnetic fields or dynamos with highly fluctuating magnetic fields are realized. In systems like the Earth's core aperiodic reversals of the poloidal field may occur in connection with periodic toroidal dynamo waves.

  18. 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.

  19. Planetary magnetism. [emphasizing dynamo theories

    NASA Technical Reports Server (NTRS)

    Stevenson, D.

    1974-01-01

    The origin and maintenance of planetary magnetic fields are discussed. The discussion is not limited to dynamo theories, although these are almost universally favored. Thermoelectric currents are found to be a possible alternative for Jupiter. Two energy sources for dynamos are considered: convection and precessionally induced fluid flow. The earth is the most favorable planet for precessionally driven dynamo, although Neptune is a possibility. Jupiter is likely to have a convectionally driven dynamo, as may Saturn, but the relevant properties of Saturn are not yet well known. Conclusions for each planet are given.

  20. SOLAR PARITY ISSUE WITH FLUX-TRANSPORT DYNAMO

    SciTech Connect

    Hotta, H.; Yokoyama, T.

    2010-05-10

    We investigated the dependence of the solar magnetic parity between the hemispheres on two important parameters, the turbulent diffusivity and the meridional flow, by means of axisymmetric kinematic dynamo simulations based on the flux-transport dynamo model. It is known that the coupling of the magnetic field between hemispheres due to turbulent diffusivity is an important factor for the solar parity issue, but the detailed criterion for the generation of the dipole field has not been investigated. Our conclusions are as follows. (1) The stronger diffusivity near the surface is more likely to cause the magnetic field to be a dipole. (2) The thinner layer of the strong diffusivity near the surface is also more apt to generate a dipolar magnetic field. (3) The faster meridional flow is more prone to cause the magnetic field to be a quadrupole, i.e., symmetric about the equator. These results show that turbulent diffusivity and meridional flow are crucial for the configuration of the solar global magnetic field.

  1. 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. PMID:21867314

  2. Planetary Dynamos: Investigations of Saturn and Ancient Mars

    SciTech Connect

    Stanley, Sabine

    2012-04-18

    Magnetic field observations by spacecraft missions have provided vital information on planetary dynamos. The four giant planets as well as Earth, Mercury and Ganymede have observable magnetic fields generated by active dynamos. In contrast, Moon and Mars only have remanent crustal fields from dynamo action in their early histories. A variety of magnetic field morphologies and intensities can be found in the solar system. We have found that some of the differences between planetary magnetic fields can be explained as the result of the presence of boundary thermal variations or stably-stratified layers. In this talk, I will discuss how dynamos are affected by these complications and discuss the implications for Mars’ magnetic dichotomy and Saturn’s extremely axisymmetric magnetic field.

  3. Magnetized turbulent dynamo in protogalaxies

    NASA Astrophysics Data System (ADS)

    Malyshkin, Leonid M.

    The prevailing theory for the origin of cosmic magnetic fields is that they have been amplified from insignificant seed values to their present values by the turbulent dynamo inductive action driven by the plasma turbulent motions 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 the 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 that has been well developed in the past. However, the applicability of the kinematic dynamo theory to protogalaxies is rather limited. The reason is that in protogalaxies the temperature is very high, and the viscosity is dominated by ions. As the magnetic field strength grows in time because of the dynamo action, 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, and the magnetic field starts to strongly affect the turbulent motions on the viscous scales. Thus, in protogalaxies the back reaction sets in much earlier, at field strengths much lower than those which correspond to energy equipartition between the field and the turbulence; and the turbulent dynamo becomes what we call the magnetized turbulent dynamo. The main purpose of this thesis is to 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 time larger than that in the kinematic dynamo theory, and this could lead to the dynamo creation of cluster fields. We also briefly discuss how the Braginskii viscosity can aid the development of the inverse cascade of magnetic energy, which happens after the energy equipartition time.

  4. Mean-Field Theory of the Solar Dynamo

    NASA Astrophysics Data System (ADS)

    Schmitt, D.

    The generation of the solar magnetic field is generally ascribed to dynamo processes in the convection zone. The dynamo effects, differential rotation (Omega-effect) and helical turbulence (alpha-effect) are explained, and the basic properties of the mean-field dynamo equations are discussed in view of the observed properties of the solar cycle. Problems of the classical picture of a dynamo in the convection zone (fibril state of magnetic flux, field strength, magnetic buoyancy, polarity rules, differential rotation and butterfly diagram) are addressed and some alternatives to overcome these problems are presented. A possibility to make up for the missing radial gradient of rotation in the convection zone is an alpha^2-Omega-dynamo with an anisotropic alpha-tensor. Dynamo solutions then might have the characteristics of the butterfly diagram. Another approach involves meridional circulation as the cause of the migration of a dynamo wave. Another suggestion is that the solar dynamo operates in the overshoot region at the base of the convection zone where strong fields, necessary to explain the polarity rules, can be stored and radial gradients in the angular velocity occur. As an alternative to the turbulent alpha-effect a dynamic alpha-effect based on magnetostrophic waves driven by a magnetic buoyancy instability of a magnetic flux layer is introduced. Model calculations which use the internal rotation of the Sun as deduced from helioseismology only show solar cycle behaviour if the turbulent diffusivity is reduced in the layer and the alpha-effect is concentrated near the equator. Another possibility is a combined model. The non-uniform rotation and most of the azimuthal magnetic flux are confined to a thin layer at the bottom of the convection zone where turbulent diffusion is greatly reduced, with the convective region above containing only weak fields for which the alpha-effect and turbulent diffusion operate in the conventional manner. The dynamo takes on the

  5. Mean-field theory of the solar dynamo

    NASA Astrophysics Data System (ADS)

    Schmitt, Dieter

    The generation of the solar magnetic field is generally ascribed to dynamo processes in the convection zone. The dynamo effects, differential rotation (Ω-effect and helical turbulence (α-effect are explained, and the basic properties of the mean-field dynamo equations are discussed in view of the observed properties of the solar cycle. Problems of the classical picture of a dynamo in the convection zone (fibril state of magnetic flux, field strength, magnetic buoyancy, polarity rules, differential rotation and butterfly diagram) are addressed and some alternatives to overcome these problems are presented. A possibility to make up for the missing radial gradient of rotation in the convection zone is an α2 Ω-dynamo with an anisotropic a-tensor. Dynamo solutions then might have the characteristics of the butterfly diagram. Another approach involves meridional circulation as the cause of the migration of a dynamo wave. Another suggestion is that the solar dynamo operates in the overshoot region at the base of the convection zone where strong fields, necessary to explain the polarity rules, can be stored and radial gradients in the angular velocity occur. As an alternative to the turbulent α-effect a dynamic α-effect based on magnetostrophic waves driven by a magnetic buoyancy instability of a magnetic flux layer is introduced. Model calculations which use the internal rotation of the Sun as deduced from helioseismology only show solar cycle behaviour if the turbulent diffusivity is reduced in the layer and the a-effect is concentrated near the equator. Another possibility is a combined model. The non-uniform rotation and most of the azimuthal magnetic flux are confined to a thin layer at the bottom of the convection zone where turbulent diffusion is greatly reduced, with the convective region above containing only weak fields for which the α-effect and turbulent diffusion operate in the conventional manner. The dynamo takes on the character of a surface wave at

  6. Magnetized Turbulent Dynamo in Protogalaxies

    SciTech Connect

    Leonid Malyshkin; Russell M. Kulsrud

    2002-01-28

    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 temperature 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.

  7. Fluid Dynamics Prize Lecture: Homogeneous Dynamos in Planets and in the Laboratory

    NASA Astrophysics Data System (ADS)

    Busse, F. H.

    2000-11-01

    Numerical simulations of the dynamo problem of the generation of magnetic fields by convection flows in rotating spherical fluid shells have been extended to a sufficiently large parameter regime such that extrapolation to the condition of planetary cores have become feasible. Besides dipolar fields, hemispherical and quadrupolar fields are preferred in various regimes of the parameter space. In the latter two cases oscillating time dependances are always found inspite of the chaotic nature of the dynamos. Subcritical dynamo states are typical and multiple dynamo states are possible. On the experimental side the homogeneous dynamo process has recently been demonstrated without the use of ferromagnetic material in Riga and Karlsruhe. Further experiments at other laboratories are expected to realize dynamos under conditions of strong turbulence.

  8. Dynamo models for Jupiter and Saturn

    NASA Astrophysics Data System (ADS)

    Jones, C. A.

    2015-10-01

    In 2016 and 2017, the interiors of Jupiter and Saturn will be probed by the Juno and Cassini missions, respectively. Both will measure the planetary gravity and magnetic fields with unprecedented accuracy. In addition, Juno will probe Jupiter's deep atmosphere by radiometry in search of its elusive water. Altogether, the observational constraints used to construct interiors models will be improved extremely significantly. In parallel, the complexity of these models has been increasing steadily, due to the realization that their central core could erode over time, that double diffusive convection could set in and that the region in which helium separates from hydrogen is probably extended. Deriving much better constraints on the central core masses and global compositions of these planets will therefore require efforts to better examine the interplay between thermal cooling, mixing of elements, interior rotation, equations of state and dynamo generation. I will review the work in this direction. I will also show how seismology can ideally complement the constraints derived from the gravity field measurements.

  9. 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.

  10. DYNAMO EFFICIENCY WITH SHEAR IN HELICAL TURBULENCE

    SciTech Connect

    Leprovost, Nicolas; Kim, Eun-jin

    2009-05-10

    To elucidate the influence of shear flow on the generation of magnetic fields through the modification of turbulence property, we consider the case where a large-scale magnetic field is parallel to a large-scale shear flow without direct interaction between the two in the kinematic limit where the magnetic field does not backreact on the velocity. By nonperturbatively incorporating the effect of shear in a helically forced turbulence, we show that turbulence intensity and turbulent transport coefficients (turbulent viscosity, {alpha} and {beta} effect) are enhanced by a weak shear, while strongly suppressed for strong shear. In particular, {beta} is shown to be much more strongly suppressed than {alpha} effect. We discuss its important implications for dynamo efficiency, i.e., on the scaling of the dynamo number with differential rotation.

  11. Solar Dynamo and Toroidal Field Instabilities

    NASA Astrophysics Data System (ADS)

    Bonanno, Alfio

    2013-10-01

    The possibility of non-axisymmetric (kink) instabilities of a toroidal field seated in the tachocline is much discussed in the literature. In this work, the basic properties of kink and quasi-interchange instabilities, produced by mixed toroidal and poloidal configuration, will be briefly reviewed. In particular, it will be shown that the unstable modes are strongly localized near the Equator and not near the Poles as often claimed in the literature. Based on the results of recent numerical simulations, it is argued that a non-zero helicity can already be produced at a non-linear level. A mean-field solar dynamo is then constructed with a positive α-effect in the overshoot layer localized near the Equator, and a meridional circulation with deep return flow. Finally, the possibility that the solar cycle is driven by an αΩ dynamo generated by the negative subsurface shear in the supergranulation layer will also be discussed.

  12. 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. PMID:23066077

  13. Dynamos in Terrestrial Exoplanets as Magnetic Shields

    NASA Astrophysics Data System (ADS)

    Driscoll, Peter; Olson, Peter

    2010-05-01

    In order to retain large amounts of water and maintain a habitable surface over long time-scales a magnetic field may be required to shield the atmosphere from mass loss and the surface from harmful stellar radiation. Terrestrial exoplanets in the 1-10 Earth-mass regime orbiting inside of 3 AU with an Earth-like composition, referred to as Super-Earths, are expected to have large, mostly Iron cores that could sustain a convectively driven dynamo. We present a model to estimate the maximum self-sustained magnetic moment of a terrestrial dynamo given the total mass and core-mass fraction. Assuming the magnetic field is self-sustained by a convectively driven dynamo we estimate the magnetic moment using a dynamo scaling law, which relies on dynamical properties of the planetary interior, such as the convective heat flux at the core-mantle boundary and size of the dynamo region. To estimate these properties we model the internal structure of the planet using a sub-solidus, mobile lid convection profile for the mantle and a thermal convection profile for the core. We present models for 1-10 Earth-masses and a range of core-mass fractions. In order to maintain a strong magnetic field we maximize the energy available to drive the dynamo by allowing the core-mantle boundary temperature to be at the perovskite solidus, denoted as the "optimal" state for magnetic field generation. We estimate an optimal Earth-mass planet can maintain a core heat flow of 30 TW, which implies a surface field intensity and magnetic moment of about twice that of the Earth. For a 10 Earth-mass planet that is 65% core by mass (Super-Mercury) we find a core heat flow of 180 TW, and a surface field intensity and magnetic moment of about 6 and 25 times that of the Earth, respectively. We demonstrate that exoplanets with large cores that produce strong magnetic fields can act to shield the surface from stellar radiation, minimizing atmospheric volatile loss and maintaining a habitable surface over

  14. The Older Generation: What Is Due, What Is Owed.

    ERIC Educational Resources Information Center

    Lowy, Louis

    Intergenerational conflict has always existed, but it is increasing as life spans increase. Generations of older adult children now coexist with a sizable older parental generation. The major task of coping with intergenerational conflict involves four major tension areas: (1) distance and closeness; (2) submission and dominance; (3) connectedness…

  15. A THREE-DIMENSIONAL BABCOCK-LEIGHTON SOLAR DYNAMO MODEL

    SciTech Connect

    Miesch, Mark S.; Dikpati, Mausumi

    2014-04-10

    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) 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.

  16. Multiscale Properties of the Local Dynamo on the Sun

    NASA Astrophysics Data System (ADS)

    Kitiashvili, Irina; Kosovichev, Alexander G.; Mansour, Nagi N; Wray, Alan A

    2014-06-01

    Dynamics of the quiet Sun represents a background ('salt-and-pepper') state for powerful manifestations of solar activity. Current numerical simulations have shown that small-scale turbulent dynamics can strongly couple with processes on larger scales, such as formation of pores and sunspots. We perform 3D MHD radiative simulations of top layers of the convection zone and the low atmosphere, taking into account effects of turbulence, magnetic fields, ionization and excitation of all abundant elements. To model the dynamo process we carry a series of the simulations with various initial weak levels of magnetic field perturbations. The results show that an initial, randomly distributed ('seed') magnetic field of 1 micro-gauss, greatly amplifies by subsurface turbulent dynamics. The self generated magnetic field (dynamo) reaches 2 kG magnetic levels in the photosphere. The local dynamo process primary operates 1 Mm below the surface where the magnetic fields are amplified by helical flows. These dynamo-generated magnetic fields are transported by downflows into deeper layers. The process of the magnetic field amplification has a substantially multiscale character, during which self-organized turbulent helical flows work coherently on scales much larger then the turbulent scales. We discuss the apparent contradiction of our results with current paradigm that local dynamo can generate magnetic fields only on the small turbulent scales. We compare our results with other simulations and observations.

  17. Hall-magnetohydrodynamic small-scale dynamos.

    PubMed

    Gómez, Daniel O; Mininni, Pablo D; Dmitruk, Pablo

    2010-09-01

    Magnetic field generation by dynamo action is often studied within the theoretical framework of magnetohydrodynamics (MHD). However, for sufficiently diffuse media, the Hall effect may become non-negligible. We present results from three-dimensional simulations of the Hall-MHD equations subjected to random nonhelical forcing. We study the role of the Hall effect in the dynamo efficiency for different values of the Hall parameter. For small values of the Hall parameter, the small-scale dynamo is more efficient, displaying faster growth and saturating at larger amplitudes of the magnetic field. For larger values of the Hall parameter, saturation of the magnetic field is reached at smaller amplitudes than in the MHD case. We also study energy transfer rates among spatial scales and show that the Hall effect produces a reduction of the direct energy cascade at scales larger than the Hall scale, therefore leading to smaller energy dissipation rates. Finally, we present results stemming from simulations at large magnetic Prandtl numbers, which is the relevant regime in the hot and diffuse interstellar medium. In the range of magnetic Prandtl numbers considered, the Hall effect moves the peak of the magnetic energy spectrum as well as other relevant magnetic length scales toward the Hall scale. PMID:21230195

  18. 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.

  19. Magnetospheric Feedback Effects on Mercury's Dynamo

    NASA Astrophysics Data System (ADS)

    Gomez Perez, N.; Heyner, D.; Wicht, J.; Solomon, S. C.; Glassmeier, K.

    2010-12-01

    The internal magnetic field of Mercury has been sampled by the Mariner 10 and MESSENGER spacecraft during a combined total of five flybys to date. The measurements are consistent with a magnetic dipole moment of ~ 250 nT RM3, where RM is the radius of Mercury. The action of high solar wind pressure at Mercury’s solar distance on such a weak internal field produces a small magnetosphere for which the dayside magnetopause is unusually close to the surface of the planet (at a planetocentric distance of about 1.5 RM). Because of this small magnetosphere and Mercury’s relatively thin silicate mantle, it has been proposed that magnetospheric currents may influence the internal dynamo process. From numerical simulations, we have previously demonstrated that magnetic field sources external to the dynamo-generating region may modify core dynamics and that this magnetospheric feedback may have influenced the history of Mercury’s dipole field. Here we combine new results from two types of numerical simulations. First, we estimate the magnitude of magnetospheric surface currents with a semi-empirical Earth model adapted to Mercury’s conditions. These currents are calculated for a range of internal dipole moments to establish the functional dependence of the feedback magnitude on internal field amplitude. Second, we implement this feedback function in the internal dynamo model. Earlier magnetospheric feedback models, such as those by Glassmeier and others and Heyner and others, demonstrated that this process is able to sustain an extremely weak magnetic field. Our new, more realistic feedback function leads to slower secular variation than in previous dynamic feedback models, but the secular variation is still typically faster than for isolated dynamos that neglect the external field altogether. Most generally, magnetospheric feedback is able to stabilize a weak dipole field with characteristics that are consistent in magnitude and form with measurements at Mercury.

  20. On the Role of Tachoclines in Solar and Stellar Dynamos

    NASA Astrophysics Data System (ADS)

    Guerrero, G.; Smolarkiewicz, P. K.; de Gouveia Dal Pino, E. M.; Kosovichev, A. G.; Mansour, N. N.

