A new fast reconnection model in a collisionless regime
Tsiklauri, David
2008-11-15
Based on the first principles [i.e., (i) by balancing the magnetic field advection with the term containing electron pressure tensor nongyrotropic components in the generalized Ohm's law; (ii) using the conservation of mass; and (iii) assuming that the weak magnetic field region width, where electron meandering motion supports electron pressure tensor off-diagonal (nongyrotropic) components, is of the order of electron Larmor radius] a simple model of magnetic reconnection in a collisionless regime is formulated. The model is general, resembling its collisional Sweet-Parker analog in that it is not specific to any initial configuration, e.g., Harris-type tearing unstable current sheet, X-point collapse or otherwise. In addition to its importance from the fundamental point of view, the collisionless reconnection model offers a much faster reconnection rate [M{sub c{sup '}}{sub less}=(c/{omega}{sub pe}){sup 2}/(r{sub L,e}L)] than Sweet-Parker's classical one (M{sub sp}=S{sup -1/2}). The width of the diffusion region (current sheet) in the collisionless regime is found to be {delta}{sub c{sup '}}{sub less}=(c/{omega}{sub pe}){sup 2}/r{sub L,e}, which is independent of the global reconnection scale L and is only prescribed by microphysics (electron inertial length, c/{omega}{sub pe}, and electron Larmor radius, r{sub L,e}). Amongst other issues, the fastness of the reconnection rate alleviates, e.g., the problem of interpretation of solar flares by means of reconnection, as for the typical solar coronal parameters the obtained collisionless reconnection time can be a few minutes, as opposed to Sweet-Parker's equivalent value of less than a day. The new theoretical reconnection rate is compared to the Magnetic Reconnection Experiment device experimental data by Yamada et al. [Phys. Plasmas 13, 052119 (2006)] and Ji et al. [Geophys. Res. Lett. 35, 13106 (2008)], and a good agreement is obtained.
Reversible collisionless magnetic reconnection
Ishizawa, A.; Watanabe, T.-H.
2013-10-15
Reversible magnetic reconnection is demonstrated for the first time by means of gyrokinetic numerical simulations of a collisionless magnetized plasma. Growth of a current-driven instability in a sheared magnetic field is accompanied by magnetic reconnection due to electron inertia effects. Following the instability growth, the collisionless reconnection is accelerated with development of a cross-shaped structure of current density, and then all field lines are reconnected. The fully reconnected state is followed by the secondary reconnection resulting in a weakly turbulent state. A time-reversed simulation starting from the turbulent state manifests that the collisionless reconnection process proceeds inversely leading to the initial state. During the reversed reconnection, the kinetic energy is reconverted into the original magnetic field energy. In order to understand the stability of reversed process, an external perturbation is added to the fully reconnected state, and it is found that the accelerated reconnection is reversible when the deviation of the E × B streamlines due to the perturbation is comparable with or smaller than a current layer width.
Fast collisionless reconnection and electron heating in strongly magnetized plasmas.
Loureiro, N F; Schekochihin, A A; Zocco, A
2013-07-12
Magnetic reconnection in strongly magnetized (low-beta), weakly collisional plasmas is investigated by using a novel fluid-kinetic model [Zocco and Schekochihin, Phys. Plasmas 18, 102309 (2011)] which retains nonisothermal electron kinetics. It is shown that electron heating via Landau damping (linear phase mixing) is the dominant dissipation mechanism. In time, electron heating occurs after the peak of the reconnection rate; in space, it is concentrated along the separatrices of the magnetic island. For sufficiently large systems, the peak reconnection rate is cE(∥)(max) ≈ 0.2v(A)B(y,0), where v(A) is the Alfvén speed based on the reconnecting field B(y,0). The island saturation width is the same as in magnetohydrodynamics models except for small systems, when it becomes comparable to the kinetic scales. PMID:23889411
Linear theory for fast collisionless magnetic reconnection in the lower-hybrid frequency range
NASA Astrophysics Data System (ADS)
Jovanović, D.; Shukla, P. K.
2005-05-01
A linear theory is presented for the interplay between the fast collisionless magnetic reconnection and the lower-hybrid waves that has been observed in recent computer simulations [J. F. Drake, M. Swisdak, C. Cattell et al., Science 299, 873 (2003)]. In plasma configurations with a strong guide field and anisotropic electron temperature, the electron dynamics is described within the framework of standard electron magnetohydrodynamic equations, accounting also for the effects of the electron polarization and ion motions in the presence of perpendicular electric fields. In the linear phase, we find two types of instabilities of a thin current sheet with steep edges, corresponding to its filamentation (or tearing) and bending. Using a surface-wave formalism for the perturbations whose wavelength is larger than the thickness of the current sheet, the corresponding growth rates are calculated as the contributions of singularities in the plasma dispersion function. These are governed by the electron inertia and the linear coupling of the reconnecting magnetic field with local plasma modes propagating in the perpendicular direction that are subject to the Buneman instability. The linear surface wave instability may be particularly important as a secondary instability, dissipating the thin current sheets that develop in the course of the fast reconnection in the shear-Alfvén and kinetic-Alfvén regimes, and providing the anomalous resistivity for the growth of magnetic islands beyond the shear-Alfvén and kinetic-Alfvén scales.
Collisionless reconnection in Jupiter's magnetotail
NASA Astrophysics Data System (ADS)
Zimbardo, G.
1991-04-01
Collisionless reconnection in Jupiter's magnetotail is quantitatively studied for the first time. It is proposed that the same tearing mechanism which works in the earth magnetotail also works in Jupiter's. It is shown that collisionless reconnection may occur around 60 R(J) downtail.
Nonlinear Collisionless Magnetic Reconnection
Grasso, D.; Tassi, E.; Borgogno, D.; Pegoraro, F.
2008-10-15
We review some recent results that have been obtained in the investigation of collisionless reconnection in two and three dimensional magnetic configurations with a strong guide field in regimes of interest for laboratory plasmas. First, we adopt a two-field plasma model where two distinct regimes, laminar and turbulent, can be identified. Then, we show that these regimes may combine when we consider a more general four-field model, where perturbation of the magnetic and velocity fields are allowed also along the ignorable coordinate.
Does the Rate of Collisionless Magnetic Reconnection Depend on the Dissipation Mechanism?
NASA Technical Reports Server (NTRS)
Aunai, Nicolas; Hesse, Michael; Black, Carrie; Evans, Rebekah; Kuznetsova, Maria
2012-01-01
The importance of the electron dissipation effect on the reconnection rate is investigated in the general case of asymmetric collisionless magnetic reconnection. Contrary to the standard collisionless reconnection model, it is found that the reconnection rate, and the macroscopic evolution of the reconnecting system, crucially depend on the nature of the dissipation mechanism and that the Hall effect alone is not able to sustain fast reconnection.
Recent advances in collisionless magnetic reconnection
NASA Astrophysics Data System (ADS)
Porcelli, F.; Borgogno, D.; Califano, F.; Grasso, D.; Ottaviani, M.; Pegoraro, F.
2002-12-01
One of the recurring problems in magnetic reconnection is the identification of the appropriate generalized Ohm's law. In weakly collisional plasmas with a strong magnetic guide field component, a fluid model may be adopted, where electron inertia and the electron pressure gradient play important roles. In the absence of collisions, electron inertia provides the mechanism for magnetic field-line breaking. Electron compressibility alters significantly the structure of the reconnection region and allows for faster reconnection rates, which are consistent with the fast relaxation times of sawtooth oscillations in tokamak plasmas. The Hall term may also become important when the guide field is weak. The very possibility of nonlinear, irreversible magnetic reconnection in the absence of dissipation is addressed. We show that in a collisionless plasma, magnetic islands can grow and reach a saturated state in a coarse-grained sense. Magnetic energy is transferred to kinetic energy in smaller and smaller spatial scale lengths through a phase mixing process. The same model is then applied to the interpretation of driven reconnection events in the vicinity of a magnetic X-line observed in the VTF experiment at MIT. The reconnection is driven by externally induced plasma flows in a background magnetic configuration that has a hyperbolic null in the reconnection plane and a magnetic guide field component perpendicular to that plane. In the limit where the guide field is strong, assuming the external drive to be sufficiently weak for a linear approximation to hold, a dynamic evolution of the system is obtained which does not reach a stationary state. The reconnection process develops in two phases: an initial phase, whose characteristic rate is a fraction of the Alfvén frequency, and a later one, whose rate is determined by the electron collision frequency.
Collisionless Reconnection and Electron Demagnetization
NASA Astrophysics Data System (ADS)
Scudder, J. D.
Observable, dimensionless properties of the electron diffusion region of collisionless magnetic reconnection are motivated and benchmarked in two and three dimensional Particle In Cell (PIC) simulations as appropriate for measurements with present state of the art spacecraft. The dimensionless quantities of this paper invariably trace their origin to breaking the magnetization of the thermal electrons. Several observable proxies are also motivated for the rate of frozen flux violation and a parameter \\varLambda _{\\varPhi } that when greater than unity is associated with close proximity to the analogue of the saddle point region of 2D reconnection usually called the electron diffusion region. Analogous regions to the electron diffusion region of 2D reconnection with \\varLambda _{\\varPhi } > 1 have been identified in 3D simulations. 10-20 disjoint diffusion regions are identified and the geometrical patterns of their locations illustrated. First examples of associations between local observables based on electron demagnetization and global diagnostics (like squashing) are also presented. A by product of these studies is the development of a single spacecraft determinations of gradient scales in the plasma.