    2016-03-01

    Rotational shear layers at the boundary between radiative and convective zones, tachoclines, play a key role in the process of magnetic field generation in solar-like stars. We present two sets of global simulations of rotating turbulent convection and dynamo. The first set considers a stellar convective envelope only; the second one, aiming at the formation of a tachocline, also considers the upper part of the radiative zone. Our results indicate that the resulting properties of the mean flows and dynamo, such as the growth rate, saturation energy, and mode, depend on the Rossby number (Ro). For the first set of models either oscillatory (with ˜2 yr period) or steady dynamo solutions are obtained. The models in the second set naturally develop a tachocline, which in turn leads to the generation of a strong mean magnetic field. Since the field is also deposited in the stable deeper layer, its evolutionary timescale is much longer than in the models without a tachocline. Surprisingly, the magnetic field in the upper turbulent convection zone evolves on the same timescale as the deep field. These models result in either an oscillatory dynamo with a ˜30 yr period or a steady dynamo depending on Ro. In terms of the mean-field dynamo coefficients computed using the first-order smoothing approximation, the field evolution in the oscillatory models without a tachocline seems to be consistent with dynamo waves propagating according to the Parker-Yoshimura sign rule. In the models with tachoclines the dynamics is more complex and involves other transport mechanisms as well as tachocline instabilities.

  1. 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.

  2. Simulations of Galactic Dynamos

    NASA Astrophysics Data System (ADS)

    Brandenburg, Axel

    We review our current understanding of galactic dynamo theory, paying particular attention to numerical simulations both of the mean-field equations and the original three-dimensional equations relevant to describing the magnetic field evolution for a turbulent flow. We emphasize the theoretical difficulties in explaining non-axisymmetric magnetic fields in galaxies and discuss the observational basis for such results in terms of rotation measure analysis. Next, we discuss nonlinear theory, the role of magnetic helicity conservation and magnetic helicity fluxes. This leads to the possibility that galactic magnetic fields may be bi-helical, with opposite signs of helicity and large and small length scales. We discuss their observational signatures and close by discussing the possibilities of explaining the origin of primordial magnetic fields.

  3. The Solar Dynamo Zoo

    NASA Astrophysics Data System (ADS)

    Egeland, Ricky; Soon, Willie H.; Baliunas, Sallie L.; Hall, Jeffrey C.; Pevtsov, Alexei A.; Henry, Gregory W.

    2016-05-01

    We present composite time series of Ca II H & K line core emission indices of up to 50 years in length for a set of 27 solar-analog stars (spectral types G0-G5; within ~10% of the solar mass) and the Sun. These unique data are available thanks to the long-term dedicated efforts of the Mount Wilson Observatory HK project, the Lowell Observatory Solar-Stellar Spectrograph, and the National Solar Observatory/Air Force Research Laboratory/Sacremento Peak K-line monitoring program. The Ca II H & K emission originates in the lower chromosphere and is strongly correlated with the presence of magnetic plage regions in the Sun. These synoptic observations allow us to trace the patterns long-term magnetic variability and explore dynamo behavior over a wide range of rotation regimes and stellar evolution timescales.

  4. 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

  5. Shear dynamo, turbulence, and the magnetorotational instability

    NASA Astrophysics Data System (ADS)

    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, known 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 small-scale 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

  6. Effects of Large-scale Non-axisymmetric Perturbations in the Mean-field Solar Dynamo.

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

    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⊙ 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 of the axisymmetric and non-axisymmetric fields depends on the turbulent magnetic Reynolds number Rm. In the range of Rm = 104-106 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.

  7. The Origin of Galactic and Metagalactic Magnetic Fields: The Black Hole Accretion Disk Dynamo

    NASA Astrophysics Data System (ADS)

    Colgate, S.; Li, H.; Beckley, H.; Finn, J.; Pariev, V.

    1998-12-01

    The frequent star-disk collisions in the accretion disk forming the massive black hole of galaxies is an efficient, robust mechanism for generating the alpha -helicity in the alpha -Omega dynamo. In the rotating frame of the disk, these collisions result in axially directed, expanding plumes which "un-twist" due to their increased moment of inertia. We have demonstrated this flow field and its feasibility in the laboratory in preparation for a dynamo experiment using liquid sodium. The near-infinite number of rotations of the inner accretion disk and hence, amplification, ensures that this dynamo will saturate at the maximum accretion disk stress, where Bmax ~ 50 kG, corresponding to a luminosity, L ~ 10(46) ergs/s. The magnetic flux expelled as a collimated Poynting flux is sufficient to explain the magnetic flux of the galaxy as well as that of galactic clusters. The dissipation of a fraction of this magnetic energy is the likely source of the AGN/quasar phenomena.

  8. Spontaneous toroidal flow generation due to negative effective momentum diffusivity

    SciTech Connect

    McMillan, Ben F.

    2015-02-15

    Spontaneous structure formation, and in particular, zonal flows, is observed in a broad range of natural and engineered systems, often arising dynamically as the saturated state of a linear instability. Flows in tokamaks are known to self-organise on small scales, but large scale toroidal flows also arise even when externally applied torques are zero. This has previously been interpreted as the result of small externally imposed breaking of a symmetry. However, we show that for large enough field line pitch, a robust spontaneous symmetry breaking occurs, leading to the generation of strong toroidal flow structures; parameters are typical of Spherical Tokamak discharges with reversed shear profiles. The short wavelength dynamics are qualitatively similar to the growth of poloidal flow structures, and toroidal flow gradients nonlinearly saturate at levels where the shearing rate is comparable to linear growth rate. On long wavelengths, we measure Prandtl numbers of around zero for these systems, in conjunction with the formation of structured toroidal flows, and we show that this is consistent with a model of momentum transport where fluxes act to reinforce small flow gradients: the effective momentum diffusivity is negative. Toroidal flow structures are largely unaffected by collisional damping, so this may allow toroidal bulk flows of order the ion thermal velocity to be maintained with zero momentum input. This phenomenon also provides a mechanism for the generation of localised meso-scale structures like transport barriers.

  9. Changes in runoff generation due to conversion of catchment vegetation

    NASA Astrophysics Data System (ADS)

    Vilhar, Urša; Kestnar, Klemen; Šraj, Mojca

    2015-04-01

    In Central Europe, many pure Norway spruce stands, established on primary beech sites, were converted into mixed stands over the last 60 years. The conversion of forest management from Norway spruce monocultures into mixed deciduous-coniferous forests changed the forest structure dramatically. This changes could influence the hydrological processes on the catchment scale, associated with changes in runoff generation. In this study, the effect of forest management on the runoff in mixed deciduous-coniferous stands on Pohorje mountains in NE Slovenia were investigated. Two small forested experimental catchments of Oplotnica River on Pohorje were compared with similar size and shape but different share of Norway spruce Picea abies (L. Karst) and European beech Fagus sylvatica (L.). Measured stream flows, throughfall, stemflow and the mixture of forests were compared in the period 2008 till 2013 for both catchments. Hydrological models in the HEC-HMS program were built for both catchmenta, calibrated and validated using measured data. Precipitation losses were estimated by the Soil Conservation Service (SCS) method, while precipitation was converted into surface runoff using the SCS synthetic unit hydrograph procedure. The measured seasonal throughfall and stream flow was lower in the catchment with higher share of spruce in the mixed spruce-beech forest. Modeled precipitation losses in the river basins were 92% and 95% of total precipitation, respectively. The results indicate higher interception, infiltration and accumulation of precipitation in the catchment with higher share of spruce in the mixed spruce-beech forest. Forest management practices should aim towards decreased surface runoff in alpine catchments. Therefore implementation of hydrology-oriented sylvicultural measures via a more accurate prediction of the impacts of tree species conversion on runoff generation in this type of alpine catchments is discussed.

  10. An iron snow dynamo explains Mercury's peculiar field

    NASA Astrophysics Data System (ADS)

    Christensen, U. R.; Wicht, J.

    2014-12-01

    The Messenger mission confirmed that Mercury's magnetic field is relatively weak and dominantly dipolar, but also showed the presence of a strong axial quadrupole term. This can be described equivalently by an offset of the dipole along the rotation axis. Furthermore, nonzonal field components could not be unambiguously identified. If Mercury's core contains more than a few percent of sulfur, crystallization may start at the core-mantle boundary rather than at the center. In the outer parts of the core iron snow would form, sink and remelt deeper down where it enriches the fluid in iron and drives compositional convection from above. The snow forming layer grows inward over time and a gradient in sulfur concentration develops which strongly stabilizes this layer against convective overturn. We study this scenario in MHD dynamo models. Aside from geodynamo-like dipolar solutions we find hemispherical dynamos. Here magnetic field is generated predominantly in either the the northern or the southern hemisphere. The axial dipole and axial quadrupole are of comparable strength at the upper boundary of the unstable dynamo region. Systematic studies show that the hemispherical solutions are favored by slow rotation and by a thick stable layer above the dynamo. A thick layer also axisymmetrizes and weakens the field at the boundary of the core. Mercury's observed dipole moment and the quadrupole-to-dipole ratio can approximately be matched by a hemispherical dynamo when the stable layer thickness exceeds half of the core radius.

  11. Hemispherical Parker waves driven by thermal shear in planetary dynamos

    NASA Astrophysics Data System (ADS)

    Dietrich, W.; Schmitt, D.; Wicht, J.

    2013-11-01

    Planetary and stellar magnetic fields are thought to be sustained by helical motions (α-effect) and, if present, differential rotation (Ω-effect). In the Sun, the strong differential rotation in the tachocline is responsible for an efficient Ω-effect creating a strong axisymmetric azimuthal magnetic field. This is a prerequisite for Parker dynamo waves that may be responsible for the solar cycle. In the liquid iron cores of terrestrial planets, the Coriolis force organizes convection into columns with a strong helical flow component. These likely dominate magnetic field generation while the Ω-effect is of secondary importance. Here we use numerical simulations to show that the planetary dynamo scenario may change when the heat flux through the outer boundary is higher in one hemisphere than in the other. A hemispherical dynamo is promoted that is dominated by fierce thermal wind responsible for a strong Ω-effect. As a consequence Parker dynamo waves are excited equivalent to those predicted for the Sun. They obey the same dispersion relation and propagation characteristics. We suggest that Parker waves may therefore also play a role in planetary dynamos for all scenarios where zonal flows become an important part of convective motions.

  12. Measurement of the dynamo effect in a plasma

    SciTech Connect

    Ji, H.; Prager, S.C.; Almagri, A.F.; Sarff, J.S.; Hirano, Y.; Toyama, H.

    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) 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.

  13. HEATING THE SOLAR ATMOSPHERE BY THE SELF-ENHANCED THERMAL WAVES CAUSED BY THE DYNAMO PROCESSES

    SciTech Connect

    Dumin, Yurii V. E-mail: dumin@izmiran.ru

    2012-05-20

    We discuss a possible mechanism for heating the solar atmosphere by the ensemble of thermal waves, generated by the photospheric dynamo and propagating upward with increasing magnitudes. These waves are self-sustained and amplified due to the specific dependence of the efficiency of heat release by Ohmic dissipation on the ratio of the collisional to gyrofrequencies, which in its turn is determined by the temperature profile formed in the wave. In the case of sufficiently strong driving, such a mechanism can increase the plasma temperature by a few times, i.e., it may be responsible for heating the chromosphere and the base of the transition region.

  14. Synthesis of small and large scale dynamos

    NASA Astrophysics Data System (ADS)

    Subramanian, Kandaswamy

    Using a closure model for the evolution of magnetic correlations, we uncover an interesting plausible saturated state of the small-scale fluctuation dynamo (SSD) and a novel analogy between quantum mechanical tunnelling and the generation of large-scale fields. Large scale fields develop via the α-effect, but as magnetic helicity can only change on a resistive timescale, the time it takes to organize the field into large scales increases with magnetic Reynolds number. This is very similar to the results which obtain from simulations using the full MHD equations.

  15. The Dynamo Clinical Trial

    NASA Astrophysics Data System (ADS)

    Ayres, Thomas R.

    2016-04-01

    The Dynamo Clinical Trial evaluates long-term stellar magnetic health through periodic X-ray examinations (by the Chandra Observatory). So far, there are only three subjects enrolled in the DTC: Alpha Centauri A (a solar-like G dwarf), Alpha Cen B (an early K dwarf, more active than the Sun), and Alpha Canis Majoris A (Procyon, a mid-F subgiant similar in activity to the Sun). Of these, Procyon is a new candidate, so it is too early to judge how it will fare. Of the other two, Alpha Cen B has responded well, with a steady magnetic heartbeat of about 8 years duration. The sickest of the bunch, Alpha Cen A, was in magnetic cardiac arrest during 2005-2010, but has begun responding to treatment in recent years, and seems to be successfully cycling again, perhaps achieving a new peak of magnetic health in the 2016 time frame. If this is the case, it has been 20 years since A's last healthful peak, significantly longer than the middle-aged Sun's 11-year magnetic heartbeat, but perhaps in line with Alpha Cen A's more senescent state (in terms of "relative evolutionary age," apparently an important driver of activity). (By the way, don't miss the exciting movie of the Alpha Cen stars' 20-year X-ray dance.)

  16. TURBULENT SMALL-SCALE DYNAMO ACTION IN SOLAR SURFACE SIMULATIONS

    SciTech Connect

    Graham, Jonathan Pietarila; Cameron, Robert; Schuessler, Manfred

    2010-05-10

    We demonstrate that a magneto-convection simulation incorporating essential physical processes governing solar surface convection exhibits turbulent small-scale dynamo action. By presenting a derivation of the energy balance equation and transfer functions for compressible magnetohydrodynamics, we quantify the source of magnetic energy on a scale-by-scale basis. We rule out the two alternative mechanisms for the generation of the small-scale magnetic field in the simulations: the tangling of magnetic field lines associated with the turbulent cascade and Alfvenization of small-scale velocity fluctuations ('turbulent induction'). Instead, we find that the dominant source of small-scale magnetic energy is stretching by inertial-range fluid motions of small-scale magnetic field lines against the magnetic tension force to produce (against Ohmic dissipation) more small-scale magnetic field. The scales involved become smaller with increasing Reynolds number, which identifies the dynamo as a small-scale turbulent dynamo.

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

    DOE PAGESBeta

    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

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

    SciTech Connect

    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 by 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.

  19. Formation of sheet plumes, current coils, and helical magnetic fields in a spherical magnetohydrodynamic dynamo

    NASA Astrophysics Data System (ADS)

    Miyagoshi, Takehiro; Kageyama, Akira; Sato, Tetsuya

    2011-07-01

    Aiming at understanding of magnetic field generation process in rapidly rotating stars and planets represented by the Earth, computer simulations of magnetohydrodynamic (MHD) dynamo were performed in a rotating spherical shell geometry. Thermal convection and dynamo process with Ekman number of the order of 10-7 were studied. New structures of convection motion, dynamo-generated electrical current, and magnetic field are found. The flow is organized as a set of thin, sheet-like plumes. The current is made of small-scale coil structure with magnetic flux tubes within each of the coil. These flux tubes are connected each other to form a large scale helical magnetic field structure.

  20. Turbulent dynamo action in axisymmetric linear machine

    NASA Astrophysics Data System (ADS)

    Kabantsev, A. A.; Reva, V. B.; Sokolov, V. G.

    1997-11-01

    Generation of large-scale magnetic fields by turbulent motion of an electrically conducting fluids plays an important role not only for astrophysical applications, but also for magnetic fusion confinement phenomena. The well-studied turbulent dynamo α-effect comes from helical properties of turbulent motion. Under this dynamo the mean electric current is produced in the direction parallel or antiparallel to the mean magnetic field. In particular, the α-effect leads to the generation of plasma current along the magnetic field in reversed field pinches. We have shown that the α-effect takes place also in axisymmetric linear machines. In axisymmetric mirror traps AMBAL-M and MAL (BINP) the electrostatic turbulence, having mean helicity h≈ 6\\cdot 10^6 m/s^2, caused as a result of unstable differential rotation of plasma column in crossed E×B fields. By manipulating the trap's magnetic and plasma conditions, we can obtain both the parallel and the antiparallel B electric current to the order of 100 A/cm^2 (total current up to 6 kA) in the plasma. The measured mean electromotive force F_em= has linear growth with turbulent diffusion coefficient DT and reaches up to 50 V/m.

  1. Numerical simulations of turbulent dynamos

    NASA Astrophysics Data System (ADS)

    Brandenburg, Axel

    Using a periodic box calculation it is shown that, owing to helicity conservation, a large scale field can only develop on a resistive timescale. This behaviour can be reproduced by a mean-field dynamo with α and ηt quenchings that are equally strong and "catastrophic".

  2. Realistic Modeling of Local Dynamo Processes on the Sun

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

    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 10-6 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 that 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.

  3. Realistic Modeling of Local Dynamo Processes on the Sun

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

    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 10‑6 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 that 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.

  4. Dipolar dynamos in stratified systems

    NASA Astrophysics Data System (ADS)

    Raynaud, R.; Petitdemange, L.; Dormy, E.

    2015-04-01

    Observations of low-mass stars reveal a variety of magnetic field topologies ranging from large-scale, axial dipoles to more complex magnetic fields. At the same time, three-dimensional spherical simulations of convectively driven dynamos reproduce a similar diversity, which is commonly obtained either with Boussinesq models or with more realistic models based on the anelastic approximation, which take into account the variation of the density with depth throughout the convection zone. Nevertheless, a conclusion from different anelastic studies is that dipolar solutions seem more difficult to obtain as soon as substantial stratifications are considered. In this paper, we aim at clarifying this point by investigating in more detail the influence of the density stratification on dipolar dynamos. To that end, we rely on a systematic parameter study that allows us to clearly follow the evolution of the stability domain of the dipolar branch as the density stratification is increased. The impact of the density stratification both on the dynamo onset and the dipole collapse is discussed and compared to previous Boussinesq results. Furthermore, our study indicates that the loss of the dipolar branch does not ensue from a specific modification of the dynamo mechanisms related to the background stratification, but could instead result from a bias as our observations naturally favour a certain domain in the parameter space characterized by moderate values of the Ekman number, owing to current computational limitations. Moreover, we also show that the critical magnetic Reynolds number of the dipolar branch is scarcely modified by the increase of the density stratification, which provides an important insight into the global understanding of the impact of the density stratification on the stability domain of the dipolar dynamo branch.