Collisionless Reconnection in an Electron-Positron Plasma
Bessho, N.; Bhattacharjee, A.
2005-12-09
Electromagnetic particle-in-cell simulations of fast collisionless reconnection in a two-dimensional electron-positron plasma (without an equilibrium guide field) are presented. A generalized Ohm's law in which the Hall current cancels out exactly is given. It is suggested that the key to fast reconnection in this plasma is the localization caused by the off-diagonal components of the pressure tensors, which produce an effect analogous to a spatially localized resistivity.
Collisionless reconnection in an electron-positron plasma.
Bessho, N; Bhattacharjee, A
2005-12-01
Electromagnetic particle-in-cell simulations of fast collisionless reconnection in a two-dimensional electron-positron plasma (without an equilibrium guide field) are presented. A generalized Ohm's law in which the Hall current cancels out exactly is given. It is suggested that the key to fast reconnection in this plasma is the localization caused by the off-diagonal components of the pressure tensors, which produce an effect analogous to a spatially localized resistivity. PMID:16384388
Magnetic reconnection in collisionless plasmas - Prescribed fields
NASA Technical Reports Server (NTRS)
Burkhart, G. R.; Drake, J. F.; Chen, J.
1990-01-01
The structure of the dissipation region during magnetic reconnection in collisionless plasma is investigated by examining a prescribed two-dimensional magnetic x line configuration with an imposed inductive electric field E(y). The calculations represent an extension of recent MHD simulations of steady state reconnection (Biskamp, 1986; Lee and Fu, 1986) to the collisionless kinetic regime. It is shown that the structure of the x line reconnection configuration depends on only two parameters: a normalized inductive field and a parameter R which represents the opening angle of the magnetic x lines.
New Expression for Collisionless Magnetic Reconnection Rate
NASA Technical Reports Server (NTRS)
Klimas, Alexander J.
2014-01-01
For 2D, symmetric, anti-parallel, collisionless magnetic reconnection, a new expression for the reconnection rate in the electron diffusion region is introduced. It is shown that this expression can be derived in just a few simple steps from a physically intuitive starting point; the derivation is given in its entirety and the validity of each step is confirmed. The predictions of this expression are compared to the results of several long-duration, open-boundary PIC reconnection simulations to demonstrate excellent agreement.
New expression for collisionless magnetic reconnection rate
Klimas, Alex
2015-04-15
For 2D, symmetric, anti-parallel, collisionless magnetic reconnection, new expressions for the reconnection rate in the electron diffusion region are introduced. It is shown that these expressions can be derived in just a few simple steps from a physically intuitive starting point; the derivations are given in their entirety, and the validity of each step is confirmed. The predictions of these expressions are compared to the results of several long-duration, open-boundary particle-in-cell reconnection simulations to demonstrate excellent agreement.
Collisionless Magnetic Reconnection in Space Plasmas
NASA Astrophysics Data System (ADS)
Treumann, Rudolf A.; Baumjohann, Wolfgang
2013-12-01
Magnetic reconnection, the merging of oppositely directed magnetic fields that leads to field reconfiguration, plasma heating, jetting and acceleration, is one of the most celebrated processes in collisionless plasmas. It requires the violation of the frozen-in condition which ties gyrating charged particles to the magnetic field inhibiting diffusion. Ongoing reconnection has been identified in near-Earth space as being responsible for the excitation of substorms, magnetic storms, generation of field aligned currents and their consequences, the wealth of auroral phenomena. Its theoretical understanding is now on the verge of being completed. Reconnection takes place in thin current sheets. Analytical concepts proceeded gradually down to the microscopic scale, the scale of the electron skin depth or inertial length, recognizing that current layers that thin do preferentially undergo spontaneous reconnection. Thick current layers start reconnecting when being forced by plasma inflow to thin. For almost half a century the physical mechanism of reconnection has remained a mystery. Spacecraft in situ observations in combination with sophisticated numerical simulations in two and three dimensions recently clarified the mist, finding that reconnection produces a specific structure of the current layer inside the electron inertial (also called electron diffusion) region around the reconnection site, the X line. Onset of reconnection is attributed to pseudo-viscous contributions of the electron pressure tensor aided by electron inertia and drag, creating a complicated structured electron current sheet, electric fields, and an electron exhaust extended along the current layer. We review the general background theory and recent developments in numerical simulation on collisionless reconnection. It is impossible to cover the entire field of reconnection in a short space-limited review. The presentation necessarily remains cursory, determined by our taste, preferences, and kn
NASA Astrophysics Data System (ADS)
Treumann, R. A.; Baumjohann, W.
2015-10-01
The present review concerns the relevance of collisionless reconnection in the astrophysical context. Emphasis is put on recent developments in theory obtained from collisionless numerical simulations in two and three dimensions. It is stressed that magnetic reconnection is a universal process of particular importance under collisionless conditions, when both collisional and anomalous dissipation are irrelevant. While collisional (resistive) reconnection is a slow, diffusive process, collisionless reconnection is spontaneous. On any astrophysical time scale, it is explosive. It sets on when electric current widths become comparable to the leptonic inertial length in the so-called lepton (electron/positron) "diffusion region", where leptons de-magnetise. Here, the magnetic field contacts its oppositely directed partner and annihilates. Spontaneous reconnection breaks the original magnetic symmetry, violently releases the stored free energy of the electric current, and causes plasma heating and particle acceleration. Ultimately, the released energy is provided by mechanical motion of either the two colliding magnetised plasmas that generate the current sheet or the internal turbulence cascading down to lepton-scale current filaments. Spontaneous reconnection in such extended current sheets that separate two colliding plasmas results in the generation of many reconnection sites (tearing modes) distributed over the current surface, each consisting of lepton exhausts and jets which are separated by plasmoids. Volume-filling factors of reconnection sites are estimated to be as large as {<}10^{-5} per current sheet. Lepton currents inside exhausts may be strong enough to excite Buneman and, for large thermal pressure anisotropy, also Weibel instabilities. They bifurcate and break off into many small-scale current filaments and magnetic flux ropes exhibiting turbulent magnetic power spectra of very flat power-law shape W_b∝ k^{-α } in wavenumber k with power becoming as
Effects of electron inertia in collisionless magnetic reconnection
Andrés, Nahuel Gómez, Daniel; Martin, Luis; Dmitruk, Pablo
2014-07-15
We present a study of collisionless magnetic reconnection within the framework of full two-fluid MHD for a completely ionized hydrogen plasma, retaining the effects of the Hall current, electron pressure and electron inertia. We performed 2.5D simulations using a pseudo-spectral code with no dissipative effects. We check that the ideal invariants of the problem are conserved down to round-off errors. Our numerical results confirm that the change in the topology of the magnetic field lines is exclusively due to the presence of electron inertia. The computed reconnection rates remain a fair fraction of the Alfvén velocity, which therefore qualifies as fast reconnection.
Effects of electron inertia in collisionless magnetic reconnection
NASA Astrophysics Data System (ADS)
Andrés, Nahuel; Martin, Luis; Dmitruk, Pablo; Gómez, Daniel
2014-07-01
We present a study of collisionless magnetic reconnection within the framework of full two-fluid MHD for a completely ionized hydrogen plasma, retaining the effects of the Hall current, electron pressure and electron inertia. We performed 2.5D simulations using a pseudo-spectral code with no dissipative effects. We check that the ideal invariants of the problem are conserved down to round-off errors. Our numerical results confirm that the change in the topology of the magnetic field lines is exclusively due to the presence of electron inertia. The computed reconnection rates remain a fair fraction of the Alfvén velocity, which therefore qualifies as fast reconnection.
Accessing the new collisionless reconnection regime in laboratory experiment
NASA Astrophysics Data System (ADS)
Olson, Joseph; Egedal, Jan; Greess, Samuel; Wallace, John; Clark, Michael; Forest, Cary
2015-11-01
The Terrestrial Reconnection Experiment (TREX), the largest dedicated reconnection experiment to date, is currently in operation at the Wisconsin Plasma Astrophysics Laboratory (WiPAL). In its inaugural run, TREX demonstrated its ability to operate in what has traditionally been called the collisionless reconnection regime by observing the out-of-plane magnetic field characteristic of Hall reconnection. Additionally, TREX is projected to access even more collisionless parameters in which electron pressure anisotropy develops, greatly influencing the dynamics of the reconnection process beyond two fluid effects. For example, spacecraft observations and kinetic simulations show that large-scale current layers are driven by this pressure anisotropy. In the last year, TREX has undergone upgrades to its plasma heating, reconnection drive, and diagnostic suite in order to study these features exclusive to truly collisionless reconnection. Preliminary results from the newly optimized experimental runs will be presented. Supported in part by DoE grant DE-SC0010463.
Nonlinear gyrofluid simulations of collisionless reconnection
Grasso, D.; Tassi, E.; Waelbroeck, F. L.
2010-08-15
The Hamiltonian gyrofluid model recently derived by Waelbroeck et al. [Phys. Plasmas 16, 032109 (2009)] is used to investigate nonlinear collisionless reconnection with a strong guide field by means of numerical simulations. Finite ion Larmor radius gives rise to a cascade of the electrostatic potential to scales below both the ion gyroradius and the electron skin depth. This cascade is similar to that observed previously for the density and current in models with cold ions. In addition to density cavities, the cascades create electron beams at scales below the ion gyroradius. The presence of finite ion temperature is seen to modify, inside the magnetic island, the distribution of the velocity fields that advect two Lagrangian invariants of the system. As a consequence, the fine structure in the electron density is confined to a layer surrounding the separatrix. Finite ion Larmor radius effects produce also a different partition between the electron thermal, potential, and kinetic energy, with respect to the cold-ion case. Other aspects of the dynamics such as the reconnection rate and the stability against Kelvin-Helmholtz modes are similar to simulations with finite electron compressibility but cold ions.
Collisionless Three-dimensional Reconnection In Impulsive Solar Flares
NASA Astrophysics Data System (ADS)
Somov, Boris V.; Kosugi, Takeo; Sakao, Taro
1998-04-01
Two subclasses of impulsive solar flares, observed with the Hard X-Ray Telescope (HXT) onboard Yohkoh, have been discovered by Sakao et al. The two subclasses can be characterized as more impulsive (MI) and less impulsive (LI) flares, the former having a shorter total duration of the impulsive phase in the hard X-ray emission than the latter. We assume that in both subclasses, the collisionless three-dimensional reconnection process occurs at the separator with a longitudinal magnetic field. The high-temperature turbulent-current sheet (HTTCS), located along the separator, generates accelerated particles and fast outflows of ``superhot'' (T >= 30 MK) plasma. Powerful anomalous heat-conductive fluxes along the reconnected field lines maintain a high temperature in the superhot plasma. The difference between the LI and MI flares presumably appears because the footpoint separation (the distance between two brightest hard X-ray sources) increases in time in the LI flares, but decreases in the MI flares. According to our model, in the LI flares the three-dimensional reconnection process accompanies an increase in the longitudinal magnetic field at the separator. In contrast, in the MI flares the reconnection proceeds with a decrease of the longitudinal field; hence, the reconnection rate is higher in the MI flares. Since reconnection in the MI flares proceeds with a decrease of the longitudinal field, the reconnected field lines become shorter in this process. As the reconnected lines become shorter, accelerated electron beams arrive at the upper chromosphere faster. So, in the MI flares chromospheric evaporation begins earlier than in the LI flares. The evaporation process driven by accelerated electron beams generates upflows of ``warm'' (T <= 10 MK) plasma that interacts with downflows of superhot plasma and can switch off the accumulation of superhot plasma in the MI flares during the impulsive phase. In the LI flares, however, an observable amount of superhot
Multiscale dynamics based on kinetic simulation of collisionless magnetic reconnection
NASA Astrophysics Data System (ADS)
Fujimoto, Keizo; Takamoto, Makoto
2016-07-01
Magnetic reconnection is a natural energy converter which allows explosive energy release of the magnetic field energy into plasma kinetic energy. The reconnection processes inherently involve multi-scale process. The breaking of the field lines takes place predominantly in a small region called the diffusion region formed near the x-line, while the fast plasma jets resulting from reconnection extend to a distance far beyond the ion kinetic scales from the x-line. There has been a significant gap in understanding of macro-scale and micro-scale processes. The macro-scale model of reconnection has been developed using the magnetohydrodynamics (MHD) equations, while the micro-scale processes around the x-line have been based on kinetic equations including the ion and electron inertia. The problem is that these two kinds of model have significant discrepancies. It has been believed without any guarantee that the microscopic model near the x-line would connect to the macroscopic model far downstream of the x-line. In order to bridge the gap between the macro and micro-scale processes, we have performed large-scale particle-in-cell simulations with the adaptive mesh refinement. The simulation results suggest that the microscopic processes around the x-line do not connect to the previous MHD model even in the region far downstream of the x-line. The slow mode shocks and the associated plasma acceleration do not appear at the exhaust boundary of kinetic reconnection. Instead, the ions are accelerated due to the Speiser motion in the current layer extending to a distance beyond the kinetic scales. The different acceleration mechanisms between the ions and electrons lead to the Hall current system in broad area of the exhaust. Therefore, the previous MHD model could be inappropriate for collisionless magnetic reconnection. Ref. K. Fujimoto & M. Takamoto, Phys. Plasmas, 23, 012903 (2016).
Reconnection properties in collisionless plasma with open boundary conditions
Sun, H. E.; Ma, Z. W.; Huang, J.
2014-07-15
Collisionless magnetic reconnection in a Harris current sheet with different initial thicknesses is investigated using a 21/2 -D Darwin particle-in-cell simulation with the magnetosonic open boundary condition. It is found that the thicknesses of the ion dissipation region and the reconnection current sheet, when the reconnection rate E{sub r} reaches its first peak, are independent of the initial thickness of the current sheet; while the peak reconnection rate depends on it. The peak reconnection rate increases with decrease of the current sheet thickness as E{sub r}∼a{sup −1/2}, where a is the initial current sheet half-thickness.
Electron nongyrotropy in the context of collisionless magnetic reconnection
Aunai, Nicolas; Hesse, Michael; Kuznetsova, Maria
2013-09-15
Collisionless magnetized plasmas have the tendency to isotropize their velocity distribution function around the local magnetic field direction, i.e., to be gyrotropic, unless some spatial and/or temporal fluctuations develop at the particle gyroscales. Electron gyroscale inhomogeneities are well known to develop during the magnetic reconnection process. Nongyrotropic electron velocity distribution functions have been observed to play a key role in the dissipative process breaking the field line connectivity. In this paper, we present a new method to quantify the deviation of a particle population from gyrotropy. The method accounts for the full 3D shape of the distribution and its analytical formulation allows fast numerical computation. Regions associated with a significant degree of nongyrotropy are shown, as well as the kinetic origin of the nongyrotropy and the fluid signature it is associated with. Using the result of 2.5D Particle-In-Cell simulations of magnetic reconnection in symmetric and asymmetric configurations, it is found that neither the reconnection site nor the topological boundaries are generally associated with a maximized degree of nongyrotropy. Nongyrotropic regions do not correspond to a specific fluid behavior as equivalent nongyrotropy is found to extend over the electron dissipation region as well as in non-dissipative diamagnetic drift layers. The localization of highly nongyrotropic regions in numerical models and their correlation with other observable quantities can, however, improve the characterization of spatial structures explored by spacecraft missions.
Collisionless reconnection driven bulk heating of electrons on TREX
NASA Astrophysics Data System (ADS)
Olson, Joseph; Egedal, Jan; Greess, Samuel; Wallace, John; Clark, Michael; Forest, Cary
2015-11-01
The mechanism for particle heating during magnetic reconnection is an open topic in plasma physics research. Addressing this issue is a major concern for theory, observation, and experiment alike. Recently, a new model has been proposed to explain the bulk heating of electrons during collisionless reconnection, predicting that the heating scales inversely with the plasma beta. The new Terrestrial Reconnection Experiment (TREX) aims to examine this energy partition in a laboratory plasma. By reducing the collisionality in the experiment, the upstream electron pressure should become anisotropic due to adiabatic trapping, making TREX the first reconnection experiment able to access the necessary parameters to study these plasma dynamics. Preliminary analysis from the TREX magnetic flux probe array will be presented, characterizing the electron diffusion region in for collisionless magnetic reconnection. Supported in part by DoE grant DE-SC0010463.
Bellan, Paul M.
2014-10-15
If either finite electron inertia or finite resistivity is included in 2D magnetic reconnection, the two-fluid equations become a pair of second-order differential equations coupling the out-of-plane magnetic field and vector potential to each other to form a fourth-order system. The coupling at an X-point is such that out-of-plane even-parity electric and odd-parity magnetic fields feed off each other to produce instability if the scale length on which the equilibrium magnetic field changes is less than the ion skin depth. The instability growth rate is given by an eigenvalue of the fourth-order system determined by boundary and symmetry conditions. The instability is a purely growing mode, not a wave, and has growth rate of the order of the whistler frequency. The spatial profile of both the out-of-plane electric and magnetic eigenfunctions consists of an inner concave region having extent of the order of the electron skin depth, an intermediate convex region having extent of the order of the equilibrium magnetic field scale length, and a concave outer exponentially decaying region. If finite electron inertia and resistivity are not included, the inner concave region does not exist and the coupled pair of equations reduces to a second-order differential equation having non-physical solutions at an X-point.
Vlasov simulations of collisionless magnetic reconnection without background density
NASA Astrophysics Data System (ADS)
Schmitz, H.; Grauer, R.