  5. MAGNETIC WREATHS AND CYCLES IN CONVECTIVE DYNAMOS

    SciTech Connect

    Nelson, Nicholas J.; Toomre, Juri; Brown, Benjamin P.; Brun, Allan Sacha

    2013-01-10

    Solar-type stars exhibit a rich variety of magnetic activity. Seeking to explore the convective origins of this activity, we have carried out a series of global three-dimensional magnetohydrodynamic simulations with the anelastic spherical harmonic code. Here we report on the dynamo mechanisms achieved as the effects of artificial diffusion are systematically decreased. The simulations are carried out at a nominal rotation rate of three times the solar value (3 {Omega}{sub Sun }), but similar dynamics may also apply to the Sun. Our previous simulations demonstrated that convective dynamos can build persistent toroidal flux structures (magnetic wreaths) in the midst of a turbulent convection zone and that high rotation rates promote the cyclic reversal of these wreaths. Here we demonstrate that magnetic cycles can also be achieved by reducing the diffusion, thus increasing the Reynolds and magnetic Reynolds numbers. In these more turbulent models, diffusive processes no longer play a significant role in the key dynamical balances that establish and maintain the differential rotation and magnetic wreaths. Magnetic reversals are attributed to an imbalance in the poloidal magnetic induction by convective motions that is stabilized at higher diffusion levels. Additionally, the enhanced levels of turbulence lead to greater intermittency in the toroidal magnetic wreaths, promoting the generation of buoyant magnetic loops that rise from the deep interior to the upper regions of our simulated domain. The implications of such turbulence-induced magnetic buoyancy for solar and stellar flux emergence are also discussed.

  6. Optimal Length Scale for a Turbulent Dynamo

    NASA Astrophysics Data System (ADS)

    Sadek, Mira; Alexakis, Alexandros; Fauve, Stephan

    2016-02-01

    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 2 π L . The flows considered are the laminar and turbulent A B C flows forced at different forcing wave numbers kf, where the turbulent case is simulated using a subgrid turbulence model. At the smallest allowed forcing wave number kf=kmin=1 /L the laminar critical magnetic Reynolds number R mclam is more than an order of magnitude smaller than the turbulent critical magnetic Reynolds number Rmc turb due to the hindering effect of turbulent fluctuations. We show that this hindering effect is almost suppressed when the forcing wave number kf is increased above an optimum wave number kfL ≃4 for which Rmc turb is minimum. At this optimal wave number, Rmc turb is smaller by more than a factor of 10 than the case forced in kf=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.

  7. 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. PMID:26943538

  8. STELLAR DYNAMOS AND CYCLES FROM NUMERICAL SIMULATIONS OF CONVECTION

    SciTech Connect

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

    2013-09-20

    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 relatively 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.

  9. AN AZIMUTHAL DYNAMO WAVE IN SPHERICAL SHELL CONVECTION

    SciTech Connect

    Cole, Elizabeth; Käpylä, Petri J.; Mantere, Maarit J.; Brandenburg, Axel

    2014-01-10

    We report the discovery of an azimuthal dynamo wave of a low-order (m = 1) mode in direct numerical simulations (DNS) of turbulent convection in spherical shells. Such waves are predicted by mean-field dynamo theory and have been obtained previously in mean-field models. An azimuthal dynamo wave has been proposed as a possible explanation for the persistent drifts of spots observed on several rapidly rotating stars, as revealed through photometry and Doppler imaging. However, this has been judged unlikely because evidence for such waves from DNS has been lacking. Here we present DNS of large-scale magnetic fields showing a retrograde m = 1 mode. Its pattern speed is nearly independent of latitude and does not reflect the speed of the differential rotation at any depth. The extrema of magnetic m = 1 structures coincide reasonably well with the maxima of m = 2 structures of the temperature. These results provide direct support for the observed drifts being due to an azimuthal dynamo wave.

  10. 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.

  11. Numerical dynamo action in cylindrical containers

    NASA Astrophysics Data System (ADS)

    Nore, Caroline; Castanon Quiroz, Daniel; Guermond, Jean-Luc; Léorat, Jacques; Luddens, Francky

    2015-06-01

    The purpose of this paper is to present results from numerical simulations of dynamo action in relation with two magnetohydrodynamics (MHD) experiments using liquid sodium in cylindrical containers. The first one is the von Kármán sodium (VKS) experiment from Cadarache (France), the second one is a precession-driven dynamo experiment from the DREsden sodium facility for DYNamo and thermohydraulic studies (DRESDYN). Contribution to the topical issue "Electrical Engineering Symposium (SGE 2014) - Elected submissions", edited by Adel Razek

  12. Ancient dynamos of terrestrial planets more sensitive to core-mantle boundary heat flows

    NASA Astrophysics Data System (ADS)

    Hori, K.; Wicht, J.; Dietrich, W.

    2014-08-01

    The early dynamos of Earth and Mars probably operated without an inner core being present. They were thus exclusively driven by secular cooling and radiogenic heating, whereas the present geodynamo is thought to be predominantly driven by buoyancy fluxes which arise from the release of latent heat and the compositional enrichment associated with inner core solidification. The impact of the inner core growth on the ancient geodynamo has been discussed extensively but is still controversial. The Martian dynamo stopped operating more than 4 Gyr ago but left its signature in the form of a strong crustal magnetization that is much stronger in the southern than in the northern hemisphere. This dichotomy can, for example, be explained by a dynamo predominantly operating in the southern hemisphere due to a heterogeneous heat flux through the core-mantle boundary (CMB). The early Martian dynamo may also have operated without an inner core being present. Here we explore the impact of lateral CMB heat flux variations on dynamos with and without an inner core by comparing numerical dynamos driven by homogeneous internal sources or by bottom buoyancy sources, arising from the inner core boundary (ICB). Three different CMB heat-flux patterns are tested that either break the northern/southern or the azimuthal symmetry. In the dynamos driven by internal heating a rather small CMB heat-flux heterogeneity suffices to break internal symmetries and leads to boundary-induced structures and different field strengths. The effect is much smaller for dynamos driven by ICB buoyancy sources. Our results indicate that the field intensity and morphology of the ancient dynamos of Earth or Mars were more variable and more sensitive to the thermal CMB structure than the geodynamo after onset of inner core growth.

  13. Broken Symmetries and Magnetic Dynamos

    NASA Technical Reports Server (NTRS)

    Shebalin, John V.

    2007-01-01

    Phase space symmetries inherent in the statistical theory of ideal magnetohydrodynamic (MHD) turbulence are known to be broken dynamically to produce large-scale coherent magnetic structure. Here, results of a numerical study of decaying MHD turbulence are presented that show large-scale coherent structure also arises and persists in the presence of dissipation. Dynamically broken symmetries in MHD turbulence may thus play a fundamental role in the dynamo process.

  14. Torsional oscillations in dynamo simulations

    NASA Astrophysics Data System (ADS)

    Wicht, Johannes; Christensen, Ulrich R.

    2010-06-01

    Cylinders aligned with the planetary rotation axis have a special significance in the dynamics of planetary dynamo regions. The azimuthal Lorentz forces on these geostrophic cylinders is expected to cancel to a large degree, establishing the so-called Taylor state. Deviations from this state take the form of torsional oscillations (TOs) that are supposed to represent important fast flow variations. These oscillations have reportedly been identified in the secular variation signal from the top of Earth's core. We have performed several dynamo simulations at different parameters to check whether Taylor state and TOs can also be identified in a numerical model. Taylor states are approached when viscous effects are small at Ekman numbers of E = 3 × 10-5 or below and Reynolds stresses are kept low by choosing moderate Rayleigh numbers. One-dimensional magnetic Alfvén waves that travel towards the boundaries then become prominent in the motion of the geostrophic cylinders. These waves obey the TO theory but are also damped and modified by other effects. For example, fast variations of likely convective origin remain important in all our simulations. Reynolds stresses may play a more sizable role for the dynamics in Earth's dynamo region than commonly assumed. They may also contribute to the motions of geostrophic cylinders and severely reduce the significance of TOs for the fast core dynamics. The amplitude of TOs amounts to not more than a few percent of the total flow amplitude in the simulations, which renders these motions insignificant for the long-term dynamo process.

  15. Effects of magnetospheric feedback on dynamo action

    NASA Astrophysics Data System (ADS)

    Gomez Perez, N.

    2013-05-01

    The observation of the magnetic field at Mercury over thirty years ago was surprising and unexpected. First, the presence of a global magnetic field in a slow rotator was unforeseen, and second, its magnitude was two orders of magnitude lower than one would expect for a dynamo in a planet the size of Mercury. There are various theories that explain the magnetic field at Mercury, some contemplating the absence of an internal dynamo. With the new data collected by the MESSENGER mission we now expect a dynamo in Mercury's deep interior. This has left fewer theories that can explain the Hermean dynamo. In this paper we will present the feedback dynamo theory which accounts for the interaction of the internal dynamo with the active magnetosphere of the planet. We have found that this interaction may lead to ultra-weak dynamos as required for Mercury. We will analyze under which conditions dynamos are affected by this feedback and how different feedback dynamos compare to spacecraft measurements of Mercury's magnetic field.

  16. A core dynamo in Vesta?

    NASA Astrophysics Data System (ADS)

    Formisano, M.; Federico, C.; De Angelis, S.; De Sanctis, M. C.; Magni, G.

    2016-05-01

    A recent study of Fu et al. analysed the remaining magnetization in the eucrite meteorite Allan Hills A81001, which mostly likely has been produced during the cooling phase of the life of the asteroid Vesta, arguing that an ancient dynamo in the advective liquid metallic core could be set in. Using petrographic and paleomagnetic arguments, Fu et al. estimated a surface magnetic field of at least 2 μT. In this work, we verify the possibility that an early core dynamo took place in Vesta by analysing four different possible fully differentiated configurations of Vesta, characterized by different chondritic compositions, with the constraints on core size and density provided by Ermakov et al. We only incorporate the thermal convection, by neglecting the effects of the compositional convection, so our results in terms of magnetic Reynolds number and duration of the dynamo can be interpreted as a lower bound. The presence of a magnetic field would make Vesta a peculiar object of the Solar system, a `small-Earth', since it has also a differentiated structure like Earth and the magnetic field has preserved Vesta from the space weathering.

  17. Experimental Bullard-von Karman dynamo: MHD saturated regimes

    NASA Astrophysics Data System (ADS)

    Miralles, Sophie; Plihon, Nicolas; Pinton, Jean-François

    2014-05-01

    The dynamo instability, converting kinetic energy into magnetic energy, creates the magnetic fields of many astrophysical bodies for which the flows are highly turbulent. Those turbulent fluctuations restricts the range of parameters of numerical and theoretical predictions. As laboratory experiments are closer from natural parameters, this approach is favored in this work. In the past decades, dynamo action has been observed in experiments involving laminar flows [1] or fully turbulent flows [2] in liquid sodium. Nevertheless, the saturation of the velocity field by the Lorentz force due to the dynamo magnetic field is weak in those experiment because the control parameter is always close to the threshold of the instability (which is not the case in astrophysical situations). The details of the mechanism of the back reaction of Lorentz force on the flow are not known. We present here an experimental semi-synthetic dynamo, for which a fluid turbulent induction mechanism ('omega' effect) is associated to an external amplification applying a current into a pair of coils. The flow, called von-Karman, is produced by the counter rotation of two coaxial propellers in a cylindrical tank filled with liquid gallium. The resulting flow is highly turbulent (Re > 10 ^ 5). The amplification, mimicking a turbulent 'alpha' effect, allow to observe the dynamo instability at low magnetic Reynolds number (Rm ~ 2), far below the threshold of natural homogeneous dynamo. This experiment reaches non linear regimes, for which the saturation is a MHD process, at control parameter several times the critical value. The instability grows through an on-off intermittent regime evolving into a full MHD saturated regime for which the Lorentz force is in balance with the inertial one. The power budget is strongly modified by the dynamo magnetic field and we give an insight of the estimated rate of conversion of kinetic energy into magnetic one from experimental data. Very rich regimes such as

  18. Kinematic Solar Dynamo with Spot Deposition

    NASA Astrophysics Data System (ADS)

    Karak, Bidya Binay; Miesch, Mark S.

    2016-05-01

    We have recently developed a kinematic dynamo model by including the observed differential rotation and the meridional flow. This model includes the emergence of sunspots from the deep-seated toroidal field and their subsequent decay at the surface, i.e., the Babcock-Leighton process for the generation of poloidal field.We shall show that this model reproduces most of the basic features of the solar magnetic cycle including the polarity reversals, 11 years periodicity, equatorward migration of sunspots at low latitudes and the poleward migration of the radial field at the surface. This model also produces the observed cycle variations when the fluctuations in the active-region tilt are included. North-south asymmetries of cycles from this model will also be demonstrated.

  19. Status of the Madison Plasma Dynamo Experiment

    NASA Astrophysics Data System (ADS)

    Wallace, John; Clark, Mike; Kendrick, Roch; Forest, Cary

    2010-11-01

    Construction is underway to build a new experimental facility for investigating self-generation of magnetic fields in plasma and a broader range of flow driven MHD instabilities. The Madison Plasma Dynamo Experiment (MPDX) consists of a 3 meter diameter spherical vacuum chamber lined with a series of high strength neodymium permanent magnet rings in a cusp confinement geometry which provides for a large, unmagnetized and hot plasma. Plasma will be produced by a combination of lanthanum hexaboride cathodes and electron cyclotron heating. The plasma will be stirred from the magnetized edge via electrode and ExB flows. This poster will (1) give an overview of the physics goals and required plasma parameters, (2) describe the engineering design of the facility including laboratory infrastructure, vacuum chamber, diagnostics, and heating systems, and (3) give a status report on the construction schedule. The construction is being funded by the NSF Major Research Instrumentation program.

  20. Status of the Madison Plasma Dynamo Experiment

    NASA Astrophysics Data System (ADS)

    Wallace, John; Clark, Mike; Collins, Cami; Katz, Noam; Weisberg, Dave; Forest, Cary

    2012-10-01

    Construction of the Madison Plasma Dynamo Experiment (MPDX) is complete. This facility creates large, un-magnetized, fast flowing, hot plasma for investigating magnetic field self-generation and flow driven MHD instabilities. A 3 meter diameter spherical vacuum chamber lined with a series of high strength samarium cobalt magnets provides plasma confinement. The plasma will be stirred from the magnetized edge using electrodes to produce JxB flows. Plasma sources will include lanthanum hexaboride cathodes and electron cyclotron heating utilizing five 20KW magnetrons. This poster will describe the operational status of the facility including laboratory infrastructure, cast aluminum vacuum chamber, magnets, stirring electrodes, sources, diagnostics and currently produced plasma parameters. Construction was funded by the NSF Major Research Instrumentation program.

  1. DYNAMO ACTION AND MAGNETIC CYCLES IN F-TYPE STARS

    SciTech Connect

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

    2013-11-10

    Magnetic activity and differential rotation are commonly observed features on main-sequence F-type stars. We seek to make contact with such observations and to provide a self-consistent picture of how differential rotation and magnetic fields arise in the interiors of these stars. The three-dimensional magnetohydrodynamic anelastic spherical harmonic code is employed to simulate global-scale convection and dynamo processes in a 1.2 M{sub ☉} F-type star at two rotation rates. The simulations are carried out in spherical shells that encompass most of the convection zone and a portion of the stably stratified radiative zone below it, allowing us to explore the effects a stable zone has upon the morphology of the global-scale magnetic fields. We find that dynamo action with a high degree of time variation occurs in the star rotating more rapidly at 20 Ω{sub ☉}, with the polarity of the mean field reversing on a timescale of about 1600 days. Between reversals, the magnetic energy rises and falls with a fairly regular period, with three magnetic energy cycles required to complete a reversal. The magnetic energy cycles and polarity reversals arise due to a linking of the polar-slip instability in the stable region and dynamo action present in the convection zone. For the more slowly rotating case (10 Ω{sub ☉}), persistent wreaths of magnetism are established and maintained by dynamo action. Compared to their hydrodynamic progenitors, the dynamo states here involve a marked reduction in the exhibited latitudinal differential rotation, which also vary during the course of a cycle.

  2. Cosmological and astrophysical consequences from the magnetic dynamo equation in torsioned spacetime and teleparallel gravity

    NASA Astrophysics Data System (ADS)

    de Andrade, L. C. G.

    2016-01-01

    A generalized dynamo equation in the first order torsion Garcia de Andrade L C (2012 Phys. Lett. B 711 143) has previously been derived. From this equation it is shown that for the 10 kpc scale, torsion gravity is not able to help seed galactic dynamos since the dynamo time is not long enough to take into account structure formation. In this paper, the dynamo equation is extended to second-order torsion terms—but unfortunately, the situation is even worse and the torsion does not seem to help dynamo efficiency. Nevertheless, in the intergalactic magnetic field scale of 1 mpc, the efficiency of the self-induction equation with torsion changes, and even in the first-order torsion case, one obtains large-scale magnetic fields with 109 yr dynamo efficiency. Dynamo efficiency in the case of interstellar matter (ISM) reaches a diffusion time of 1013 yr. This seems to be in contrast with a recent investigation by Bamba et al (2012 J. Cosmol. Astropart. Phys. JCAP05(2010)08) where they obtained, from another type of torsion theory called teleparallelism (A Einstein, Math Annalen (1922)), a large scale intergalactic magnetic field of 10-9 G. If this is not a model-dependent result, there is an apparent contradiction that has to be addressed. It is shown that for dynamo efficiency in astrophysical flow without shear, a strong seed field of 10-11 G is obtained, which is suitable for seeding galactic dynamos. As an example of a non-parity-violating dynamo equation, a magnetic field of the order of 10-27G is obtained as a seed field for the galactic dynamo from the theory of Einstein’s unified teleparallelism. This shows that in certain gravity models, torsion is able to enhance cosmological magnetic fields in view of obtaining better dynamo efficiency. To better compare our work with Bamba et al (2012 J. Cosmol. Astropart. Phys. JCAP05(2010)08), we consider the slow decay of magnetic fields in the teleparallel model. This observation is due to an anonymous referee who

  3. The Combined Effect of Precession and Convection on the Dynamo Action

    NASA Astrophysics Data System (ADS)

    Wei, Xing

    2016-08-01

    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 result may explain why the geodynamo has been maintained for such a long time compared to the Martian dynamo.

  4. 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.

  5. 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

  6. Gas generation mechanism due to electrolyte decomposition in commercial lithium-ion cell

    NASA Astrophysics Data System (ADS)

    Kumai, Kazuma; Miyashiro, Hajime; Kobayashi, Yo; Takei, Katsuhito; Ishikawa, Rikio

    To elucidate the gas generation mechanism due to electrolyte decomposition in commercial lithium-ion cells after long cycling, we developed a device which can accurately determine the volume of generated gas in the cell. Experiments on Li xC 6/Li 1- xCoO 2 cells using electrolytes such as 1 M LiPF 6 in propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are presented and discussed. In the nominal voltage range (4.2-2.5 V), compositional change due mainly to ester exchange reaction occurs, and gaseous products in the cell are little. Generated gas volume and compositional change in the electrolyte are detected largely in overcharged cells, and we discussed that gas generation due to electrolyte decomposition involves different decomposition reactions in overcharged and overdischarged cells.