2008-02-01
A standard starting point for the simulation of collisionless reconnection is the Harris equilibrium which is made up of a current sheet that separates two regions of opposing magnetic field. Magnetohydrodynamic simulations of collisionless reconnection usually include a homogeneous background density for reasons of numerical stability. While, in some cases, this is a realistic assumption, the background density may introduce new effects both due to the more involved structure of the distribution function or due to the fact that the Alfvèn speed remains finite far away from the current sheet. We present a fully kinetic Vlasov simulation of the perturbed Harris equilibrium using a Vlasov code. Parameters are chosen to match the Geospace Environment Modeling (GEM) Magnetic Reconnection Challenge but excluding the background density. This allows to compare with earlier simulations [Schmitz H, Grauer R. Kinetic Vlasov simulations of collisionless magnetic reconnection. Phys Plasmas 2006;13:092309] which include the background density. It is found that the absence of a background density causes the reconnection rate to be higher. On the other hand, the time until the onset of reconnection is hardly affected. Again the off diagonal elements of the pressure tensor are found to be important on the X-line but with modified importance for the individual terms.
New Measure of the Dissipation Region in Collisionless Magnetic Reconnection
NASA Technical Reports Server (NTRS)
Zenitani, Seiji; Hesse, Michael; Klimas, Alex; Kuznetsova, Masha
2012-01-01
A new measure to identify a small-scale dissipation region in collisionless magnetic reconnection is proposed. The energy transfer from the electromagnetic field to plasmas in the electron s rest frame is formulated as a Lorentz-invariant scalar quantity. The measure is tested by two-dimensional particle-in-cell simulations in typical configurations: symmetric and asymmetric reconnection, with and without the guide field. The innermost region surrounding the reconnection site is accurately located in all cases. We further discuss implications for nonideal MHD dissipation.
New Measure of the Dissipation Region in Collisionless Magnetic Reconnection
Zenitani, Seiji; Hesse, Michael; Klimas, Alex; Kuznetsova, Masha
2011-05-13
A new measure to identify a small-scale dissipation region in collisionless magnetic reconnection is proposed. The energy transfer from the electromagnetic field to plasmas in the electron's rest frame is formulated as a Lorentz-invariant scalar quantity. The measure is tested by two-dimensional particle-in-cell simulations in typical configurations: symmetric and asymmetric reconnection, with and without the guide field. The innermost region surrounding the reconnection site is accurately located in all cases. We further discuss implications for nonideal MHD dissipation.
Catastrophic onset of fast magnetic reconnection with a guide field
NASA Astrophysics Data System (ADS)
Cassak, P. A.; Drake, J. F.; Shay, M. A.
2007-05-01
It was recently shown that the slow (collisional) Sweet-Parker and the fast (collisionless) Hall magnetic reconnection solutions simultaneously exist for a wide range of resistivities; reconnection is bistable [Cassak, Shay, and Drake, Phys. Rev. Lett., 95, 235002 (2005)]. When the thickness of the dissipation region becomes smaller than a critical value, the Sweet-Parker solution disappears and fast reconnection ensues, potentially explaining how large amounts of magnetic free energy can accrue without significant release before the onset of fast reconnection. Two-fluid numerical simulations extending the previous results for anti-parallel reconnection (where the critical thickness is the ion skin depth) to component reconnection with a large guide field (where the critical thickness is the thermal ion Larmor radius) are presented. Applications to laboratory experiments of magnetic reconnection and the sawtooth crash are discussed.
The Diffusion Region in Collisionless Magnetic Reconnection
NASA Technical Reports Server (NTRS)
Hesse, Michael; Neukirch, Thomas; Schindler, Karl; Kuznetsova, Masha; Zenitani, Seiji
2011-01-01
A review of present understanding of the dissipation region in magnetic reconnection is presented. The review focuses on results of the thermal inertia-based dissipation mechanism but alternative mechanisms are mentioned as well. For the former process, a combination of analytical theory and numerical modeling is presented. Furthermore, a new relation between the electric field expressions for anti-parallel and guide field reconnection is developed.
The role of microturbulence on collisionless reconnection. [in magnetospheric plasmas
NASA Technical Reports Server (NTRS)
Papadopoulos, K.
1980-01-01
The linear, non-linear and anomalous transport properties associated with various microinstabilities driven by cross field currents in reconnecting geometries are reviewed. An assessment of their role in collisionless tearing based on analytic theory, computer simulations and experimental evidence, supports the dominant role of lower hybrid waves. The relevance of microturbulence on macroscopic stationary and time dependent models of merging is presented. It is concluded that a fluid-numerical simulation approach that includes (at each space and time step) the effects of anomalous transport in a self consistent manner, similar to the one used for laboratory collisionless shocks, represents the best method for studying and modeling the details of the reconnection process.
Collisionless Reconnection in Global Modeling of Magnetospheric Dynamics
NASA Astrophysics Data System (ADS)
Kuznetsova, M. M.; Hesse, M.; Rastaetter, L.; Gombosi, T.; de Zeeuw, D.; Toth, G.
2006-12-01
Recent advances in small-scale kinetic modeling of magnetic reconnection significantly improved our understanding of physical mechanisms controlling the dissipation in the vicinity of the reconnection site in collisionless plasma. However the progress in studies of small-scale geometries was not very helpful for large scale simulations. Global magnetosphere simulations usually include non-ideal processes in terms of numerical dissipation and/or ad hoc anomalous resistivity. To understand the role of magnetic reconnection in global evolution of magnetosphere and to place spacecraft observations into global context it is desirable to perform global simulations with physically motivated model of dissipation that are capable to reproduce reconnection rates observed in kinetic models. In our efforts to bridge the gap between small scale kinetic modeling and global simulations we introduced an approach that allows to quantify the interaction between large-scale global magnetospheric dynamics and microphysical processes in diffusion regions near reconnection sites. We utilized the global MHD code BATSRUS and incorporate primary mechanism controlling the dissipation in the vicinity of the reconnection site in terms of non-gyrotropic corrections to the induction equation. We demonstrated that nongyrotropic effects can significantly alter the global magnetosphere evolution. Our approach allowed for the first time to model loading/unloading cycle in response to steady southward IMF driving. We will extend our approach to cases with nonzero IMF By and analyze the effects of solar wind parameters and ionospheric conductance on reconnection rate and global magnetosphere dynamics.
Collisionless Reconnection with Weak Slow Shocks Under Anisotropic MHD Approximation
NASA Astrophysics Data System (ADS)
Hirabayashi, K.; Hoshino, M.
2014-12-01
Magnetic reconnection accompanied by a pair of slow-mode shock waves, known as Petschek's theory, has been widely studied as an efficient mechanism to convert magnetically stored energy to thermal and/or kinetic energy in plasmas. Satellite observations in the Earth's magnetotail, on the other hand, report that the detection of slow shocks is rare compared with the theory. As an important step to bridge the gap between the observational fact and the Petschek-type reconnection, we performed one- and two- dimensional collisionless magnetohydrodynamic (MHD) simulations of magnetic reconnection paying special attention to the effect of temperature anisotropy. In high-beta plasmas such as a plasma sheet in the magnetotail, it is expected that even weak temperature anisotropy can greatly modify the dynamics. We demonstrate that the slow shocks do exist in the reconnection layer even under the anisotropic temperature. The resultant shocks, however, are weaker than those in isotropic MHD in terms of plasma compression. In addition, the amount of magnetic energy released across the shock is extremely small, that is, the shock is no longer switch-off type. In spite of the weakness of the shocks, the reconnection rates measured by the inflow velocities are kept at the same level as the isotropic cases. Once the slow shock forms, the downstream plasma is heated in highly anisotropic manner, and the firehose-sense anisotropy affects the wave structure in the system. In particular, it is remarkable that the sequential order of propagation of slow shocks and rotational discontinuities reverses depending upon the magnitude of a superposed guide field. Our result is consistent with the rareness of the slow shock detection in the magnetotail, and implies that shocks do not necessarily play an important role. Furthermore, a variety of wave structure of a reconnection layer shown here will help interpretation of observational data in collisionless reconnection.
Electron Force Balance in Steady Collisionless-Driven Reconnection
Li Bin; Horiuchi, Ritoku
2008-11-21
Steady collisionless-driven reconnection in an open system is investigated by means of full-particle simulations. A long thin electron current sheet extends towards the outflow direction when the system relaxes to a steady state. Although the pressure tensor term along the reconnection electric field contributes to the violation of the electron frozen-in condition, a new force balance in the inflow direction is realized between the Lorentz and electrostatic forces, which is quite different from that in Harris equilibrium. The strong electrostatic field is generated through the combined effect of the Hall term and a driving inflow. This new force balance is more evident in the three-dimensional case due to the growth of an instability along the reconnection electric field. It is also found that the normalized charge density is in proportion to the square of the electron Alfven velocity averaged over the electron dissipation region.
ONSET OF FAST MAGNETIC RECONNECTION IN PARTIALLY IONIZED GASES
Malyshkin, Leonid M.; Zweibel, Ellen G. E-mail: zweibel@astro.wisc.edu
2011-10-01
We consider quasi-stationary two-dimensional magnetic reconnection in a partially ionized incompressible plasma. We find that when the plasma is weakly ionized and the collisions between the ions and the neutral particles are significant, the transition to fast collisionless reconnection due to the Hall effect in the generalized Ohm's law is expected to occur at much lower values of the Lundquist number, as compared to a fully ionized plasma case. We estimate that these conditions for fast reconnection are satisfied in molecular clouds and in protostellar disks.
Toward a transport model of collisionless magnetic reconnection
NASA Astrophysics Data System (ADS)
Kuznetsova, Masha M.; Hesse, Michael; Winske, Dan
2000-04-01
An absence of theoretical justification for the magnitude of resistivity is one of the major limitations of large-scale simulations of magnetic reconnection in collisionless magnetospheric plasma. We took advantage of the results of recent progress in kinetic modeling of collisionless dissipation in the vicinity of the magnetically neutral X point aiming to find ways to represent small-scale kinetic effects in large-scale models. The study was based on a combination of hybrid and particle methods and on analytical analysis. A comprehensive hybrid simulation code which incorporates the leading terms in electron dynamics responsible for breaking the frozen magnetic flux constraint (electron bulk flow inertia and nongyrotropic pressure effects) was utilized. The results of the comprehensive hybrid model were found to be in excellent quantitative agreement with the results of full particle simulations with similar setups. Both simulations demonstrated that the actual reconnection electric field is determined primarily by kinetic quasi-viscous effects and less by electron bulk flow inertia. An analytical expression for the quasi-viscous reconnection electric field averaged over the nongyrotropic region was obtained. Similar behavior of the evaluated quasi-viscous electric field and actual reconnection electric field taken from the simulations was demonstrated. Conventional hybrid simulations with simple nongyrotropic corrections to the electric field where also performed. The model was further reduced for utilization in MHD models. Analytical expressions for the time evolution of the reconnected flux evaluated from the MHD model modified by nongyrotropic corrections appeared to be in very good agreement with the results of comprehensive kinetic simulations. The evaluated averaged quasi-viscous electric field can be substituted into large-scale simulation models.
Kinetic Vlasov simulations of collisionless magnetic reconnection
Schmitz, H.; Grauer, R.
2006-09-15
A fully kinetic Vlasov simulation of the Geospace Environment Modeling Magnetic Reconnection Challenge is presented. Good agreement is found with previous kinetic simulations using particle in cell (PIC) codes, confirming both the PIC and the Vlasov code. In the latter the complete distribution functions f{sub k} (k=i,e) are discretized on a numerical grid in phase space. In contrast to PIC simulations, the Vlasov code does not suffer from numerical noise and allows a more detailed investigation of the distribution functions. The role of the different contributions of Ohm's law are compared by calculating each of the terms from the moments of the f{sub k}. The important role of the off-diagonal elements of the electron pressure tensor could be confirmed. The inductive electric field at the X line is found to be dominated by the nongyrotropic electron pressure, while the bulk electron inertia is of minor importance. Detailed analysis of the electron distribution function within the diffusion region reveals the kinetic origin of the nongyrotropic terms.
Does fast magnetic reconnection exist?
NASA Technical Reports Server (NTRS)
Priest, E. R.; Forbes, T. G.
1992-01-01
The main features of the Priest-Forbes (1986) and Priest-Lee (1990) models of magnetic reconnection in astrophysical plasmas are discussed, and the Priest-Lee model is generalized to include inflow pressure gradients and thus different regimes of reconnection. It is shown that different scaling results can be obtained depending on the boundary conditions. These results are compared to the ones observed in the numerical experiments of Biskamp (1986) and Lee and Fu (1986). It is concluded that numerical experiments with suitably designed boundary conditions are likely to exhibit fast reconnection, and that such reconnection is a common process in astrophysical and space plasmas.
Scaling of Magnetic Reconnection in Relativistic Collisionless Pair Plasmas
NASA Technical Reports Server (NTRS)
Liu, Yi-Hsin; Guo, Fan; Daughton, William; Li, Hui; Hesse, Michael
2015-01-01
Using fully kinetic simulations, we study the scaling of the inflow speed of collisionless magnetic reconnection in electron-positron plasmas from the non-relativistic to ultra-relativistic limit. In the anti-parallel configuration, the inflow speed increases with the upstream magnetization parameter sigma and approaches the speed of light when sigma is greater than O(100), leading to an enhanced reconnection rate. In all regimes, the divergence of the pressure tensor is the dominant term responsible for breaking the frozen-in condition at the x-line. The observed scaling agrees well with a simple model that accounts for the Lorentz contraction of the plasma passing through the diffusion region. The results demonstrate that the aspect ratio of the diffusion region, modified by the compression factor of proper density, remains approximately 0.1 in both the non-relativistic and relativistic limits.
Scaling of magnetic reconnection in relativistic collisionless pair plasmas.
Liu, Yi-Hsin; Guo, Fan; Daughton, William; Li, Hui; Hesse, Michael
2015-03-01
Using fully kinetic simulations, we study the scaling of the inflow speed of collisionless magnetic reconnection in electron-positron plasmas from the nonrelativistic to ultrarelativistic limit. In the antiparallel configuration, the inflow speed increases with the upstream magnetization parameter σ and approaches the speed of light when σ>O(100), leading to an enhanced reconnection rate. In all regimes, the divergence of the pressure tensor is the dominant term responsible for breaking the frozen-in condition at the x line. The observed scaling agrees well with a simple model that accounts for the Lorentz contraction of the plasma passing through the diffusion region. The results demonstrate that the aspect ratio of the diffusion region, modified by the compression factor of proper density, remains ∼0.1 in both the nonrelativistic and relativistic limits. PMID:25793820
The Impact of Geometrical Constraints on Collisionless Magnetic Reconnection
NASA Technical Reports Server (NTRS)
Hesse, Michael; Aunai, Nico; Kuznetsova, Masha; Frolov, Rebekah; Black, Carrrie
2012-01-01
One of the most often cited features associated with collisionless magnetic reconnection is a Hall-type magnetic field, which leads, in antiparallel geometries, to a quadrupolar magnetic field signature. The combination of this out of plane magnetic field with the reconnection in-plane magnetic field leads to angling of magnetic flux tubes out of the plane defined by the incoming magnetic flux. Because it is propagated by Whistler waves, the quadrupolar field can extend over large distances in relatively short amounts of time - in fact, it will extend to the boundary of any modeling domain. In reality, however, the surrounding plasma and magnetic field geometry, defined, for example, by the overall solar wind flow, will in practice limit the extend over which a flux tube can be angled out of the main plain. This poses the question to what extent geometric constraints limit or control the reconnection process and this is the question investigated in this presentation. The investigation will involve a comparison of calculations, where open boundary conditions are set up to mimic either free or constrained geometries. We will compare momentum transport, the geometry of the reconnection regions, and the acceleration if ions and electrons to provide the current sheet in the outflow jet.
Slow shock formation and temperature anisotropy in collisionless magnetic reconnection
NASA Astrophysics Data System (ADS)
Higashimori, K.; Hoshino, M.
2011-12-01
We perform a two-dimensional simulation by using an electromagnetic hybrid code to study the formation of slow-mode shocks in collisionless magnetic reconnection in low beta plasmas, and we argue that one of important agents of the formation of slow shocks is the ion temperature anisotropy enhanced at the shock downstream region. As magnetic reconnection develops, it is known that the parallel temperature along the magnetic field becomes large in association with the anisotropic PSBL ion beams, and this temperature anisotropy has a tendency to suppress the formation of slow shock. Although preceding studies on magnetic reconnection with kinetic codes have shown such ion temperature anisotropy along the reconnection layer, the direct relation between formation of slow shocks and the ion temperature anisotropy has not been investigated. Based on our simulation result, we found that the slow shock formation is suppressed due to the large temperature anisotropy near the X-type region, but the downstream ion temperature anisotropy relaxes with increasing the distance from the magnetic neutral point. As a result, two pairs of current structures, which are the strong evidence of dissipation of magnetic field in slow shocks, are formed at the distance |x| > 115 λ i from the neutral point.
On the relationship between quadrupolar magnetic field and collisionless reconnection
Smets, R. Belmont, G.; Aunai, N.; Boniface, C.
2014-06-15
Using hybrid simulations, we investigate the onset of fast reconnection between two cylindrical magnetic shells initially close to each other. This initial state mimics the plasma structure in High Energy Density Plasmas induced by a laser-target interaction and the associated self-generated magnetic field. We clearly observe that the classical quadrupolar structure of the out-of-plane magnetic field appears prior to the reconnection onset. Furthermore, a parametric study reveals that, with a non-coplanar initial magnetic topology, the reconnection onset is delayed and possibly suppressed. The relation between the out-of-plane magnetic field and the out-of-plane electric field is discussed.