  7. Intermittent magnetic field excitations in the Madison Dynamo Experiment

    NASA Astrophysics Data System (ADS)

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

    2006-10-01

    Determining the onset conditions for magnetic field growth in magnetohydrodynamics is fundamental to understanding how astrophysical dynamos such as the Earth, the Sun, and the galaxy self-generate magnetic fields. The role of turbulence in modifying these onset conditions is studied in the Madison Dynamo Experiment. A turbulent flow of liquid sodium, composed primarily of two counter-rotating helical vortices, is generated by impellers. Laser Doppler velocimetry measurements of the flow in an identical-scale water experiment demonstrate that the turbulence is isotropic, though not homogeneous, with particularly long-lived eddies in the shear layer between the two flow cells. The magnetic field induced when an axial field is applied shows intermittent periods of growth and has a spatial structure consistent with the fastest growing magnetic eigenmode predicted by a laminar kinematic dynamo model of the mean flow. Turbulent fluctuations of the velocity field change the flow geometry such that the eigenmode growth rate is temporarily positive, thus generating the magnetic bursts. It is found from ensemble averaging that the bursts gain strength and frequency with increased impeller rotation rate, though they become shorter so that each burst remains a rare, random event. Nornberg et al., Phys. Rev. Lett., in press (2006), physics/0606239.

  8. A shell model for turbulent dynamos

    NASA Astrophysics Data System (ADS)

    Nigro, G.; Perrone, D.; Veltri, P.

    2011-06-01

    A self-consistent nonlinear dynamo model is presented. The nonlinear behavior of the plasma at small scale is described by using a MHD shell model for fields fluctuations; this allow us to study the dynamo problem in a large parameter regime which characterizes the dynamo phenomenon in many natural systems and which is beyond the power of supercomputers at today. The model is able to reproduce dynamical situations in which the system can undergo transactions to different dynamo regimes. In one of these the large-scale magnetic field jumps between two states reproducing the magnetic polarity reversals. From the analysis of long time series of reversals we infer results about the statistics of persistence times, revealing the presence of hidden long-time correlations in the chaotic dynamo process.

  9. Dynamo Effect in the Kraichnan Magnetohydrodynamic Turbulence

    NASA Astrophysics Data System (ADS)

    Arponen, Heikki; Horvai, Peter

    2007-10-01

    The existence of a dynamo effect in a simplified magnetohydrodynamic model of turbulence is considered when the magnetic Prandtl number approaches zero or infinity. The magnetic field is interacting with an incompressible Kraichnan-Kazantsev model velocity field which incorporates also a viscous cutoff scale. An approximate system of equations in the different scaling ranges can be formulated and solved, so that the solution tends to the exact one when the viscous and magnetic-diffusive cutoffs approach zero. In this approximation we are able to determine analytically the conditions for the existence of a dynamo effect and give an estimate of the dynamo growth rate. Among other things we show that in the large magnetic Prandtl number case the dynamo effect is always present. Our analytical estimates are in good agreement with previous numerical studies of the Kraichnan-Kazantsev dynamo by Vincenzi (J. Stat. Phys. 106:1073-1091, 2002).

  10. 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. PMID:22282809

  11. Mechanically-forced dynamos (Invited)

    NASA Astrophysics Data System (ADS)

    Le Bars, M.

    2013-12-01

    It is a commonly accepted hypothesis that convection is responsible for planetary dynamos. However, the validity of the convective dynamo model can be questioned in various planets and moons as well as in asteroids, where the constraints from thermal evolution and compositional core models are sometimes difficult to reconcile with available data from paleomagnetism and in situ measurements. Over the last few years, researches have thus been pursued to find alternative mechanisms for sustaining intense three-dimensional motions in liquid cores, a necessary ingredient for planetary dynamo. In particular, mechanical forcings driven by libration, precession, nutation and tides, have received a renewed interest, following the first studies by Malkus in the 60's. A huge reservoir of energy is available in the rotational and orbital motions of all planetary systems. If planetary bodies were completely rigid and rotating at a constant spin rate, their fluid layers in the absence of convection would also behave rigidly and follow the spin of their boundaries. But small periodic perturbations of the shape of the core/mantle boundary (i.e. dynamic tides) and/or small periodic perturbations of the direction of the spin vector (i.e. precession and nutation) and/or small periodic perturbations of the spin rate (i.e. libration) systematically perturb this rigid state. Then, each of these small perturbations is capable of triggering instabilities in fluid layers, conveying energy from the spin and orbital motions to drive intense three-dimensional flows in the liquid cores. With the view to establish a general framework for planetary applications, I will present here the basic physical ingredients of these instabilities, which involve a resonance between the considered mechanical forcing and two inertial waves of the core. I will then review the numerical and experimental validations of this generic principle, and the few magnetohydrodynamic validations of their dynamo capacity

  12. 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.

  13. Theory and Simulation of Magnetohydrodynamic Dynamos and Faraday Rotation for Plasmas of General Composition

    NASA Astrophysics Data System (ADS)

    Park, Kiwan

    2013-03-01

    Many astrophysical phenomena depend on the underlying dynamics of magnetic fields. The observations of accretion disks and their jets, stellar coronae, and the solar corona are all best explained by models where magnetic fields play a central role. Understanding these phenomena requires studying the basic physics of magnetic field generation, magnetic energy transfer into radiating particles, angular momentum transport, and the observational implications of these processes. Each of these topics comprises a large enterprise of research. However, more practically speaking, the nonlinearity in large scale dynamo is known to be determined by magnetic helicity(>), the topological linked number of knotted magnetic field. Magnetic helicity, which is also observed in solar physics, has become an important tool for observational and theoretical study. The first part of my work addresses one aspect of the observational implications of magnetic fields, namely Faraday rotation. It is shown that plasma composition affects the interpretation of Faraday rotation measurements of the field, and in turn how this can be used to help constrain unknown plasma composition. The results are applied to observations of astrophysical jets. The thesis then focuses on the evolution of magnetic fields. In particular, the dynamo amplification of large scale magnetic fields is studied with an emphasis on the basic physics using both numerical simulations and analytic methods. In particular, without differential rotation, a two and three scale mean field (large scale value + fluctuation scales) dynamo theory and statistical methods are introduced. The results are compared to magnetohydrodynamic (MHD) simulations of the Pencil code, which utilizes high order finite difference methods. Simulations in which the energy is initially driven into the system in the form of helical kinetic energy (via kinetic helicity) or helical magnetic energy (via magnetic helicity) reveal the exponential growth of

  14. 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. PMID:27466124

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

    SciTech Connect

    Khalzov, I. V.; Brown, B. P.; Schnack, D. D.; Forest, C. B.; Ebrahimi, F.

    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 NIMROD 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.

  16. Radially dependent large-scale dynamos in global cylindrical shear flows and the local cartesian limit

    NASA Astrophysics Data System (ADS)

    Ebrahimi, F.; Blackman, E. G.

    2016-06-01

    For cylindrical differentially rotating plasmas, we study large-scale magnetic field generation from finite amplitude non-axisymmetric perturbations by comparing numerical simulations with quasi-linear analytic theory. When initiated with a vertical magnetic field of either zero or finite net flux, our global cylindrical simulations exhibit the magnetorotational instability (MRI) and large-scale dynamo growth of radially alternating mean fields, averaged over height and azimuth. This dynamo growth is explained by our analytic calculations of a non-axisymmetric fluctuation-induced electromotive force that is sustained by azimuthal shear of the fluctuating fields. The standard `Ω effect' (shear of the mean field by differential rotation) is unimportant. For the MRI case, we express the large-scale dynamo field as a function of differential rotation. The resulting radially alternating large-scale fields may have implications for angular momentum transport in discs and corona. To connect with previous work on large-scale dynamos with local linear shear and identify the minimum conditions needed for large-scale field growth, we also solve our equations in local Cartesian coordinates. We find that large-scale dynamo growth in a linear shear flow without rotation can be sustained by shear plus non-axisymmetric fluctuations - even if not helical, a seemingly previously unidentified distinction. The linear shear flow dynamo emerges as a more restricted version of our more general new global cylindrical calculations.

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

    SciTech Connect

    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, 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.

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

    NASA Astrophysics Data System (ADS)

    Wright, Nicholas J.; Drake, Jeremy J.

    2016-07-01

    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.

  19. A STUDY OF THE DYNAMO TRANSITION IN A SELF-CONSISTENT NONLINEAR DYNAMO MODEL

    SciTech Connect

    Nigro, Giuseppina; Veltri, Pierluigi

    2011-10-20

    We develop a nonlinear dynamo model that couples the evolution of a large-scale magnetic field with the turbulent dynamics of a magnetofluid system in the small scale by electromotive force. Because the dynamo effect takes place in astrophysical objects characterized by a range of dynamical parameters (Reynolds numbers, Prandtl number, etc.) which is beyond the current possibilities of direct numerical simulations, we describe the nonlinear behavior of the system at small scales by using a shell model. Under specific conditions of the turbulent state, the field fluctuations at small scales are able to trigger the dynamo instability. The stability curve derived from our simulations allows us to gain some insight not only into the regime of parameters analyzed up to this point but also for very large Prandtl numbers. Moreover, from our analysis, it is shown that the large-scale dynamo transition displays a hysteretic behavior revealing its subcritical nature. The system, undergoing dynamo transition, can reach different dynamo regimes depending on the Reynolds numbers of the magnetic flow. This points out the critical role that turbulence plays in the dynamo phenomenon. Moreover, in this Letter, we show the presence of the natural ordering of dynamo regimes (oscillatory-reversing-steady dynamos) observed in the large-scale magnetic field for increasing magnetic Reynolds numbers. However, the signature of these regimes is also found in the small-scale dynamo by looking at the scaling properties of magnetic fluctuation energy as a function of magnetic Reynolds number.

  20. Aurora on Uranus - A Faraday disc dynamo mechanism

    NASA Astrophysics Data System (ADS)

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

    1983-10-01

    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'.

  1. Magnetic cycle of LQ Hydrae: observational indications and dynamo model

    NASA Astrophysics Data System (ADS)

    Kitchatinov, L. L.; Jardine, M.; Donati, J.-F.

    2000-11-01

    We present a model for the differential rotation and dynamo activity of the young rapidly rotating K0 dwarf LQ Hya (Prot=1.6d). As might be expected from observations of the similar rapid rotator AB Dor, the predicted differential rotation is small (~=0.8per cent) but extremely efficient in generating magnetic fields. The dynamo, which is of a distributed type, produces a globally axisymmetric field with radial and azimuthal components that are of the same magnitude and display a phase-lag in their evolution of about π/2. This is consistent with the long-term Zeeman-Doppler imaging study by Donati. The latitudinal distribution of flux is, however, a little different from that observed and the cycle period of 3.2yr is somewhat shorter than suggested by the observations.

  2. 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

  3. INTERIOR STRUCTURE OF WATER PLANETS: IMPLICATIONS FOR THEIR DYNAMO SOURCE REGIONS

    SciTech Connect

    Yunsheng Tian, Bob; Stanley, Sabine

    2013-05-10

    Recent discoveries of water-rich, sub-Neptunian- to Neptunian-massed exoplanets with short-period orbits present a new parameter space for the study of exoplanetary dynamos. We explore the geometry of the dynamo source region within this parameter space using 1D interior structure models. We model planets with four chemically distinct layers that consist of (1) an iron core, (2) a silicate layer, (3) an H{sub 2}O layer, and (4) an H/He envelope. By varying the total planetary mass in the range of 1-19 M{sub Circled-Plus }, the mass fraction of the H/He envelope between 0.1% and 5.1%, and the equilibrium temperature between 100 K and 1000 K, a survey of the parameter space for potential dynamo source region geometries is conducted. We find that due to the nature of the phase diagram of water at pressure and temperature conditions of planetary interiors, two different dynamo source region geometries are obtainable. Specifically, we find that smaller planets, and planets with thicker H/He envelopes, are likely to be in the regime of a thick-shelled dynamo. Massive planets, and planets with thin H/He envelopes, are likely to be in the regime of a thin-shelled dynamo. Also, small variations of these parameters can produce large interior structure differences. This implies the potential to constrain these parameters based on observations of a planet's magnetic field signature.

  4. Dynamo Driven By Inertial Instabilities, Application to the Moon

    NASA Astrophysics Data System (ADS)

    Cebron, D.; Hollerbach, R.; Vantieghem, S.; Noir, J.; Schaeffer, N.

    2014-12-01

    Large-scale planetary or stellar magnetic fields generated by a dynamo effect are mostly attributed to flows forced by buoyancy forces in electrically conducting fluid layers. However, large-scale, turbulent flows may also be driven by the combined action of boundary topography (i.e. departure from spherical geometry), and mechanical forcings (e.g. libration, tides). This has been previously proposed to explain the magnetic data we have on the star τ-boo, Mars, or the early Moon. In this work, we use theoretical analysis and global magneto-hydrodynamic simulations to show, for the first time, that: (i) the tidal forcing can generate a (dipole-dominated) large-scale magnetic field in global simulations, an hypothesis previously assumed by Le Bars et al. (2011) in their model of lunar magnetic history. (ii) latitudinal libration (i.e. an oscillation of the figure axis with respect to the mean rotation axis) can excite inertial instabilities, which may have driven dynamos in telluric bodies such as the Early Moon. We discuss our results in the light of magnetic observations. In particular, we propose here a new possible mechanism for the early Moon dynamo, based on latitudinal libration driven instabilities. This new scenario is evaluated by comparing the associated estimates of the surface magnetic field strength with the recent paleo-magnetic lunar measurements.

  5. Helicity and the ALPHA-EFFECT:DYNAMO Theory and Observations

    NASA Astrophysics Data System (ADS)

    Kuzanyan, Kirill M.

    The best available tracers of the alpha-effect in the solar convection zone are current helicity and twist of the photospheric magnetic fields obtained by vector magnetographic observations. Here we present results of systematic studies of the current helicity and twist of averaged over a series of solar active regions. The data analysis enables us to reveal latitudinal dependence of the effect which is antisymmetric over the solar equator. Consideration of individual rotation rates of active regions versus the solar internal differential rotation law indicates that the radial structure of the alpha-effect is likely sign-changing. These properties are in agreement with theoretical estimates and numerical simulations of flows in the solar convection zone and model assumptions of dynamo theory. The fine structure of observational signatures of the alpha-effect indicates that the magnetic field generation mainly occurs near the bottom of the convection zone. We revealed some cyclic evolution of current helicity over the solar cycle that is in accord with dynamo models under certain parameter range. Thus knowledge on the spatially-temporal structure of signatures of the alpha-effect leads to further improvement of dynamo theory in description of the mechanism of generation of solar magnetic fields.

  6. A dynamo explanation for Mercury's anomalous magnetic field

    NASA Astrophysics Data System (ADS)

    Cao, Hao; Aurnou, Jonathan M.; Wicht, Johannes; Dietrich, Wieland; Soderlund, Krista M.; Russell, Christopher T.

    2014-06-01

    Recent MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) measurements have shown that Mercury's magnetic field is axial-dominant, yet strongly asymmetric with respect to the equator: the field strength in the Northern Hemisphere is approximately 3 times stronger than that in the Southern Hemisphere. Here we show that convective dynamo models driven by volumetric buoyancy with north-south symmetric thermal boundaries are capable of generating quasi-steady north-south asymmetric magnetic fields similar to Mercury's. This symmetry breaking is promoted and stabilized when the core-mantle boundary heat flux is higher at the equator than at high latitudes. The equatorially asymmetric magnetic field generation in our dynamo models corresponds to equatorially asymmetric kinetic helicity, which results from mutual excitation of two different modes of columnar convection. Our dynamo model can be tested by future assessment of Mercury's magnetic field from MESSENGER and BepiColombo as well as through investigations on Mercury's lower mantle temperature heterogeneity and buoyancy forcing in Mercury's core.

  7. Geometrodynamical Fluid Theory Applied to Dynamo Flows in Planetary Interiors

    NASA Astrophysics Data System (ADS)

    Lewis, Kayla; Miramontes, Diego; Scofield, Dillon

    2015-11-01

    Due to their reliance on a Newtonian viscous stress model, the traditional Navier-Stokes equations are of parabolic type; this in turn leads to acausal behavior of solutions to these equations, e.g., a localized disturbance at any point instantaneously affects the solution arbitrarily far away. Geometrodynamical fluid theory (GFT) avoids this problem through a relativistically covariant formulation of the flow equations. Using GFT, we derive the magnetohydrodynamic equations describing the balance of energy-momentum appropriate for dynamo flows in planetary interiors. These equations include interactions between magnetic and fluid vortex fields. We derive scaling laws from these equations and compare them with scaling laws derived from the traditional approach. Finally, we discuss implications of these scalings for flows in planetary dynamos.

  8. Dependence of magnetic cycle parameters on period of rotation in non-linear solar-type dynamos

    NASA Astrophysics Data System (ADS)

    Pipin, V. V.

    2015-08-01

    Parameters of magnetic activity on the solar-type stars depend on the properties of the dynamo processes operating in stellar convection zones. We apply non-linear mean-field axisymmetric α2Ω dynamo models to calculate of the magnetic cycle parameters, such as the dynamo cycle period, the total magnetic flux and the Poynting magnetic energy flux on the surface of solar analogues with the rotation periods from 15 to 30 d. The models take into account the principal non-linear mechanisms of the large-scale dynamo, such as the magnetic helicity conservation, magnetic buoyancy and effects of magnetic forces on the angular momentum balance inside the convection zones. Also, we consider two types of the dynamo models. The distributed (D-type) models employ the standard α-effect distributed on the whole convection zone. The `boundary' (B-type) models employ the non-local α-effect, which is confined to the boundaries of the convection zone. Both the D- and B-type models show that the dynamo-generated magnetic flux increases with the increase of the stellar rotation rate. It is found that for the considered range of the rotational periods the magnetic helicity conservation is the most significant effect for the non-linear quenching of the dynamo. This quenching is more efficient in the B-type than in the D-type dynamo models. The D-type dynamo reproduces the observed dependence of the cycle period on the rotation rate for the Sun analogues. For the solar analogue rotating with a period of 15 d, we find non-linear dynamo regimes with multiply cycles.

  9. 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.

  10. Statistical simulation of the magnetorotational dynamo

    SciTech Connect

    Squire, J.; Bhattacharjee, A.