Asymmetric evolution of magnetic reconnection in collisionless accretion disk
Shirakawa, Keisuke Hoshino, Masahiro
2014-05-15
An evolution of a magnetic reconnection in a collisionless accretion disk is investigated using a 2.5 dimensional hybrid code simulation. In astrophysical disks, magnetorotational instability (MRI) is considered to play an important role by generating turbulence in the disk and contributes to an effective angular momentum transport through a turbulent viscosity. Magnetic reconnection, on the other hand, also plays an important role on the evolution of the disk through a dissipation of a magnetic field enhanced by a dynamo effect of MRI. In this study, we developed a hybrid code to calculate an evolution of a differentially rotating system. With this code, we first confirmed a linear growth of MRI. We also investigated a behavior of a particular structure of a current sheet, which would exist in the turbulence in the disk. From the calculation of the magnetic reconnection, we found an asymmetric structure in the out-of-plane magnetic field during the evolution of reconnection, which can be understood by a coupling of the Hall effect and the differential rotation. We also found a migration of X-point whose direction is determined only by an initial sign of J{sub 0}×Ω{sub 0}, where J{sub 0} is the initial current density in the neutral sheet and Ω{sub 0} is the rotational vector of the background Keplerian rotation. Associated with the migration of X-point, we also found a significant enhancement of the perpendicular magnetic field compared to an ordinary MRI. MRI-Magnetic reconnection coupling and the resulting magnetic field enhancement can be an effective process to sustain a strong turbulence in the accretion disk and to a transport of angular momentum.
A twenty-moment model for collisionless guide field reconnection
NASA Astrophysics Data System (ADS)
Ng, Jonathan; Hakim, Ammar; Bhattacharjee, Amitava
2015-11-01
The integration of kinetic effects in fluid models is an important problem in global simulations of the Earth's magnetosphere and space weather modelling. Here we introduce a new fluid model and closure for collisionless magnetic reconnection and more general applications. It has recently been shown that electron pressure anisotropy is important in setting the structure of the reconnection region, and a closure based on the drift kinetic equation using a distribution of trapped and passing particles has been derived. We extend the model and present a general expression for moments of the distribution function. By evolving the heat flux tensor and closing at the fourth velocity moment, we obtain a self-consistent set of fluid equations, which includes the evolution of the off-diagonal elements of the pressure tensor. The model is implemented in a two-fluid code and the results are compared to PIC simulations of guide field reconnection. This work was supported by NSF Grant No. AGS-1338944, DOE Contract DE-AC02-09CH11466.
The link between shocks, turbulence, and magnetic reconnection in collisionless plasmas
Karimabadi, H.; Omelchenko, Y. A.; Roytershteyn, V.; Vu, H. X.; Scudder, J.; Daughton, W.; Dimmock, A.; Nykyri, K.; Wan, M.; Sibeck, D.; Tatineni, M.; Majumdar, A.; Loring, B.; Geveci, B.
2014-06-15
Global hybrid (electron fluid, kinetic ions) and fully kinetic simulations of the magnetosphere have been used to show surprising interconnection between shocks, turbulence, and magnetic reconnection. In particular, collisionless shocks with their reflected ions that can get upstream before retransmission can generate previously unforeseen phenomena in the post shocked flows: (i) formation of reconnecting current sheets and magnetic islands with sizes up to tens of ion inertial length. (ii) Generation of large scale low frequency electromagnetic waves that are compressed and amplified as they cross the shock. These “wavefronts” maintain their integrity for tens of ion cyclotron times but eventually disrupt and dissipate their energy. (iii) Rippling of the shock front, which can in turn lead to formation of fast collimated jets extending to hundreds of ion inertial lengths downstream of the shock. The jets, which have high dynamical pressure, “stir” the downstream region, creating large scale disturbances such as vortices, sunward flows, and can trigger flux ropes along the magnetopause. This phenomenology closes the loop between shocks, turbulence, and magnetic reconnection in ways previously unrealized. These interconnections appear generic for the collisionless plasmas typical of space and are expected even at planar shocks, although they will also occur at curved shocks as occur at planets or around ejecta.
The link between shocks, turbulence, and magnetic reconnection in collisionless plasmas
NASA Astrophysics Data System (ADS)
Karimabadi, H.; Roytershteyn, V.; Vu, H. X.; Omelchenko, Y. A.; Scudder, J.; Daughton, W.; Dimmock, A.; Nykyri, K.; Wan, M.; Sibeck, D.; Tatineni, M.; Majumdar, A.; Loring, B.; Geveci, B.
2014-06-01
Global hybrid (electron fluid, kinetic ions) and fully kinetic simulations of the magnetosphere have been used to show surprising interconnection between shocks, turbulence, and magnetic reconnection. In particular, collisionless shocks with their reflected ions that can get upstream before retransmission can generate previously unforeseen phenomena in the post shocked flows: (i) formation of reconnecting current sheets and magnetic islands with sizes up to tens of ion inertial length. (ii) Generation of large scale low frequency electromagnetic waves that are compressed and amplified as they cross the shock. These "wavefronts" maintain their integrity for tens of ion cyclotron times but eventually disrupt and dissipate their energy. (iii) Rippling of the shock front, which can in turn lead to formation of fast collimated jets extending to hundreds of ion inertial lengths downstream of the shock. The jets, which have high dynamical pressure, "stir" the downstream region, creating large scale disturbances such as vortices, sunward flows, and can trigger flux ropes along the magnetopause. This phenomenology closes the loop between shocks, turbulence, and magnetic reconnection in ways previously unrealized. These interconnections appear generic for the collisionless plasmas typical of space and are expected even at planar shocks, although they will also occur at curved shocks as occur at planets or around ejecta.
The Link Between Shocks, Turbulence, and Magnetic Reconnection in Collisionless Plasmas
NASA Technical Reports Server (NTRS)
Karimabadi, H.; Roytershteyn, V.; Vu, H. X.; Omelchenko, Y. A.; Scudder, J.; Daughton, W.; Dimmock, A.; Nykyri, K.; Wan, M.; Sibeck, D.; Tatineni, M.; Majumdar, A.; Loring, B.; Geveci, B.
2014-01-01
Global hybrid (electron fluid, kinetic ions) and fully kinetic simulations of the magnetosphere have been used to show surprising interconnection between shocks, turbulence and magnetic reconnection. In particular collisionless shocks with their reflected ions that can get upstream before retransmission can generate previously unforeseen phenomena in the post shocked flows: (i) formation of reconnecting current sheets and magnetic islands with sizes up to tens of ion inertial length. (ii) Generation of large scale low frequency electromagnetic waves that are compressed and amplified as they cross the shock. These 'wavefronts' maintain their integrity for tens of ion cyclotron times but eventually disrupt and dissipate their energy. (iii) Rippling of the shock front, which can in turn lead to formation of fast collimated jets extending to hundreds of ion inertial lengths downstream of the shock. The jets, which have high dynamical pressure, 'stir' the downstream region, creating large scale disturbances such as vortices, sunward flows, and can trigger flux ropes along the magnetopause. This phenomenology closes the loop between shocks, turbulence and magnetic reconnection in ways previously unrealized. These interconnections appear generic for the collisionless plasmas typical of space, and are expected even at planar shocks, although they will also occur at curved shocks as occur at planets or around ejecta.
Collisionless reconnection in two-dimensional magnetotail equilibria
NASA Technical Reports Server (NTRS)
Pritchett, P. L.; Coroniti, F. V.; Pellat, R.; Karimabadi, H.
1991-01-01
A two-dimensional particle simulation model based on the Darwin approximation to Maxwell's equations for studying collisionless reconnection in the magnetotail has been developed. Simulations of the pure ion tearing mode in a thin current sheet with normal B(z) field component demonstrate that in this limit this mode grows more slowly than expected based on previous analytic estimates. The saturation level of the tearing instability greatly surpasses estimates based on a simple trapping argument. The effect of the normal field component on the evolution of the tearing instability is considered. It is found that a normal field of even a few percent on axis strongly inhibits the growth of the instability.
Do dispersive waves play a role in collisionless magnetic reconnection?
Liu, Yi-Hsin; Daughton, W.; Li, H.; Karimabadi, H.; Peter Gary, S.
2014-02-15
Using fully kinetic simulations, we demonstrate that the properly normalized reconnection rate is fast ∼0.1 for guide fields up to 80× larger than the reconnecting field and is insensitive to both the system size and the ion to electron mass ratio. These results challenge conventional explanations of reconnection based on fast dispersive waves, which are completely absent for sufficiently strong guide fields. In this regime, the thickness of the diffusion layer is set predominantly by the electron inertial length with an inner sublayer that is controlled by finite gyro-radius effects. As the Alfvén velocity becomes relativistic for very strong guide fields, the displacement current becomes important and strong deviations from charge neutrality occur, resulting in the build-up of intense electric fields which absorb a portion of the magnetic energy release. Over longer time scales, secondary magnetic islands are generated near the active x-line while an electron inertial scale Kelvin-Helmholtz instability is driven within the outflow. These secondary instabilities give rise to time variations in the reconnection rate but do not alter the average value.
Mechanisms for fast flare reconnection
NASA Technical Reports Server (NTRS)
Vanhoven, G.; Deeds, D.; Tachi, T.
1988-01-01
Normal collisional-resistivity mechanisms of magnetic reconnection have the drawback that they are too slow to explain the fast rise of solar flares. Two methods are examined which are proposed for the speed-up of the magnetic tearing instability: the anomalous enhancement of resistivity by the injection of MHD turbulence and the increase of Coulomb resistivity by radiative cooling. The results are described for nonlinear numerical simulations of these processes which show that the first does not provide the claimed effects, while the second yields impressive rates of reconnection, but low saturated energy outputs.
Physics of collisionless reconnection in a stressed X-point collapse
Tsiklauri, D.; Haruki, T.
2008-10-15
Recently, magnetic reconnection during collisionless, stressed, X-point collapse was studied using kinetic, 2.5-dimensional, fully electromagnetic, relativistic particle-in-cell numerical code [D. Tsiklauri and T. Haruki, Phys. Plasmas 14, 112905 (2007)]. Here we finalize the investigation of this topic by addressing key outstanding physical questions: (i) Which term in the generalized Ohm's law is responsible for the generation of the reconnection electric field? (ii) How does the time evolution of the reconnected flux vary with the ion-electron mass ratio? (iii) What is the exact energy budget of the reconnection process; i.e., in which proportion initial (mostly magnetic) energy is converted into other forms of energy? (iv) Are there any anisotropies in the velocity distribution of the accelerated particles? The following points have been established. (i) A reconnection electric field is generated by the electron pressure tensor off-diagonal terms, resembling to the case of tearing unstable Harris current sheet studied by the GEM reconnection challenge. (ii) For m{sub i}/m{sub e}>>1, the time evolution of the reconnected flux is independent of ion-electron mass ratio. In addition, in the case of m{sub i}/m{sub e}=1, we show that reconnection proceeds slowly as the Hall term is zero; when m{sub i}/m{sub e}>>1 (i.e., the Hall term is nonzero) reconnection is fast and we conjecture that this is due to magnetic field being frozen into electron fluid, which moves significantly faster than ion fluid. (iii) Within one Alfven time, somewhat less than half ({approx}40%) of the initial total (roughly magnetic) energy is converted into the kinetic energy of electrons, and somewhat more than half ({approx}60%) into kinetic energy of ions (similar to solar flare observations). (iv) In the strongly stressed X-point case, in about one Alfven time, a full isotropy in all three spatial directions of the velocity distribution is seen for superthermal electrons (also commensurate
NASA Astrophysics Data System (ADS)
Olson, J.; Egedal, J.; Greess, S.; Myers, R.; Clark, M.; Endrizzi, D.; Flanagan, K.; Milhone, J.; Peterson, E.; Wallace, J.; Weisberg, D.; Forest, C. B.
2016-06-01
The spontaneous formation of magnetic islands is observed in driven, antiparallel magnetic reconnection on the Terrestrial Reconnection Experiment. We here provide direct experimental evidence that the plasmoid instability is active at the electron scale inside the ion diffusion region in a low collisional regime. The experiments show the island formation occurs at a smaller system size than predicted by extended magnetohydrodynamics or fully collisionless simulations. This more effective seeding of magnetic islands emphasizes their importance to reconnection in naturally occurring 3D plasmas.
Suppression of collisionless magnetic reconnection in asymmetric current sheets
NASA Astrophysics Data System (ADS)
Liu, Yi-Hsin; Hesse, Michael
2016-06-01
Using fully kinetic simulations, we study the suppression of asymmetric reconnection in the limit where the diamagnetic drift speed ≫ Alfvén speed and the magnetic shear angle is moderate. We demonstrate that the slippage between electrons and the magnetic flux mitigates the suppression and can even result in fast reconnection that lacks one of the outflow jets. Through comparing a case where the diamagnetic drift is supported by the temperature gradient with a companion case that has a density gradient instead, we identify a robust suppression mechanism. The drift of the x-line is slowed down locally by the asymmetric nature of the x-line, and then the x-line is run over and swallowed by the faster-moving following flux.
Suppression of Collisionless Magnetic Reconnection in Asymmetric Current Sheets
NASA Technical Reports Server (NTRS)
Liu, Yi-Hsin; Hesse, Michael
2016-01-01
Using fully kinetic simulations, we study the suppression of asymmetric reconnection in the limit where the diamagnetic drift speed >> Alfven speed and the magnetic shear angle is moderate. We demonstrate that the slippage between electrons and the magnetic flux mitigates the suppression and can even result in fast reconnection that lacks one of the outflow jets. Through comparing a case where the diamagnetic drift is supported by the temperature gradient with a companion case that has a density gradient instead, we identify a robust suppression mechanism. The drift of the x-line is slowed down locally by the asymmetric nature of the x-line, and then the x-line is run over and swallowed by the faster-moving following flux.
FAST MAGNETIC RECONNECTION AND SPONTANEOUS STOCHASTICITY
Eyink, Gregory L.; Lazarian, A.; Vishniac, E. T.
2011-12-10
Magnetic field lines in astrophysical plasmas are expected to be frozen-in at scales larger than the ion gyroradius. The rapid reconnection of magnetic-flux structures with dimensions vastly larger than the gyroradius requires a breakdown in the standard Alfven flux-freezing law. We attribute this breakdown to ubiquitous MHD plasma turbulence with power-law scaling ranges of velocity and magnetic energy spectra. Lagrangian particle trajectories in such environments become 'spontaneously stochastic', so that infinitely many magnetic field lines are advected to each point and must be averaged to obtain the resultant magnetic field. The relative distance between initial magnetic field lines which arrive at the same final point depends upon the properties of two-particle turbulent dispersion. We develop predictions based on the phenomenological Goldreich and Sridhar theory of strong MHD turbulence and on weak MHD turbulence theory. We recover the predictions of the Lazarian and Vishniac theory for the reconnection rate of large-scale magnetic structures. Lazarian and Vishniac also invoked 'spontaneous stochasticity', but of the field lines rather than of the Lagrangian trajectories. More recent theories of fast magnetic reconnection appeal to microscopic plasma processes that lead to additional terms in the generalized Ohm's law, such as the collisionless Hall term. We estimate quantitatively the effect of such processes on the inertial-range turbulence dynamics and find them to be negligible in most astrophysical environments. For example, the predictions of the Lazarian and Vishniac theory are unchanged in Hall MHD turbulence with an extended inertial range, whenever the ion skin depth {delta}{sub i} is much smaller than the turbulent integral length or injection-scale L{sub i} .
Aunai, Nicolas; Hesse, Michael; Black, Carrie; Evans, Rebekah; Kuznetsova, Maria
2013-04-15
Numerical studies implementing different versions of the collisionless Ohm's law have shown a reconnection rate insensitive to the nature of the non-ideal mechanism occurring at the X line, as soon as the Hall effect is operating. Consequently, the dissipation mechanism occurring in the vicinity of the reconnection site in collisionless systems is usually thought not to have a dynamical role beyond the violation of the frozen-in condition. The interpretation of recent studies has, however, led to the opposite conclusion that the electron scale dissipative processes play an important dynamical role in preventing an elongation of the electron layer from throttling the reconnection rate. This work re-visits this topic with a new approach. Instead of focusing on the extensively studied symmetric configuration, we aim to investigate whether the macroscopic properties of collisionless reconnection are affected by the dissipation physics in asymmetric configurations, for which the effect of the Hall physics is substantially modified. Because it includes all the physical scales a priori important for collisionless reconnection (Hall and ion kinetic physics) and also because it allows one to change the nature of the non-ideal electron scale physics, we use a (two dimensional) hybrid model. The effects of numerical, resistive, and hyper-resistive dissipation are studied. In a first part, we perform simulations of symmetric reconnection with different non-ideal electron physics. We show that the model captures the already known properties of collisionless reconnection. In a second part, we focus on an asymmetric configuration where the magnetic field strength and the density are both asymmetric. Our results show that contrary to symmetric reconnection, the asymmetric model evolution strongly depends on the nature of the mechanism which breaks the field line connectivity. The dissipation occurring at the X line plays an important role in preventing the electron current layer
Explosive magnetic reconnection caused by an X-shaped current-vortex layer in a collisionless plasma
Hirota, M.; Hattori, Y.; Morrison, P. J.
2015-05-15
A mechanism for explosive magnetic reconnection is investigated by analyzing the nonlinear evolution of a collisionless tearing mode in a two-fluid model that includes the effects of electron inertia and temperature. These effects cooperatively enable a fast reconnection by forming an X-shaped current-vortex layer centered at the reconnection point. A high-resolution simulation of this model for an unprecedentedly small electron skin depth d{sub e} and ion-sound gyroradius ρ{sub s}, satisfying d{sub e}=ρ{sub s}, shows an explosive tendency for nonlinear growth of the tearing mode, where it is newly found that the explosive widening of the X-shaped layer occurs locally around the reconnection point with the length of the X shape being shorter than the domain length and the wavelength of the linear tearing mode. The reason for the onset of this locally enhanced reconnection is explained theoretically by developing a novel nonlinear and nonequilibrium inner solution that models the local X-shaped layer, and then matching it to an outer solution that is approximated by a linear tearing eigenmode with a shorter wavelength than the domain length. This theoretical model proves that the local reconnection can release the magnetic energy more efficiently than the global one and the estimated scaling of the explosive growth rate agrees well with the simulation results.