    2014-08-01

    We analyze turbulence and dynamo induced by the magnetorotational instability (MRI) using quasi-linear statistical simulation methods. We find that homogenous turbulence is unstable to a large scale dynamo instability, which saturates to an inhomogenous equilibrium with a very strong dependence on the magnetic Prandtl number (Pm). Despite its enormously reduced nonlinearity, the quasi-linear model exhibits the same qualitative scaling of angular momentum transport with Pm as fully nonlinear turbulence. This demonstrates the relationship of recent convergence problems to the large scale dynamo and suggests possible methods for studying astrophysically relevant regimes at very low or high Pm.

  11. 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

  12. Consistent Scaling Laws in Anelastic Spherical Shell Dynamos

    NASA Astrophysics Data System (ADS)

    Yadav, Rakesh K.; Gastine, Thomas; Christensen, Ulrich R.; Duarte, Lúcia D. V.

    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 analyze 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.

  13. CONSISTENT SCALING LAWS IN ANELASTIC SPHERICAL SHELL DYNAMOS

    SciTech Connect

    Yadav, Rakesh K.; Gastine, Thomas; Christensen, Ulrich R.; Duarte, Lucia D. V.

    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 analyze 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.

  14. Magnetic Cycles and Buoyant Loops in Convective Dynamos

    NASA Astrophysics Data System (ADS)

    Nelson, Nicholas J.

    2013-01-01

    Solar-type stars display a rich spectrum of magnetic activity. Seeking to explore convective dynamo action in solar-like stars with the anelastic spherical harmonic (ASH) code, we have carried out a series of global 3-D MHD simulations. Here we report on the dynamo mechanisms realized in a series of numerical models of a sun-like star which explore the effects of decreasing diffusion. While these models nominally rotate at three times the current solar rate (3Ω), the results may be more widely applicable as both these simulations and the solar convection zone achieve similar levels of rotationally constrained convection. Previous simulations at 3Ω have shown that convective dynamos can build persistent wreath-like structures of strong toroidal magnetic field in the convection zone (Brown et al. 2010). Here we find that magnetic reversals and cycles can be realized at 3Ω by decreasing the explicit diffusion and thereby making the resolved flows more turbulent. In these more turbulent models, diffusive processes no longer play a primary role in the key dynamical balances which maintain differential rotation and generate the global-scale wreaths. With reduced resistive diffusion of magnetic fields, the axisymmetric poloidal fields can no longer achieve a steady state and this triggers reversals in global magnetic polarity. Additionally, the enhanced levels of turbulence lead to greater intermittency in the toroidal magnetic wreaths, which can create buoyant magnetic loops that rise from the deep interior to the upper regions of our simulated domain. Turbulence-enabled magnetic buoyancy in our most turbulent simulation yields large numbers of buoyant loops, enabling us to examine the distribution of the characteristics of buoyant magnetic loops, such as twist, tilt angle, and relation to axisymmetric fields. These models provide a pathway towards linking convective dynamo models and the emergence of magnetic flux in the Sun and sun-like stars.

  15. Effects of Tachocline in Solar-Stellar Dynamo Simulations

    NASA Astrophysics Data System (ADS)

    Kosovichev, Alexander; Guerrero, Gustavo; Smolarkiewicz, Petr; Gouveia Dal Pino, Elisabete; Mansour, Nagi N.

    2015-08-01

    Using 3D implicit large-eddy simulations and comparing these with observational constrains we investigate variations of the solar rotation and dynamo-generated magnetic fields. Two sets of simulations models are explored, the first one considers the solar convective envelope only; the second one, aiming at the formation of a tachocline, considers also a fraction of the radiative core. In both kind of models the rotation profile and the dynamo solution depend on the Rossby number, however, the flux of angular momentum is also affected by the Lorentz force. The slow rotating models without tachocline reproduce remarkably well the solar differential rotation in the convection zone (CZ) including the tilted iso-rotation contours and the near-surface shear layer (NSSL). Two of these models exhibit periodic reversals of the magnetic field with a cycle of ˜ 2 yr. With this short dynamic timescale, the NSSL apparently does not affect the migration of the surface magnetic field which results to be poleward. The radial shear at the tachocline gives rise to a strong toroidal magnetic field at this location. Since it is also deposited in the stable layer below, it diffuses in longer timescales than the field at the CZ. These models result either in an oscillatory dynamo with a ˜ 30 yr cycle period or in steady fields. In all the models with magnetic cycles the evolution of the fields seems to be consistent with dynamo waves propagating according to the Parker-Yoshimura sign rule. We compare the modeling results with recent helioseismology inferences, and observations of solar and stellar differential rotation and cycles, including intermittency of the Maunder-minimum type.

  16. Dynamos, Domains, and Paleomagnetic Poles

    NASA Astrophysics Data System (ADS)

    Kent, Dennis; Pan, Yongxin

    2011-05-01

    Earth's and Planetary Interiors: Observation and Numerical Models of Paleomagnetic and Planetary Magnetism; Beijing, China, 7-11 July 2010 ; The second international Beijing Earth and Planetary Interior Symposium (BEPIS; http://www.paleomag.net/meeting) was held at the Institute of Geology and Geophysics, Chinese Academy of Sciences (CAS), just down the road from the Bird's Nest and other iconic structures of the 2008 Beijing Olympics. The symposium was organized by Rixiang Zhu (CAS, Beijing, China) and Keke Zhang (Exeter University, Exeter, UK) and brought together more than 100 scientists, including 30 graduate students from 10 countries. Thirty-nine invited talks were organized along three major themes: planetary dynamos, paleomagnetism, and mineral magnetism. The talks were held in alternating and sometimes closely interleaved sessions and were supported by 40 poster presentations.

  17. Saturation of the turbulent dynamo.

    PubMed

    Schober, J; Schleicher, D R G; Federrath, C; Bovino, S; Klessen, R S

    2015-08-01

    The origin of strong magnetic fields in the Universe can be explained by amplifying weak seed fields via turbulent motions on small spatial scales and subsequently transporting the magnetic energy to larger scales. This process is known as the turbulent dynamo and depends on the properties of turbulence, i.e., on the hydrodynamical Reynolds number and the compressibility of the gas, and on the magnetic diffusivity. While we know the growth rate of the magnetic energy in the linear regime, the saturation level, i.e., the ratio of magnetic energy to turbulent kinetic energy that can be reached, is not known from analytical calculations. In this paper we present a scale-dependent saturation model based on an effective turbulent resistivity which is determined by the turnover time scale of turbulent eddies and the magnetic energy density. The magnetic resistivity increases compared to the Spitzer value and the effective scale on which the magnetic energy spectrum is at its maximum moves to larger spatial scales. This process ends when the peak reaches a characteristic wave number k☆ which is determined by the critical magnetic Reynolds number. The saturation level of the dynamo also depends on the type of turbulence and differs for the limits of large and small magnetic Prandtl numbers Pm. With our model we find saturation levels between 43.8% and 1.3% for Pm≫1 and between 2.43% and 0.135% for Pm≪1, where the higher values refer to incompressible turbulence and the lower ones to highly compressible turbulence. PMID:26382506

  18. Soft iron and axisymetric eigenmodes in the von-Karman-Sodium dynamo

    NASA Astrophysics Data System (ADS)

    Giesecke, A.; Stefani, F.; Gerbeth, G.

    2012-04-01

    In the Cadarache von-Karman-Sodium (VKS) dynamo experiment magnetic field excitation is generated by a turbulent flow of liquid sodium. In the experiment this so called von-Karman-like flow is driven by two counter-rotating impellers that are located close to the end-caps of a cylindrical vessel. Despite of extensive numerical and experimental efforts the very nature of the VKS dynamo and its surprising properties still remain unclear. Firstly, dynamo action is obtained only when (at least one of) the flow driving impellers are made of soft iron with a relative permeability around 65. Moreover, and in apparent contradiction with Cowling's anti-dynamo theorem, the geometric structure of the observed magnetic field is dominated by an axisymmetric field. Our kinematic simulations of an axisymmetric model of the Cadarache dynamo show a close connection between the exclusive occurrence of dynamo action with soft iron impellers and the axisymmetry of the magnetic field. We observe two distinct classes of axisymmetric eigenmodes, a purely toroidal mode that is amplified by paramagnetic pumping at the fluid-disk interface and a mixed mode consisting of a poloidal and a toroidal contribution that is rather insensitive to the disk permeability. In the limit of large permeability, the purely toroidal mode is close to the onset of dynamo action with a growth-rate that is rather independent of the flow field. This mode is located near to and in the high permeability disks and becomes the leading mode when the disk permeability exceeds a critical value. However, since in our axisymmetric configuration the purely toroidal mode is decoupled from any poloidal field component no dynamo action can be expected from this mode. The purely toroidal mode and its strong amplification by paramagnetic pumping at the fluid-disks interface can be obtained only by explicitly considering the internal permeability distribution. This mode does not exist in case of highly conducting disks or in

  19. Faraday's first dynamo: An alternate analysis

    NASA Astrophysics Data System (ADS)

    Redinz, José Arnaldo

    2015-02-01

    The steady-state charge densities, electric potential, and current densities are determined analytically in the case of the first dynamo created by Michael Faraday, which consists of a conducting disk rotating between the poles of an off-axis permanent magnet. The results obtained are compared with another work that considered the same problem using a different approach. We also obtain analytical expressions for the total current on the disk and for the dynamo's electromotive force.

  20. Buoyant Magnetic Loops in a Global Dynamo Simulation of a Young Sun

    NASA Astrophysics Data System (ADS)

    Nelson, Nicholas J.; Brown, Benjamin P.; Brun, Allan Sacha; Miesch, Mark S.; Toomre, Juri

    2011-10-01

    The current dynamo paradigm for the Sun and Sun-like stars places the generation site for strong toroidal magnetic structures deep in the solar interior. Sunspots and starspots on Sun-like stars are believed to arise when sections of these magnetic structures become buoyantly unstable and rise from the deep interior to the photosphere. Here, we present the first three-dimensional global magnetohydrodynamic (MHD) simulation in which turbulent convection, stratification, and rotation combine to yield a dynamo that self-consistently generates buoyant magnetic loops. We simulate stellar convection and dynamo action in a spherical shell with solar stratification, but rotating three times faster than the current solar rate. Strong wreaths of toroidal magnetic field are realized by dynamo action in the convection zone. By turning to a dynamic Smagorinsky model for subgrid-scale turbulence, we here attain considerably reduced diffusion in our simulation. This permits the regions of strongest magnetic field in these wreaths to rise toward the top of the convection zone via a combination of magnetic buoyancy instabilities and advection by convective giant cells. Such a global simulation yielding buoyant loops represents a significant step forward in combining numerical models of dynamo action and flux emergence.

  1. Symmetry and couplings in stationary Von Kármán sodium dynamos

    NASA Astrophysics Data System (ADS)

    Boisson, J.; Aumaitre, S.; Bonnefoy, N.; Bourgoin, M.; Daviaud, F.; Dubrulle, B.; Odier, Ph; Pinton, J.-F.; Plihon, N.; Verhille, G.

    2012-01-01

    We study different types of stationary dynamos observed in the Von Kármán sodium (VKS) experiment when varying the electromagnetic boundary conditions on (and in) the impellers. The flow is driven with two impellers made of soft iron (Monchaux et al 2007 Phys Rev. Lett. 98 044502) or using one soft-iron impeller and one stainless steel impeller. The magnetic field is mapped using 40 three-dimensional probes distributed within the flow and its surroundings. Symmetry and coupling properties are then retrieved from direct probe measurements and/or from the field structure as reconstructed using the inversion procedure described by Boisson and Dubrulle (2011 New J. Phys. 13 023037). Several salient results are obtained: (i) dynamo action is not achieved unless at least one iron impeller is rotating, at a frequency larger than 15 Hz (ii) the resulting dynamo is a dipolar, mostly axisymmetric structure; and (iii) the self-sustained magnetic field properties depend on the sodium flow structure between the two impellers. We propose to interpret the stationary dynamos generation as the (constructive or destructive) superposition of two one-impeller fluid dynamos generated close to the soft-iron impellers, nonlinearly coupled through the turbulent flow, as suggested by Verhille et al (2010 New J. Phys. 12 033006). The normal form equation describing this coupling is similar to the one obtained in a theoretical model (Pétrélis et al 2009 Phys. Rev. Lett. 102 144503).

  2. BUOYANT MAGNETIC LOOPS IN A GLOBAL DYNAMO SIMULATION OF A YOUNG SUN

    SciTech Connect

    Nelson, Nicholas J.; Toomre, Juri; Brown, Benjamin P.; Brun, Allan Sacha

    2011-10-01

    The current dynamo paradigm for the Sun and Sun-like stars places the generation site for strong toroidal magnetic structures deep in the solar interior. Sunspots and starspots on Sun-like stars are believed to arise when sections of these magnetic structures become buoyantly unstable and rise from the deep interior to the photosphere. Here, we present the first three-dimensional global magnetohydrodynamic (MHD) simulation in which turbulent convection, stratification, and rotation combine to yield a dynamo that self-consistently generates buoyant magnetic loops. We simulate stellar convection and dynamo action in a spherical shell with solar stratification, but rotating three times faster than the current solar rate. Strong wreaths of toroidal magnetic field are realized by dynamo action in the convection zone. By turning to a dynamic Smagorinsky model for subgrid-scale turbulence, we here attain considerably reduced diffusion in our simulation. This permits the regions of strongest magnetic field in these wreaths to rise toward the top of the convection zone via a combination of magnetic buoyancy instabilities and advection by convective giant cells. Such a global simulation yielding buoyant loops represents a significant step forward in combining numerical models of dynamo action and flux emergence.

  3. Precession-driven dynamos in a full sphere and the role of large scale cyclonic vortices

    NASA Astrophysics Data System (ADS)

    Lin, Yufeng; Marti, Philippe; Noir, Jerome; Jackson, Andrew

    2016-06-01

    Precession has been proposed as an alternative power source for planetary dynamos. Previous hydrodynamic simulations suggested that precession can generate very complex flows in planetary liquid cores [Y. Lin, P. Marti, and J. Noir, "Shear-driven parametric instability in a precessing sphere," Phys. Fluids 27, 046601 (2015)]. In the present study, we numerically investigate the magnetohydrodynamics of a precessing sphere. We demonstrate precession driven dynamos in different flow regimes, from laminar to turbulent flows. In particular, we highlight the magnetic field generation by large scale cyclonic vortices, which has not been explored previously. In this regime, dynamos can be sustained at relatively low Ekman numbers and magnetic Prandtl numbers, which paves the way for planetary applications.

  4. Self-generated overvoltages due to open-phasing of ungrounded-wye delta transformer banks

    SciTech Connect

    Walling, R.A.; Hartana, R.K.; Ros, W.J.

    1994-12-31

    Disconnecting one phase of an ungrounded-wye delta transformer bank from the source can result in severe overvoltages due to neutral shift and ferroresonance. Neutral shift overvoltages are quantified and their impact on metal-oxide surge arresters are evaluated. Ferroresonance is shown by test data to occur when low-loss banks, 15 kV and higher, are open phased. Design and operating practices to eliminate or mitigate these self-generated overvoltages are discussed.

  5. Self-generated overvoltages due to open-phasing of ungrounded-wye delta transformer banks

    SciTech Connect

    Walling, R.A.; Hartana, R.K.; Ros, W.J.

    1995-01-01

    Disconnecting one phase of an ungrounded-wye delta transformer bank from the source can result In severe overvoltages due to neutral shirt and ferroresonance. Neutral shirt overvoltages are quantified and their Impact on metal-oxide surge arresters are evaluated. Ferroresonance is shown by test data to occur when low-loss banks, 15 kV and higher, are open phased. Design and operating practices to eliminate or mitigate these self-generated overvoltages are discussed.

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

    SciTech Connect

    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 of 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.

  7. Dynamics in large generators due to oval rotor and triangular stator shape

    NASA Astrophysics Data System (ADS)

    Lundström, Niklas L. P.; Aidanpää, Jan-Olov

    2011-02-01

    Earlier measurements in large synchronous generators indicate the existence of complex whirling motion, and also deviations of shape in both the rotor and the stator. These non-symmetric geometries produce an attraction force between the rotor and the stator, called unbalanced magnetic pull (UMP). The target of this paper is to analyse responses due to certain deviations of shape in the rotor and the stator. In particular, the perturbation on the rotor is considered to be of oval character, and the perturbations of the stator are considered triangular. By numerical and analytical methods it is concluded for which generator parameters harmful conditions, such as complicated whirling motion and high amplitudes, will occur. During maintenance of hydro power generators the shapes of the rotor and stator are frequently measured. The results from this paper can be used to evaluate such measurements and to explain the existence of complex whirling motion.

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

    SciTech Connect

    Stepanovs, Deniss; Fendt, Christian; Sheikhnezami, Somayeh 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 outer 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.

  9. CONVECTIVE BABCOCK-LEIGHTON DYNAMO MODELS

    SciTech Connect

    Miesch, Mark S.; Brown, Benjamin P.

    2012-02-20

    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 unclear 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.

  10. Dynamic consequences of electromagnetic pull due to deviations in generator shape

    NASA Astrophysics Data System (ADS)

    Lundström, Niklas L. P.; Aidanpää, Jan-Olov

    2007-03-01

    Results from earlier measurements on hydropower generators have indicated relatively large eccentricities and shape deviations in the rotor and stator. These non-symmetric geometries produce an attraction force between the rotor and the stator, called unbalanced magnetic pull (UMP). The UMP force can produce large vibrations which can be dangerous to the machine. A mathematical model is developed to describe the shapes of the rotor and stator, and the corresponding UMP is obtained through the law of energy conservation. The target of the paper is to analyse the dynamics of a generator due to shape deviations in the rotor and stator. As rotor-model, a balanced Jeffcott rotor is used. A linearization of the UMP indicates the importance of considering the nonlinear effects. The stability of some attractors are analysed and the generator dynamics are further investigated by simulating the basin of attraction. The magnitudes are approximately obtained when the shape deviations become dangerous for the generator. It is concluded which shape deviations that are more dangerous than others. In hydropower generator maintenance the shapes of the rotor and stator are frequently measured. The results from this paper can be used to evaluate such measurements and estimate the stability and robustness by simulations.

  11. TURBULENT CROSS-HELICITY IN THE MEAN-FIELD SOLAR DYNAMO PROBLEM

    SciTech Connect

    Pipin, V. V.; Kuzanyan, K. M.; Zhang, H.; Kosovichev, A. G.

    2011-12-20

    We study the dynamical and statistical properties of turbulent cross-helicity (correlation of the aligned fluctuating velocity and magnetic field components). We derive an equation governing generation and evolution of the turbulent cross-helicity and discuss its meaning for the dynamo. Using the symmetry properties of the problem we suggest a general expression for the turbulent cross-helicity. Effects of the density stratification, large-scale magnetic fields, differential rotation, and turbulent convection are taken into account. We investigate the relative contribution of these effects to the cross-helicity evolution for two kinds of dynamo models of the solar cycle: a distributed mean-field model and a flux-transport dynamo model. We show that the contribution from the density stratification follows the evolution of the radial magnetic field, while large-scale electric currents produce a more complicated pattern of the cross-helicity of comparable magnitude. The pattern of the cross-helicity evolution strongly depends on details of the dynamo mechanism. Thus, we anticipate that direct observations of the cross-helicity on the Sun may serve for the diagnostic purpose of the solar dynamo process.

  12. Wave drag due to generation of capillary-gravity surface waves

    NASA Astrophysics Data System (ADS)

    Burghelea, Teodor; Steinberg, Victor

    2002-11-01

    The onset of the wave resistance via the generation of capillary-gravity waves by a small object moving with a velocity V is investigated experimentally. Due to the existence of a minimum phase velocity Vc for surface waves, the problem is similar to the generation of rotons in superfluid helium near their minimum. In both cases, waves or rotons are produced at V>Vc due to Cherenkov radiation. We find that the transition to the wave drag state is continuous: in the vicinity of the bifurcation the wave resistance force is proportional to (V-Vc) for various fluids. This observation contradicts the theory of Raphaël and de Gennes. We also find that the reduced wave drag force for different fluids and different ball size may be scaled in such a way that all the data collapse on a single curve. The capillary-gravity wave pattern and the shape of the wave-generating region are investigated both experimentally and theoretically. Good agreement between the theory and the experimental data is found in this case.

  13. 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

  14. 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).

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

    PubMed

    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

  16. Contrast degradation in a chirped-pulse amplifier due to generation of prepulses by postpulses.

    PubMed

    Didenko, N V; Konyashchenko, A V; Lutsenko, A P; Tenyakov, S Yu

    2008-03-01

    Experiment and modeling show that the refractive index nonlinearity can significantly degrade the contrast of a chirped-pulse amplifier seeded with a pulse and a single postpulse. Multiple powerful non-equidistant pre- and postpulses are generated. For a Gaussian pulse and a hat-top beam, an incident postpulse of energy W results in a prepulse of energy 0.58B(2)W, where B is the nonlinear phase (B-integral) of the main pulse. Calculations show that level of satellites due to gain saturation is negligibly small. Experimental results for Ti:Sapphire regenerative and multipass amplifiers and prepulse generation in fused silica agree well with the theory. PMID:18542405

  17. ANALYSIS OF DISTRIBUTION FEEDER LOSSES DUE TO ADDITION OF DISTRIBUTED PHOTOVOLTAIC GENERATORS

    SciTech Connect

    Tuffner, Francis K.; Singh, Ruchi

    2011-08-09

    Distributed generators (DG) are small scale power supplying sources owned by customers or utilities and scattered throughout the power system distribution network. Distributed generation can be both renewable and non-renewable. Addition of distributed generation is primarily to increase feeder capacity and to provide peak load reduction. However, this addition comes with several impacts on the distribution feeder. Several studies have shown that addition of DG leads to reduction of feeder loss. However, most of these studies have considered lumped load and distributed load models to analyze the effects on system losses, where the dynamic variation of load due to seasonal changes is ignored. It is very important for utilities to minimize the losses under all scenarios to decrease revenue losses, promote efficient asset utilization, and therefore, increase feeder capacity. This paper will investigate an IEEE 13-node feeder populated with photovoltaic generators on detailed residential houses with water heater, Heating Ventilation and Air conditioning (HVAC) units, lights, and other plug and convenience loads. An analysis of losses for different power system components, such as transformers, underground and overhead lines, and triplex lines, will be performed. The analysis will utilize different seasons and different solar penetration levels (15%, 30%).

  18. Fluctuation dynamo amplified by intermittent shear bursts in convectively driven magnetohydrodynamic turbulence

    NASA Astrophysics Data System (ADS)

    Pratt, J.; Busse, A.; Müller, W.-C.

    2013-09-01

    Intermittent large-scale high-shear flows are found to occur frequently and spontaneously in direct numerical simulations of statistically stationary turbulent Boussinesq magnetohydrodynamic (MHD) convection. The energetic steady state of the system is sustained by convective driving of the velocity field and small-scale dynamo action. The intermittent emergence of flow structures with strong velocity and magnetic shearing generates magnetic energy at an elevated rate on time scales that are longer than the characteristic time of the large-scale convective motion. The resilience of magnetic energy amplification suggests that intermittent shear bursts are a significant driver of dynamo action in turbulent magnetoconvection.

  19. Rotation and magnetism of solar-like stars: from scaling laws to spot-dynamos

    NASA Astrophysics Data System (ADS)

    Brun, Allan Sacha

    2014-08-01

    The Sun is the archetype of magnetic star and its proximity coupled with very high accuracy observations has helped us understanding how solar-like stars (e.g with a convective envelope) redistribute angular momentum and generate a cyclic magnetic field. However most solar models have been so fine tuned that when they are applied to other solar-like stars the agreement with observations is not good enough. I will thus discuss, based on theoretical considerations and multi-D MHD stellar models, what can be considered as robust properties of solar-like star dynamics and magnetism and what is still speculative. I will derive scaling laws for differential rotation and magnetic energy as a function of stellar parameters, discuss recent results of stellar dynamo models and define the new concept of spot-dynamo, e.g. global dynamo that develops self-consistent magnetic buoyant structures that emerge at the surface.

  20. Effect of collisionality and diamagnetism on the plasma dynamo

    SciTech Connect

    Ji, H.; Yagi, Y.; Hattori, K.; Almagri, A.F.; Prager, S.C.; Hirano, Y.; Sarff, J.S.; Shimada, T.; Maejima, Y.; Hayase, K. ||

    1995-08-07

    Fluctuation-induced dynamo electric fields 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 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.

  1. The effect of collisionality and diamagnetism on the plasma dynamo

    SciTech Connect

    Ji, H.; Yagi, Y.; Hattori, K.; Hirano, Y.; Shimada, T.; Maejima, Y.; Hayase, K.; Almagri, A.F.; Prager, S.C.; Sarff, J.S.

    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.

  2. Nonmodal growth and the magnetorotational dynamo instability

    NASA Astrophysics Data System (ADS)

    Squire, Jonathan; Bhattacharjee, Amitava

    2014-10-01

    Unravelling the important dynamo processes in magnetized rotating shear flows remains fundamental in understanding turbulent transport in astrophysical disks. We consider the dynamo of the magnetorotational instability (MRI) in its simplest possible form, studying the unstratified shearing box without a mean magnetic field. Despite the lack of spectral instability, sustained turbulence and dynamo is possible in this system, with the non-normality of the linear operator playing an important role. An analysis of the MRI from this non-normal perspective has proved enlightening, illustrating that the fastest growing non-axisymmetric disturbances are very different from the eigenmodes, invariably resembling waves shearing with the background flow (shear waves). With the goal of understanding the core dynamo process, we evolve an statistical ensemble of shear waves in a quasi-linear version of the shearing box system. Among the most interesting ideas resulting from this approach is the existence of a mean field dynamo instability of homogenous background turbulence. The instability saturates at levels consistent with fully nonlinear turbulence simulations and depends strongly on magnetic Prandtl number. This work was supported by Max Planck/Princeton Center for Plasma Physics and U.S. DOE (DE-AC02-09CH11466).

  3. Stellar Surface Differential Rotation from Dynamo Models

    NASA Astrophysics Data System (ADS)

    Korhonen, H.; Elstner, D.

    2006-08-01

    We have previously published dynamo models that can reproduce the flip-flop phenomenon. In this phenomenon the main part of the stellar activity changes longitude by 180 degrees. Here we use these dynamo models for studying the stellar surface differential rotation. We use standard cross-correlation methods to study the changes in the magnetic pressure maps obtained from the dynamo calculations. As these maps can be treated the same way as the temperature maps, e.g., ones obtained with the Doppler imaging, we can use the same techniques as for real observations to analyse the maps produced by the dynamo calculations. Our investigation reveals that the input rotation used in the dynamo calculations is not always obtained with the analysis. In some cases even the sign of the surface differential rotation changes from the solar type input surface rotation to anti-solar surface rotation obtained from the analysis. This means that the spot motion is not determined by the differential rotation, but mainly by the underlying magnetic field structure. There is also some indication that in some cases the strength of the surface differential rotation varies with the activity cycle. These results could have important implications for observational studies of the stellar surface differential rotation.

  4. Convective Dynamo Simulation with a Grand Minimum

    NASA Astrophysics Data System (ADS)

    Augustson, Kyle C.; Brun, A. S.; Miesch, Mark; Toomre, Juri

    2015-01-01

    The global-scale dynamo action achieved in a simulation of a Sun-like star rotating at thrice the solar rate is assessed. The 3-D MHD Anelastic Spherical Harmonic (ASH) code, augmented with a viscosity minimization scheme, is employed to capture convection and dynamo processes in this G-type star. The simulation is carried out in a spherical shell that encompasses 3.8 density scale heights of the solar convection zone. It is found that dynamo action with a high degree of time variation occurs, with many periodic polarity reversals occurring roughly every 6.2 years. The magnetic energy also rises and falls with a regular period. The magnetic energy cycles arise from a Lorentz-force feedback on the differential rotation, whereas the processes leading to polarity reversals are more complex, appearing to arise from the interaction of convection with the mean toroidal fields. Moreover, an equatorial migration of toroidal field is found, which is linked to the changing differential rotation, and potentially to a nonlinear dynamo wave. This simulation also enters a grand minimum lasting roughly 20 years, after which the dynamo recovers its regular polarity cycles.

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

    SciTech Connect

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

    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 field strength in the region of equatorward propagation.

  6. MHD Dynamo phenomenon in our lab (Petrus Peregrinus Medal Lecture)

    NASA Astrophysics Data System (ADS)

    Gailitis, Agris

    2016-04-01

    Celestial objects generate magnetic field very like technical dynamo do. Field induces current in a moving electroconductor. The induced current amplifies magnetic field. At large enough product conductivity time's velocity time's size amplification exceeds losses and situation without magnetic field is impossible. Such scenario is obvious for technical dynamo made from insolated wire but not so for uniform conductor as in celestial bodies. Development of the idea took literally the entire 20th century. Discovery of sunspot magnetic fields at the century rise and laboratory verification at the very fall. At thirties Cowling noticed that geometrically simple shaped (axially symmetrical) field can't sustain itself. Process must be more complex, somehow spatially fragmented. At the middle of century Parker and Steenbeck saw such fragmentation in a turbulent structure of hydrodynamic flow. Shortly after his α-effect approach was made ready Steenbeck invited us to think on molten Na experiments for theory verification. The first idea was to push the Na flow through the hand-blown pipe maze. Similar industrial scale experiment after years and regardless of us was realized in Karlsruhe. Seeking for something cheaper we stopped at Ponomarenko idea - axially symmetric helical flow can't generate axi-simmetric field but it can generate azimuthally structured one. The mathematical model was modified to experimental conditions and numerically optimized. The Dynamo stand was built and it works. Even after optimization Dynamo stand exceeds usual size of hydraulic experiments. 2m3 of molten Na circulate there by means of propeller powered from 200kW motor. When circulation exceeds 0.6 m3/s (at 120°C) seemingly from nowhere appears magnetic field. Twisted field pattern slowly (about 1.5Hz) rotates round flow axis. Up to 0.1T field stay as long as stay circulation and temperature. When sodium is heated up or slowed down the field is slowly dying out. Phenomenon is much richer

  7. Overview of Recent Upgrades to the Madison Dynamo Experiment

    NASA Astrophysics Data System (ADS)

    Kaplan, Elliot; Forest, Cary; Kendrick, Roch; Parada, Carlos; Taylor, Zane; Nornberg, Mark; Spence, Erik

    2007-11-01

    The Madison Dynamo Experiment is designed to function as a simply-connected, homogeneous dynamo. A turbulent flow of liquid sodium is driven by two counter-rotating impellers in a one-meter-diameter sphere. The experiment is presently undergoing upgrades to it's magnetic diagnostics and seed field coils to better refine the measurement of turbulence driven currents. A high current amplifier, to drive the experiment's seed magnetic field coils, is under development that will be able to generate a >200 gauss sinusoidal magnetic field in the .1-5 Hz frequency band. The current wave form is generated by applying pulse-width-modulated square waves to a set of four IGBT switches in an H Bridge configuration which allows the current to flow in either direction through the external field coils. The duty cycle is determined through one of two methods: An analog circuit generates a reference sine wave and a modulating triangle wave in an intersective PWM circuit; a Labview Realtime control that uses a PID feedback loop to calculate the duty cycle. This is replacing the present system of a single IGBT turning on a DC current through the coils. The primary physics goal for this hardware is to measure the electrical skin depth of large scale magnetic perturbation and unravel the nature of the turbulent resistivity of the experiment.

  8. Towards understanding dynamo action in M dwarfs

    NASA Astrophysics Data System (ADS)

    Shulyak, D.; Sokoloff, D.; Kitchatinov, L.; Moss, D.

    2015-06-01

    Recent progress in observational studies of magnetic activity in M dwarfs urgently requires support from ideas of stellar dynamo theory. We propose a strategy to connect observational and theoretical studies. In particular, we suggest four magnetic configurations that appear relevant to dwarfs from the viewpoint of the most conservative version of dynamo theory, and discuss observational tests to identify the configurations observationally. As expected, any such identification contains substantial uncertainties. However, the situation in general looks less pessimistic than might be expected. Several identifications between the phenomenology of individual stars and dynamo models are suggested. Remarkably, all models discussed predict substantial surface magnetic activity at rather high stellar latitudes. This prediction looks unexpected from the viewpoint of our experience observing the Sun (which of course differs in some fundamental ways from these late-type dwarfs). We stress that a fuller understanding of the topic requires a long-term (at least 15 years) monitoring of M dwarfs by Zeeman-Doppler imaging.

  9. Planetary dynamos driven by helical waves - II

    NASA Astrophysics Data System (ADS)

    Davidson, P. A.; Ranjan, A.

    2015-09-01

    In most numerical simulations of the Earth's core the dynamo resides outside the tangent cylinder and may be crudely classified as being of the α2 type. In this region the flow comprises a sea of thin columnar vortices aligned with the rotation axis, taking the form of alternating cyclones and anticyclones. The dynamo is thought to be driven by these columnar vortices within which the flow is observed to be highly helical, helicity being a crucial ingredient of planetary dynamos. As noted in Davidson, one of the mysteries of this dynamo cartoon is the origin of the helicity, which is observed to be positive in the south and negative in the north. While Ekman pumping at the mantle can induce helicity in some of the overly viscous numerical simulations, it is extremely unlikely to be a significant source within planets. In this paper we return to the suggestion of Davidson that the helicity observed in the less viscous simulations owes its existence to helical wave packets, launched in and around the equatorial plane where the buoyancy flux is observed to be strong. Here we show that such wave packets act as a potent source of planetary helicity, constituting a simple, robust mechanism that yields the correct sign for h north and south of the equator. Since such a mechanism does not rely on the presence of a mantle, it can operate within both the Earth and the gas giants. Moreover, our numerical simulations show that helical wave packets dispersing from the equator produce a random sea of thin, columnar cyclone/anticyclone pairs, very like those observed in the more strongly forced dynamo simulations. We examine the local dynamics of helical wave packets dispersing from the equatorial regions, as well as the overall nature of an α2-dynamo driven by such wave packets. Our local analysis predicts the mean emf induced by helical waves, an analysis that rests on a number of simple approximations which are consistent with our numerical experiments, while our global

  10. Unscheduled load flow effect due to large variation in the distributed generation in a subtransmission network

    NASA Astrophysics Data System (ADS)

    Islam, Mujahidul

    A sustainable energy delivery infrastructure implies the safe and reliable accommodation of large scale penetration of renewable sources in the power grid. In this dissertation it is assumed there will be no significant change in the power transmission and distribution structure currently in place; except in the operating strategy and regulatory policy. That is to say, with the same old structure, the path towards unveiling a high penetration of switching power converters in the power system will be challenging. Some of the dimensions of this challenge are power quality degradation, frequent false trips due to power system imbalance, and losses due to a large neutral current. The ultimate result is the reduced life of many power distribution components - transformers, switches and sophisticated loads. Numerous ancillary services are being developed and offered by the utility operators to mitigate these problems. These services will likely raise the system's operational cost, not only from the utility operators' end, but also reflected on the Independent System Operators and by the Regional Transmission Operators (RTO) due to an unforeseen backlash of frequent variation in the load-side generation or distributed generation. The North American transmission grid is an interconnected system similar to a large electrical circuit. This circuit was not planned but designed over 100 years. The natural laws of physics govern the power flow among loads and generators except where control mechanisms are installed. The control mechanism has not matured enough to withstand the high penetration of variable generators at uncontrolled distribution ends. Unlike a radial distribution system, mesh or loop networks can alleviate complex channels for real and reactive power flow. Significant variation in real power injection and absorption on the distribution side can emerge as a bias signal on the routing reactive power in some physical links or channels that are not distinguishable

  11. The origin of the helicity hemispheric sign rule reversals in the mean-field solar-type dynamo

    NASA Astrophysics Data System (ADS)

    Pipin, V. V.; Zhang, H.; Sokoloff, D. D.; Kuzanyan, K. M.; Gao, Y.

    2013-11-01

    Observations of proxies of the magnetic helicity in the Sun over the past two solar cycles revealed reversals of the helicity hemispheric sign rule (negative in the North and positive in the South hemispheres). We apply the mean-field solar dynamo model to study the reversals of the magnetic helicity sign for the dynamo operating in the bulk of the solar convection zone. The evolution of the magnetic helicity is governed by the conservation law. We found that the reversal of the sign of the small-scale magnetic helicity follows the dynamo wave propagating inside the convection zone. Therefore, the spatial patterns of the magnetic helicity reversals reflect the processes which contribute to generation and evolution of the large-scale magnetic fields. At the surface, the patterns of the helicity sign reversals are determined by the magnetic helicity boundary conditions at the top of the convection zone. We demonstrate the impact of fluctuations in the dynamo parameters and variability in dynamo cycle amplitude on the reversals of the magnetic helicity sign rule. The obtained results suggest that the magnetic helicity of the large-scale axisymmetric field can be treated as an additional observational tracer for the solar dynamo and it probably can be used for the solar activity forecast as well.

  12. 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.

  13. The nuclear dynamo; Can a nuclear tornado annihilate nations

    SciTech Connect

    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.

  14. The dynamo basis of solar cycle precursor schemes

    NASA Astrophysics Data System (ADS)

    Charbonneau, Paul; Barlet, Guillaume

    2011-02-01

    We investigate the dynamo underpinning of solar cycle precursor schemes based on direct or indirect measures of the solar surface magnetic field. We do so for various types of mean-field-like kinematic axisymmetric dynamo models, where amplitude fluctuations are driven by zero-mean stochastic forcing of the dynamo number controlling the strength of the poloidal source term. In all stochastically forced models considered, the surface poloidal magnetic field is found to have precursor value only if it feeds back into the dynamo loop, which suggests that accurate determination of the magnetic flux budget of the solar polar fields may hold the key to dynamo model-based cycle forecasting.

  15. On the Predictability Limit of a Chaotic Flux Transport Solar Dynamo Model and its implications for Solar Magnetic Activity Forecasting

    NASA Astrophysics Data System (ADS)

    Sanchez, S. M.; Fournier, A.; Aubert, J.

    2012-12-01

    The Sun's magnetic activity cycle undoubtedly plays a major role on space weather, having a strong impact not only in satellite operation but also probably on the long-term variability of the Earth's climate. It is therefore of great importance to better understand the mechanism underlying the magnetic activity cycle - the solar dynamo. Looking into the past, historical data points both to amplitude and frequency fluctuations of the solar magnetic cycles, displaying even episodes of no sunspot observations such as the Maunder Minimum. This observational constraint together with the intrinsic non-linear turbulent regime in which the solar dynamo operates suggests the chaotic nature of the system, and predictions of solar dynamo activity based on the comprehensive set of equations describing its evolution is currently out of reach. On the other hand, solar dynamo models based on the axisymmetric approximation of the Sun's large-scale magnetic field are able to reproduce some of the fundamental features of the solar magnetic field dynamics. These models often include two main processes to transform poloidal into toroidal fields and vice-versa: first the shearing action of solar differential rotation on an initial poloidal field generates the toroidal field; secondly helical motions due to the Coriolis force on toroidal fields acts as to regenerate the poloidal field. Assuming different strategies for the latter process, these models are currently being used in magnetic activity forecasting for cycle 24. For this purpose, it is of great importance to access the predictability limit of such kind of models and what this information will imply for solar magnetic activity forecasting. In this work we consider the case of an axisymmetric flux-transport dynamo in the kinematic regime, using both Babcock-Leighton effect at the surface and alpha-effect at the base of the convection zone as the mechanisms for poloidal field regeneration in a full meridional plane. Admitting

  16. Optimizing the flow in a liquid sodium dynamo experiment

    NASA Astrophysics Data System (ADS)

    Taylor, N. Zane

    The Madison Dynamo experiment drives a turbulent flow of liquid sodium in a sphere in order to observe a MHD dynamo instability: An exponentially growing magnetic field at the expense of kinetic energy. Initial runs of the experiment observed intermittent bursts of the predicted magnetic mode, but no self-excited field was observed. It was found that turbulent fluctuations were producing large-scale magnetic fields that were a significant fraction of the magnitude of the fields induced by the mean flow. These turbulent-induced fields were solely detrimental, opposing the generation of the magnetic field produced by the mean flow. Baffles and vanes were added to the experiment to optimize the helical pitch of the mean flow and to remove the large-scale detrimental fluctuations. The observed drop in required motor power and a drop in specific measured magnetic response modes gives direct confirmation that these large detrimental eddies have been removed. A probe was developed to characterize the turbulence in the MDE after the baffles were installed and it was determined that the remaining turbulent EMF was mostly acting as an enhanced dissipation to the induced magnetic field. After these modifications, the induced magnetic field produced by the flowing sodium interacting with a seed magnetic field now closely matches laminar predictions. However, no self-excited field has been observed. A velocity inversion technique has been developed that compares internal and external field measurements with a predictive model and determines what the effective mean flow is in the experiment. Results from this velocity inversion give another metric on how optimized the flow profile is and also provide the most robust method of determining how close the experiment is to achieving a dynamo.

  17. A New Simple Dynamo Model for Stellar Activity Cycle

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

    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 solved 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.

  18. Reversals of the solar magnetic dipole in the light of observational data and simple dynamo models

    NASA Astrophysics Data System (ADS)

    Pipin, V. V.; Moss, D.; Sokoloff, D.; Hoeksema, J. T.

    2014-07-01

    Context. Observations show that the photospheric solar magnetic dipole usually does not vanish during the reversal of the solar magnetic field, which occurs in each solar cycle. In contrast, mean-field solar dynamo models predict that the dipole field does become zero. In a recent paper it was suggested that this contradiction could be explained as a large-scale manifestation of small-scale magnetic fluctuations of the surface poloidal field. Aims: Our aim is to confront this interpretation with the available observational data. Methods: Here we compare this interpretation with Wilcox Solar Observatory (WSO) photospheric magnetic field data in order to determine the amplitude of magnetic fluctuations required to explain the phenomenon and to compare the results with predictions from a simple dynamo model which takes these fluctuations into account. Results: We demonstrate that the WSO data concerning the magnetic dipole reversals are very similar to the predictions from our very simple solar dynamo model, which includes both mean magnetic field and fluctuations. The ratio between the rms value of the magnetic fluctuations and the mean field is estimated to be about 2, in reasonable agreement with estimates from sunspot data. The reversal epoch, during which the fluctuating contribution to the dipole is larger than that from the mean field, is about 4 months. The memory time of the fluctuations is about 2 months. Observations demonstrate that the rms of the magnetic fluctuations is strongly modulated by the phase of the solar cycle. This gives additional support to the concept that the solar magnetic field is generated by a single dynamo mechanism rather than also by independent small-scale dynamo action. A suggestion of a weak nonaxisymmetric magnetic field of a fluctuating nature arises from the analysis, with a lifetime of about 1 year. Conclusions: The behaviour of the magnetic dipole during the reversal epoch gives valuable information about details of solar

  19. 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.

  20. Configuration Design of Novel Manually Operated Dynamo Flashlights

    NASA Astrophysics Data System (ADS)

    Yan, Hong-Sen; Wang, Hsin-Te

    This paper synthesizes novel configurations of manually operated dynamo flashlights. Topology and motion characteristics of existing gear dynamos are modified and concluded. The structural sketches and corresponding graph representations for gear trains and dynamos with the defined induced magnetic circuits are defined. Through the concepts of generalization and specialization, the atlas of the structural sketches and graphs of the embedded gear dynamos is obtained subject to the defined design requirements and constraints. And, a systematic approach is proposed to synthesize the novel mechanisms of the embedded gear dynamos. As a result, the embedded three-link and four-link gear dynamos have 12 and 24 novel design configurations, respectively. One prototype of the embedded three-link and another of the embedded four-link gear dynamo are built.

  1. Kinematic dynamo action in square and hexagonal patterns

    NASA Astrophysics Data System (ADS)

    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.

  2. Generation and Propagation of Long Waves due to Spatial and Temporal Pressure Distributions

    NASA Astrophysics Data System (ADS)

    Metin, A. D.; Yalçıner, A. C.; Ozyurt Tarakcıoglu, G.; Zaytsev, A.

    2015-12-01

    An abnormal wave event was observed between 23 and 27 June 2014 in the Mediterranean and Black Seas. First, sea level oscillations began in Ciutadella Inlet (Spain) after midnight of 22 June. The phenomena continued with observation of strong oscillations (up to 3 m wave height) in the Adriatic Sea, Mediterranean Sea and Black Sea on 25-26 June. Finally, at noon on 27 June on a calm and sunny day, the abnormal waves suddenly struck coasts of Odessa with 1-2 m wave height injuring a number of people. This tsunami-like event which is called meteotsunami is generated by different types of meteorological disturbances such as atmospheric gravity waves, pressure jumps and squall lines and the significant consequences necessitates the research to understand, model and simulate such events accurately. Thus, using the 2014 event as a case study, the waves generated by the change of atmospheric pressure distribution is studied. A static water level drop due to high atmospheric pressure in a region and rise due to low atmospheric pressure in another region deform the water level throughout the entire sea area. To compute the sea level change, the relation between the pressure difference and change of water level from normal position (ζ=0.99ΔP) is used where ζ is the change of water level (cm) according to the pressure difference from normal pressure ΔP. This relation gives that 1 hPa (1millibar) depression in air pressure from normal water level position (under 1000millibar) creates almost 1 cm rise in mean sea level. The respective small amplitude long waves propagate along the sea which is continuously excited by the spatial and temporal changes of atmospheric pressure. And, the amplification becomes important to understand the occurrence of unexpected water level changes, especially near the coastal zone. In this study, this long wave propagation due to water surface deformation is modelled by solving nonlinear shallow water equations. The model results are compared

  3. Magnetic Helicity and the Solar Dynamo

    NASA Technical Reports Server (NTRS)

    Canfield, Richard C.

    1997-01-01

    The objective of this investigation is to open a new window into the solar dynamo, convection, and magnetic reconnection through measurement of the helicity density of magnetic fields in the photosphere and tracing of large-scale patterns of magnetic helicity in the corona.

  4. Converting DYNAMO simulations to Powersim Studio simulations

    SciTech Connect

    Walker, La Tonya Nicole; Malczynski, Leonard A.

    2014-02-01

    DYNAMO is a computer program for building and running 'continuous' simulation models. It was developed by the Industrial Dynamics Group at the Massachusetts Institute of Technology for simulating dynamic feedback models of business, economic, and social systems. The history of the system dynamics method since 1957 includes many classic models built in DYANMO. It was not until the late 1980s that software was built to take advantage of the rise of personal computers and graphical user interfaces that DYNAMO was supplanted. There is much learning and insight to be gained from examining the DYANMO models and their accompanying research papers. We believe that it is a worthwhile exercise to convert DYNAMO models to more recent software packages. We have made an attempt to make it easier to turn these models into a more current system dynamics software language, Powersim © Studio produced by Powersim AS2 of Bergen, Norway. This guide shows how to convert DYNAMO syntax into Studio syntax.

  5. 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.

  6. The RFP dynamo: MHD to kinetic regimes

    NASA Astrophysics Data System (ADS)

    Sarff, J. S.; Almagri, A. F.; den Hartog, D. J.; McCollam, K. J.; Nornberg, M. D.; Sauppe, J. P.; Sovinec, C. R.; Terry, P. W.; Triana, J. C.; Brower, D. L.; Ding, W. X.; Parke, E.

    2015-11-01

    The hallmark of magnetic relaxation in an RFP plasma is profile flattening of J0 .B0 /B2 effected by a dynamo-like emf in Ohm's law. This is well-studied in single-fluid MHD, but recent MST results and extended MHD modeling show that both and the Hall emf, - /ene , are important, revealing decoupled electron and ion motion. Since dynamo is current-related, the electron fluid emf, , captures both effects. In MST, the electron flow is dominantly Ve , 1 ~E1 ×B0 /B2 , implying ~ / B . This and the Hall emf are measured in MST for comparison in Ohm's law. A finite-pressure response is also possible, e.g., ``diamagnetic dynamo'', ∇ . /ene , associated with diamagnetic drift, and ``kinetic dynamo'' associated with collisionless streaming of electrons in a stochastic magnetic field. Correlation measurements and using FIR interferometry and Thomson scattering reveal these as small but finite in MST. A kinetic emf might be expected for any high-beta plasma with inhomogeneous pressure. Support by DOE/NSF.

  7. Extrapolating Solar Dynamo Models Throughout the Heliosphere

    NASA Astrophysics Data System (ADS)

    Cox, B. T.; Miesch, M. S.; Augustson, K.; Featherstone, N. A.

    2014-12-01

    There are multiple theories that aim to explain the behavior of the solar dynamo, and their associated models have been fiercely contested. The two prevailing theories investigated in this project are the Convective Dynamo model that arises from the pure solving of the magnetohydrodynamic equations, as well as the Babcock-Leighton model that relies on sunspot dissipation and reconnection. Recently, the supercomputer simulations CASH and BASH have formed models of the behavior of the Convective and Babcock-Leighton models, respectively, in the convective zone of the sun. These models show the behavior of the models within the sun, while much less is known about the effects these models may have further away from the solar surface. The goal of this work is to investigate any fundamental differences between the Convective and Babcock-Leighton models of the solar dynamo outside of the sun and extending into the solar system via the use of potential field source surface extrapolations implemented via python code that operates on data from CASH and BASH. The use of real solar data to visualize supergranular flow data in the BASH model is also used to learn more about the behavior of the Babcock-Leighton Dynamo. From the process of these extrapolations it has been determined that the Babcock-Leighton model, as represented by BASH, maintains complex magnetic fields much further into the heliosphere before reverting into a basic dipole field, providing 3D visualisations of the models distant from the sun.

  8. Tests of Diffusion-Free Scaling Behaviors in Numerical Dynamo Data Sets

    NASA Astrophysics Data System (ADS)

    Cheng, J. S.; Aurnou, J. M.

    2015-12-01

    In order to describe the fluid physics of the dynamo generating regions of planets, the geoscience community has largely adopted a set of scaling laws proposed in the seminal work of Christensen and Aubert (2006).[1] These scalings make use of specially-constructed parameters that are independent of fluid diffusivities, anticipating that large-scale turbulent processes will dominate the physics in planetary dynamo settings. In the work presented here, we test the validity of diffusion-free heat transfer scaling laws by constructing synthetic heat transfer datasets and examining their scaling properties alongside those proposed by Christensen and Aubert (2006). These tests demonstrate that the seemingly robust collapse of heat transfer data using diffusion-free parameters is not indicative of fully turbulent, diffusion-free physics, but is instead an a priori consequence of the way such parameters are constructed. In particular, the diffusion-free heat transfer scaling is determined by the onset of convection, which is itself determined by the viscous diffusivity of the fluid. Our results, in conjunction with those of Stelzer and Jackson (2013),[2] show that diffusion-free scalings are not validated by current-day numerical dynamo datasets, and that it still remains to be established under what conditions dynamo generation becomes free of fluid diffusivities.References [1] Christensen, U.R., Aubert, J., 2006. Geophys. J. Int. 166, 97-114.[2] Stelzer, Z., Jackson, A., 2013. Geophys. J. Int. ggt083.

  9. Destruction of large-scale magnetic field in non-linear simulations of the shear dynamo

    NASA Astrophysics Data System (ADS)

    Teed, Robert J.; Proctor, Michael R. E.

    2016-05-01

    The Sun's magnetic field exhibits coherence in space and time on much larger scales than the turbulent convection that ultimately powers the dynamo. In the past the α-effect (mean-field) concept has been used to model the solar cycle, but recent work has cast doubt on the validity of the mean-field ansatz under solar conditions. This indicates that one should seek an alternative mechanism for generating large-scale structure. One possibility is the recently proposed `shear dynamo' mechanism where large-scale magnetic fields are generated in the presence of a simple shear. Further investigation of this proposition is required, however, because work has been focused on the linear regime with a uniform shear profile thus far. In this paper we report results of the extension of the original shear dynamo model into the non-linear regime. We find that whilst large-scale structure can initially persist into the saturated regime, in several of our simulations it is destroyed via large increase in kinetic energy. This result casts doubt on the ability of the simple uniform shear dynamo mechanism to act as an alternative to the α-effect in solar conditions.

  10. 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

  11. 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

  12. Magnetic Diagnostics and Field Structure in the Madison Dynamo Experiment

    NASA Astrophysics Data System (ADS)

    Rasmus, A. M.; Clark, M.; Kaplan, E. J.; Kendrick, R. D.; Nornberg, M. D.; Rahbarnia, K.; Taylor, N. Z.; Forest, C. B.

    2010-11-01

    The Madison Dynamo Experiment(MDE) is expected to spontaneously self-generate a magnetic field in a two vortex flow geometry driven by counter rotating impellers in a 1 m diameter sphere filled with liquid sodium. This poster will focus on the spatial structure of the magnetic field associated with the dynamo eigenmodes and the turbulent fluctuations. A new internal array of Hall probes will increase the number of probe locations from 60 to 100 (in addition to 74 existing surface probes), including 40 spanning the center of the experiment. Three orthogonal measurements of the magnetic field are taken at each internal location, whereas previous internal probes took one directional data (2 directional after probe rotation on a different run). This will allow resolution of harmonic modes up to a poloidal order of l=7 and a toroidal order of m=5. Cross correlation analysis between the surface probes and internal probes will be used to determine the internal structure associated with each l and m. This work is supported by the NSF/DOE partnership in plasma physics.

  13. Magnetic Field Structure in the Madison Dynamo Experiment

    NASA Astrophysics Data System (ADS)

    Rasmus, A. M.; Clark, M.; Kaplan, E. J.; Nornberg, M. D.; Rahbarnia, K.; Taylor, N. Z.; Forest, C. B.

    2011-10-01

    The Madison Dynamo Experiment(MDE) is expected to spontaneously self-generate a magnetic field in a two vortex flow geometry driven by counter rotating impellers in a 1 m diameter sphere filled with liquid sodium. Prevoiusly an equatorial baffle was installed and has been demonstrated to reduce the largest scale turbulent-eddies. An additonal set of six rotatable baffles have been installed to optimize the helicity of the flow, lowering the critical magnetic Reynolds number. This poster will focus on the spatial structure of the magnetic field associated with the dynamo eigenmodes and the turbulent fluctuations. Singular value decomposition (SVD) and cross correlation analysis between the surface harmonics and internal probes will be used to determine the internal structure associated with each spherical harmonic. Spherical harmonic decomposition is of limited utility when analysing the equatorial array of internal probes as there is a limited angular spread (only one theta value and two phi values), whereas cross correlation and SVD allow the use of time domain data to infer internal modes excited via three-wave couplings. This work is supported by the NSF/DOE partnership in plasma physics.

  14. Reduction of Turbulent Diamagnetism in the Madison Dynamo Experiment

    NASA Astrophysics Data System (ADS)

    Kaplan, E. J.; Kendrick, R. D.; Nornberg, M. D.; Rahbarnia, K.; Rasmus, A.; Spence, E. J.; Taylor, N. Z.; Forest, C. B.

    2010-11-01

    The dynamo effect is a magnetic instability whereby a flowing conductor generates a magnetic field such as those seen in the Earth and Sun. The Madison Dynamo Experiment (MDE) is a 1 m diameter sphere filled with liquid sodium that aims to produce this effect in a flow driven by two counter rotating impellers. Previous experiments on the MDE demonstrated an induced axisymmetric magnetic dipole counter to the applied axisymmetric dipole. An antidynamo theorem exists that shows the observed diamagnetism is impossible in a two vortex flow, and is thus associated with a turbulent EMF. This poster shows results from a new campaign with an equatorial baffle installed that drasically diminishes the turbulent diamagnetism. Spherical harmonic decomposition of the induced field also shows a reduction of higher order magnetic modes associated with three mode coupling between the applied field and large scale velocity fluctuations. Numerical simulations of the sodium flow with and without baffles also indicate the possibility of reduced hydrodynamic turbulence while maintaining a two vortex flow.

  15. Quantifying paleosecular variation: Insights from numerical dynamo simulations

    NASA Astrophysics Data System (ADS)

    Lhuillier, F.; Gilder, S. A.

    2013-12-01

    Numerical dynamo simulations can be used to investigate paleosecular variation of Earth-like magnetic fields over several million-year timescales. Using a set of five numerical models integrated over the equivalent of 40-50 Myr, we generated synthetic data analogous to paleomagnetic data. We show that paleosecular variation among the five models is best discriminated by the relative variability in paleointensity (ɛ_F) and the precision parameter (k) of directions or poles. Whether the geodynamo operated in different regimes in its past can be best tested with these parameters in combination. Roughly one million years of time with 200 time-independent samples is required to achieve convergence of ɛ_F and k. The quantities ɛ_F and k correlate well with the average chron duration (μ_chr), which suggests that excursions and reversals are an integral part of palaeosecular variation. If applicable to the geodynamo, the linear dependence of k on μ_chr could help to predict μ_chr for the Earth during geologic times with no available reversal frequency data; it also predicts much higher average k for directions during superchrons (k ≈ 2500 for the Cretaceous normal superchron) than during actively reversing times (k ≈ 35 for the last 80 Myr). As such high k values are not observed, either this family of dynamo models is not applicable to the geodynamo, or the geodynamo regime acting during superchrons lies statistically within the same energy state as at present.

  16. Extending the core paradox posed by an early dynamo

    NASA Astrophysics Data System (ADS)

    Tarduno, J. A.; Cottrell, R. D.

    2014-12-01

    New, higher values of core thermal conductivity, together with the reliance of any early geodynamo on thermal convection, represent a paradox (Olson, 2013). These factors suggest either early Earth lacked an internally-generated magnetic field or novel mechanisms for driving an early dynamo. The oldest record of past field strength from in situ igneous rocks comes from 3.4 to 3.45 Ga dacites of the Barberton and Nondweni Greenstone belts of South Africa. These analyses, based on single silicate crystals containing magnetic inclusions with ideal recording characteristics, indicate a field strength within 50 to 70% of the present-day field (Tarduno et al., 2010). Much older igneous rocks are generally not viable as paleomagnetic recorders because of their high metamorphic state (amphibolite grade or higher). Here we discuss extending Earth's magnetic field history further back in time using Archean and Hadean zircons hosting magnetic inclusions that are now found in younger sedimentary units of low metamorphic grade. Thellier-Coe paleointensity data passing reliability checks from zircons of the Jack Hills belt (Western Australia) suggest the presence of a magnetic field at 3.55 Ga, thereby extending the core paradox by 100 million years. We will discuss our continuing efforts to test for the presence/absence of a dynamo field between 3.55 Ga and 4.2 Ga.

  17. 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.

  18. Measuring and Optimizing flows in the Madison Dynamo Experiment

    NASA Astrophysics Data System (ADS)

    Taylor, N. Z.; Clark, M.; Forest, C. B.; Kaplan, E. J.; Nornberg, M. D.; Rasmus, A. M.; Rahbarnia, K.

    2012-10-01

    In the Madison Dynamo Experiment, two counter-rotating impellers drive a turbulent flow of liquid sodium in a one meter-diameter sphere. One of the goals of the experiment is to observe a magnetic field grow at the expense of kinetic energy in the flow. The enormous Reynolds number of the experiment and its two vortex geometry leads to a large turbulent EMF. This poster presents results from the MDE after several upgrades were made. First, an equatorial baffle was installed to stabilize the position of the shear layer between the two counterrotating hemispheres. This reduced the scale of the largest eddies in the experiment, lowering the effective resistivity due to turbulence. Next, a probe was used to measure both the fluctuating velocity and magnetic fields, enabling a direct measurement of the turbulent EMF. This EMF is anti-parallel to the mean current, consistent with an enhanced resistivity predicted by mean field theory. Finally, vanes with adjustable orientation were installed on the vessel wall, allowing the pitch of the helical flow to be altered. Computational fluid dynamics simulations and inversion of the measured induced magnetic field are used to determine the optimum angle of these vanes to minimize the critical velocity at which the dynamo onset occurs.

  19. Whirling Dervish Dynamos: Magnetic Activity in CV Secondaries

    NASA Astrophysics Data System (ADS)

    Saar, Steven

    2003-07-01

    The mass-losing secondary stars of cataclysmic variables {CVs} are the most rapidly rotating cool dwarfs observable. Other rapid rotators show a maximal, "saturated" level of magnetic activity {e.g., X-ray emission}, but there are hints from contact binaries and young clusters that activity may be suppressed at the highest rotation rates. CV secondaries are thus important probes of magnetic dynamos at rotational extremes. Implications for CV evolution {e.g., the ``period gap", accretion variability} may also be profound. Unfortunately, study of CV secondaries is hampered by pesky accretion-related phenomena and reflection effects. As a result, little systematic work has been done. To explore activity in these stars, we therefore propose to study far-UV spectra of AM Her-type systems {which have no accretion disks} in deep photometric minima in which accretion is shut off. Magnetic-related emission from the secondary will be separated {in velocity} from residual accretion emission by observations near quadratures. Lower chromospheric irradiation due to the white dwarf primary will be removed by modeling, yielding the true level of magnetic activity on the secondary. We will compare the results to other dMe stars and draw implications for magnetic dynamos and activity at rotational extremes, and for CV evolution and behavior.

  20. Mechanism of Cisplatin-Induced Cytotoxicity Is Correlated to Impaired Metabolism Due to Mitochondrial ROS Generation

    PubMed Central

    Shim, Wooyoung; Anwar, Muhammad Ayaz; Kwon, Ji-Woong; Kwon, Hyuk-Kwon; Kim, Hyung Joong; Jeong, Hyobin; Kim, Hwan Myung; Hwang, Daehee; Kim, Hyung Sik; Choi, Sangdun

    2015-01-01

    The chemotherapeutic use of cisplatin is limited by its severe side effects. In this study, by conducting different omics data analyses, we demonstrated that cisplatin induces cell death in a proximal tubular cell line by suppressing glycolysis- and tricarboxylic acid (TCA)/mitochondria-related genes. Furthermore, analysis of the urine from cisplatin-treated rats revealed the lower expression levels of enzymes involved in glycolysis, TCA cycle, and genes related to mitochondrial stability and confirmed the cisplatin-related metabolic abnormalities. Additionally, an increase in the level of p53, which directly inhibits glycolysis, has been observed. Inhibition of p53 restored glycolysis and significantly reduced the rate of cell death at 24 h and 48 h due to p53 inhibition. The foremost reason of cisplatin-related cytotoxicity has been correlated to the generation of mitochondrial reactive oxygen species (ROS) that influence multiple pathways. Abnormalities in these pathways resulted in the collapse of mitochondrial energy production, which in turn sensitized the cells to death. The quenching of ROS led to the amelioration of the affected pathways. Considering these observations, it can be concluded that there is a significant correlation between cisplatin and metabolic dysfunctions involving mROS as the major player. PMID:26247588

  1. MHD Turbulence and Magnetic Dynamos

    NASA Technical Reports Server (NTRS)

    Shebalin, John V

    2014-01-01

    Incompressible magnetohydrodynamic (MHD) turbulence and magnetic dynamos, which occur in magnetofluids with large fluid and magnetic Reynolds numbers, will be discussed. When Reynolds numbers are large and energy decays slowly, the distribution of energy with respect to length scale becomes quasi-stationary and MHD turbulence can be described statistically. In the limit of infinite Reynolds numbers, viscosity and resistivity become zero and if these values are used in the MHD equations ab initio, a model system called ideal MHD turbulence results. This model system is typically confined in simple geometries with some form of homogeneous boundary conditions, allowing for velocity and magnetic field to be represented by orthogonal function expansions. One advantage to this is that the coefficients of the expansions form a set of nonlinearly interacting variables whose behavior can be described by equilibrium statistical mechanics, i.e., by a canonical ensemble theory based on the global invariants (energy, cross helicity and magnetic helicity) of ideal MHD turbulence. Another advantage is that truncated expansions provide a finite dynamical system whose time evolution can be numerically simulated to test the predictions of the associated statistical mechanics. If ensemble predictions are the same as time averages, then the system is said to be ergodic; if not, the system is nonergodic. Although it had been implicitly assumed in the early days of ideal MHD statistical theory development that these finite dynamical systems were ergodic, numerical simulations provided sufficient evidence that they were, in fact, nonergodic. Specifically, while canonical ensemble theory predicted that expansion coefficients would be (i) zero-mean random variables with (ii) energy that decreased with length scale, it was found that although (ii) was correct, (i) was not and the expected ergodicity was broken. The exact cause of this broken ergodicity was explained, after much

  2. Statistical Mechanics of Turbulent Dynamos

    NASA Technical Reports Server (NTRS)

    Shebalin, John V.

    2014-01-01

    Incompressible magnetohydrodynamic (MHD) turbulence and magnetic dynamos, which occur in magnetofluids with large fluid and magnetic Reynolds numbers, will be discussed. When Reynolds numbers are large and energy decays slowly, the distribution of energy with respect to length scale becomes quasi-stationary and MHD turbulence can be described statistically. In the limit of infinite Reynolds numbers, viscosity and resistivity become zero and if these values are used in the MHD equations ab initio, a model system called ideal MHD turbulence results. This model system is typically confined in simple geometries with some form of homogeneous boundary conditions, allowing for velocity and magnetic field to be represented by orthogonal function expansions. One advantage to this is that the coefficients of the expansions form a set of nonlinearly interacting variables whose behavior can be described by equilibrium statistical mechanics, i.e., by a canonical ensemble theory based on the global invariants (energy, cross helicity and magnetic helicity) of ideal MHD turbulence. Another advantage is that truncated expansions provide a finite dynamical system whose time evolution can be numerically simulated to test the predictions of the associated statistical mechanics. If ensemble predictions are the same as time averages, then the system is said to be ergodic; if not, the system is nonergodic. Although it had been implicitly assumed in the early days of ideal MHD statistical theory development that these finite dynamical systems were ergodic, numerical simulations provided sufficient evidence that they were, in fact, nonergodic. Specifically, while canonical ensemble theory predicted that expansion coefficients would be (i) zero-mean random variables with (ii) energy that decreased with length scale, it was found that although (ii) was correct, (i) was not and the expected ergodicity was broken. The exact cause of this broken ergodicity was explained, after much

  3. Double Dynamo Signatures in a Global MHD Simulation and Mean-field Dynamos

    NASA Astrophysics Data System (ADS)

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

    2016-08-01

    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 produced 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 2Ω 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.

  4. Strategies for Observing Self-excitation in the Madison Dynamo Experiment

    NASA Astrophysics Data System (ADS)

    Taylor, N. Z.; Kaplan, E. J.; Kendrick, R. D.; Nornberg, M. D.; Rahbarnia, K.; Rasmus, A. M.; Forest, C. B.; Spence, E. J.

    2010-11-01

    In the Madison Dynamo Experiment(MDE) two counter-rotating impellers drive a turbulent flow of liquid sodium in a one meter-diameter sphere. One of the goals of the experiment is to observe the spontaneous generation of magnetic field. Initial runs of the MDE saw intermittent bursts of a transverse dipole field similar to the induced field predicted by laminar kinematics, but no sustained self-excited field was observed. This poster will present recent results from the MDE after an equatorial baffle was installed to stabilize the position of the shear layer between the two counterrotating hemispheres and to help in the reduction of of large-scale turbulence and the motors were run up to maximum power. Required motor power indicates that the baffle has decreased the amount of turbulence in the flow. When run up to full power still no self-excited dynamo was observed, but there was significant amplification of the transverse dipole field with extended decay rates indicating we may be approaching the dynamo threshold. Future modifications to the experiment will also be presented exploring a subcritical dynamo transition by supplying a sufficiently strong magnetic field and the addition of poloidal baffles to optimize the helicity of the mean flow. This work is supported by the NSF/DOE partnership in plasma physics.

  5. Strategies for Observing Self-excitation in the Madison Dynamo Experiment

    NASA Astrophysics Data System (ADS)

    Taylor, N. Z.; Forest, C. B.; Kaplan, E. J.; Kendrick, R. D.; Rasmus, A. M.

    2009-11-01

    In the Madison Dynamo Experiment two counter-rotating impellers drive a turbulent flow of liquid sodium in a one meter-diameter sphere. One of the goals of the experiment is to observe the spontaneous generation of magnetic field. Initial runs of the Madison Dynamo Experiment saw intermittent bursts of a transverse dipole field similar to the induced field predicted by laminar kinematics, but no sustained self-excited field was observed. Recent numerical simulations have shown that turbulent fluctuations strongly increase the critical magnetic Reynolds number required for self-excitation, beyond the design parameters of the experiment. Three different techniques for accessing dynamos are currently being implemented on the experiment. First, the addition of an equatorial and poloidal baffles to the experiment will help in the reduction of large-scale turbulence and optimization of the helicity of the mean flow. Second, freely rotating segments of a symmetric airfoil will be added to the internal probe tubes to reduce vibration that prevented operation at full speed. Third, the externally applied field will be strengthened to explore a sub-critical dynamo transition that has recently been discovered using numerical simulations.

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

    NASA Astrophysics Data System (ADS)

    Dikpati, Mausumi; Suresh, Akshaya; Burkepile, Joan

    2016-02-01

    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.

  7. Generation of adakites in a cold subduction zone due to double subducting plates

    NASA Astrophysics Data System (ADS)

    Nakamura, Hitomi; Iwamori, Hikaru

    2013-06-01

    Adakites have been found in various tectonic settings, since the first report for the distinct lavas as a product of slab melting in Adak Island by Kay (J Volcanol Geotherm Res 4:117-132, 1978). In this study, we present geochemical data for an `adakite' and `adakitic rock' suite in central Japan with a cold subduction environment due to the two overlapping subducting plates: the Pacific plate and the Philippine sea plate. Based on the major, trace and isotopic compositions of the rocks, elemental transport from initial slab inventory at the trench to the volcanic rocks as a final product is quantitatively analyzed, considering the thermal structure, slab dehydration, elemental mobility, slab-fluid migration and melting of fluid-added mantle. The analysis demonstrates a large compositional impact of slab-fluid in the arc magma generation in central Japan. The melting conditions have been also estimated inversely by optimizing the predicted magma composition to the observed composition of volcanic rock, with the two parameters: the degree of melting and the proportion of spinel and garnet lherzolites involved in melting. Consequently, a moderately low degree of near-solidus melting of dominantly garnet lherzolite with a high fluid flux from the two overlapping slabs beneath the region has been argued to be responsible for the compositional characteristics, including the adakitic signatures, of the studied rocks. These results imply that the geochemical approach may provide useful constraints on the P- T condition of melting in the mantle wedge and the thermal structure in subduction zones, being complementary to the geophysical approach.

  8. Generation of adakites in a cold subduction zone due to double subducting plates

    NASA Astrophysics Data System (ADS)

    Nakamura, H.; Iwamori, H.

    2012-12-01

    Adakites have been found in various tectonic settings, since the first report for the distinct lavas as a product of slab melting in Adak Island by Kay (1978). In this study, we present geochemical data for an 'adakite' and 'adakitic rock' suite in central Japan with a cold subduction environment due to the two overlapping subudcting plates, the Pacific Plate and the Philippine Sea Plate. Based on the major, trace and isotopic compositions of the rocks, elemental transport from initial slab inventory at the trench to the volcanic rocks as a final product is quantitatively analyzed, considering the thermal structure, slab dehydration, elemental mobility, slab-fluid migration and melting of fluid-added mantle. The analysis demonstrates a large compositional impact of slab-fluid in the arc magma generation in central Japan. The melting conditions have been also estimated inversely by optimizing the predicted magma composition to the observed composition of volcanic rock, with the two parameters: the degree of melting and the proportion of spinel- and garnet-lherzolites involved in melting. Consequently, a low degree of melting of dominantly garnet-lherzolite with a high fluid flux from the two overlapping slabs beneath the region has been argued to be responsible for the compositional characteristics, including the adakitic signatures, of the studied rocks. These results imply that the geochemical approach may provide useful constraints on the P-T condition of melting in the mantle wedge and the thermal structure in subduction zones, being complementary to the geophysical approach. We have also applied this geochemical approach to the adjacent NE Japan where the Pacific plate subducts, which revealed the thermal regime in the mantle beneath the arc-arc transition.

  9. 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.

  10. Testing the geomagnetic dipole and reversing dynamo models over Earth's cooling history

    NASA Astrophysics Data System (ADS)

    Heimpel, Moritz; Evans, Ted

    2014-05-01

    Continental drift reconstructions rely on the assumption that Earth's mean magnetic field has been a geocentric axial dipole over geologic time. However, the coupled dynamics of mantle and core convection may have had profound effects on the magnetic field in the distant past. Previous dynamo models have linked differences between polar and equatorial mantle heat flow to apparently anomalous paleomagnetic fields, and changes in reversal frequency. Here we use the inclination test (Evans, 1976) to interpret observational magnetic field models and polarity-reversing numerical dynamos representing various convective states of the mantle and core. Dynamo models with uniform buoyancy flux represent three convective states of the mantle and core: (1) present era Earth, driven thermo-chemically at the inner core boundary; (2) mantle overturn, with elevated heat flux at the core-mantle boundary, and (3) ancient Earth prior to inner core nucleation, with buoyancy production solely at the CMB. Consistent with Earth's present magnetic field, dynamos driven by buoyancy due to inner core growth are nearly dipolar. In contrast, elevated CMB heat flow yields small to moderate inclination flattening due to a persistent octupole that reverses synchronously with the dipole. For the ancient Earth models the relatively strong octupole component tends to stabilize the dynamo and decrease the reversal frequency. Our results, along with evidence of a young inner core, imply that an entirely liquid core contributed to shallow inclinations in Precambrian time. We also run models with latitudinally variable heat flux boundary conditions to further investigate the relationship between dynamo flow fields, the octupole component, magnetic inclinations and reversal frequency. For models with increased polar CMB heat flux we find that the relative strength of the octupole component increases in proportion to latitudinal heat flux variation. On the other hand, models are very sensitive to