Wave modes facilitating fast magnetic reconnection
NASA Astrophysics Data System (ADS)
Singh, N.
2011-12-01
Whistler and kinetic Alfven waves are often invoked to explain fast magnetic reconnection in collsionless plasmas. But how these wave modes facilitate the reconnection has remained unclear. An important unanswered question deals with the meaning of the wave frequency in the context of magnetic reconnection. New measurement on a fast explosive reconnection event in the Versatile Toroidal Facility (VTF) at MIT provides an interesting example of the meaning of the wave mode and the associated frequency directly related to the time scale of the impulsive reconnection. We examine the measurements in VTF in view of the whistler wave mode, showing that the explosive growth in the reconnection is related to the thinning of the current sheet to a few electron skin depths. We further demonstrate that the fastest measured time scale (~ 3 microseconds) and the largest normalized reconnection rate (~0.35) agree with those predicted from the whistler mode dispersion relation.
NASA Astrophysics Data System (ADS)
Büchner, J.; Kuska, J.-P.
1997-01-01
Based on analytical calculations we have currently argued that spontaneous reconnection through thin collisionless current sheets is an essentially three-dimensional (3 D) process (Büchner, 1996 a, b). Since 3 D kinetic PIC codes have become available, the three dimensional nature of the collisionless current sheet decay are now illustrated by numerical simulations (Büchner and Kuska, 1996; Pritchett and Coroniti, 1996; Zhu and Winglee, 1996). While the latter two claim a coupling to a longer wavelength kink mode as a main factor, destabilizing thin current sheets in 3 D, our simulations have revealed that even shorter scale perturbations in the current direction suffice to destabilize thin sheets very quickly. Since past simulation runs, however, were limited to mass ratios near unity, the influence of the electrons was not treated adequately. We have now investigated the stability of thin collisionless current sheets including 64 times lighter negatively charged particles. We can now show that while the two-dimensional tearing instability slows down for M = M_p/m_e = 64, the three-dimensional current sheet decay is a much faster process - practically as fast as the mass ratio M = 1 3 D sheet decay, even without kinking the sheet. We further conclude that, unlike the two-dimensional tearing instability, the three-dimensional decay of thin current sheets is not controlled by the electrons. For a sheet width comparable with the ion inertial length, we also recovered signatures of the Hall effect as predicted by Vasyliunas (1975) in the mass ratio M = 64 case. The ion inertial length seems to be the critical scale at which the sheet starts to decay.
Aspects of Collisionless Magnetic Reconnection in Asymmetric Systems
NASA Technical Reports Server (NTRS)
Hesse, Michael; Aunai, Nicolas; Zenitani, Seiji; Kuznetsova, Masha; Birn, Joachim
2013-01-01
Asymmetric reconnection is being investigated by means of particle-in-cell simulations. The research has two foci: The direction of the reconnection line in configurations with nonvanishing magnetic fields; and the question why reconnection can be faster if a guide field is added to an otherwise unchanged asymmetric configuration. We find that reconnection prefers a direction, which maximizes the available magnetic energy, and show that this direction coincides with the bisection of the angle between the asymptotic magnetic fields. Regarding the difference in reconnection rates between planar and guide field models, we demonstrate that a guide field can provide essential confinement for particles in the reconnection region, which the weaker magnetic field in one of the inflow directions cannot necessarily provide.
Aspects of Collisionless Magnetic Reconnection in Asymmetric Systems
NASA Technical Reports Server (NTRS)
Hesse, Michael; Aunai, Nicolas; Zeitani, Seiji; Kuznetsova, Masha; Birn, Joachim
2013-01-01
Asymmetric reconnection is being investigated by means of particle-in-cell simulations. The research has two foci: the direction of the reconnection line in configurations with non-vanishing magnetic fields; and the question why reconnection can be faster if a guide field is added to an otherwise unchanged asymmetric configuration. We find that reconnection prefers a direction, which maximizes the available magnetic energy, and show that this direction coincides with the bisection of the angle between the asymptotic magnetic fields. Regarding the difference in reconnection rates between planar and guide field models, we demonstrate that a guide field can provide essential confinement for particles in the reconnection region, which the weaker magnetic field in one of the inflow directions cannot necessarily provide.
Aspects of collisionless magnetic reconnection in asymmetric systems
Hesse, Michael; Aunai, Nicolas; Kuznetsova, Masha; Zenitani, Seiji; Birn, Joachim
2013-06-15
Asymmetric reconnection is being investigated by means of particle-in-cell simulations. The research has two foci: the direction of the reconnection line in configurations with nonvanishing magnetic fields; and the question why reconnection can be faster if a guide field is added to an otherwise unchanged asymmetric configuration. We find that reconnection prefers a direction, which maximizes the available magnetic energy, and show that this direction coincides with the bisection of the angle between the asymptotic magnetic fields. Regarding the difference in reconnection rates between planar and guide field models, we demonstrate that a guide field can provide essential confinement for particles in the reconnection region, which the weaker magnetic field in one of the inflow directions cannot necessarily provide.
Olson, J; Egedal, J; Greess, S; Myers, R; Clark, M; Endrizzi, D; Flanagan, K; Milhone, J; Peterson, E; Wallace, J; Weisberg, D; Forest, C B
2016-06-24
The spontaneous formation of magnetic islands is observed in driven, antiparallel magnetic reconnection on the Terrestrial Reconnection Experiment. We here provide direct experimental evidence that the plasmoid instability is active at the electron scale inside the ion diffusion region in a low collisional regime. The experiments show the island formation occurs at a smaller system size than predicted by extended magnetohydrodynamics or fully collisionless simulations. This more effective seeding of magnetic islands emphasizes their importance to reconnection in naturally occurring 3D plasmas. PMID:27391729
Erratum: A Simple, Analytical Model of Collisionless Magnetic Reconnection in a Pair Plasma
NASA Technical Reports Server (NTRS)
Hesse, Michael; Zenitani, Seiji; Kuznetsova, Masha; Klimas, Alex
2011-01-01
The following describes a list of errata in our paper, "A simple, analytical model of collisionless magnetic reconnection in a pair plasma." It supersedes an earlier erratum. We recently discovered an error in the derivation of the outflow-to-inflow density ratio.
Zacharias, O.; Kleiber, R.; Borchardt, M.; Comisso, L.; Grasso, D.; Hatzky, R.
2014-06-15
The first detailed comparison between gyrokinetic and gyrofluid simulations of collisionless magnetic reconnection has been carried out. Both the linear and nonlinear evolution of the collisionless tearing mode have been analyzed. In the linear regime, we have found a good agreement between the two approaches over the whole spectrum of linearly unstable wave numbers, both in the drift kinetic limit and for finite ion temperature. Nonlinearly, focusing on the small-Δ′ regime, with Δ′ indicating the standard tearing stability parameter, we have compared relevant observables such as the evolution and saturation of the island width, as well as the island oscillation frequency in the saturated phase. The results are basically the same, with small discrepancies only in the value of the saturated island width for moderately high values of Δ′. Therefore, in the regimes investigated here, the gyrofluid approach can describe the collisionless reconnection process as well as the more complete gyrokinetic model.
Hewett, D.W.; Francis, G.E.; Max, C.E.
1990-06-29
Evidence from magnetospheric and solar flare research supports the belief that collisionless magnetic reconnection can proceed on the Alfven-wave crossing timescale. Reconnection behavior that occurs this rapidly in collisionless plasmas is not well understood because underlying mechanisms depend on the details of the ion and electron distributions in the vicinity of the emerging X-points. We use the direct implicit Particle-In-Cell (PIC) code AVANTI to study the details of these distributions as they evolve in the self-consistent E and B fields of magnetic reconnection. We first consider a simple neutral sheet model. We observe rapid movement of the current-carrying electrons away from the emerging X-point. Later in time an oscillation of the trapped magnetic flux is found, superimposed upon continued linear growth due to plasma inflow at the ion sound speed. The addition of a current-aligned and a normal B field widen the scope of our studies.
Full Two-Fluid Collisionless Magnetic Reconnection Simulations
NASA Astrophysics Data System (ADS)
Gomez, D. O.; Andres, N.; Dmitruk, P.
2015-12-01
Magnetic reconnection is an important energy conversion process in space environments such as the solar corona or planetary magnetospheres. At the theoretical level of resistive one-fluid MHD, the Sweet-Parker model leads to extremely low reconnection rates for virtually all space physics applications. Kinetic plasma effects introduce new spatial and temporal scales into the theoretical description, which are expected to increase the reconnection rates. Within the theoretical framework of two-fluid MHD, we retain the effects of the Hall current and electron inertia and neglect dissipative effects such as viscosity and electric resistivity. This level of description brings two new spatial scales into play, namely, the ion and electron inertial scales. In absence of resistive dissipation, reconnection can only be attained by the action of electron inertia. We performed 2.5D two-fluid simulations using a pseudo-spectral code which yields exact conservation (to round-off errors) of the ideal invariants. Our simulations show that when the effects of electron inertia are retained, magnetic reconnection takes place. In a stationary regime, the reconnection rate is simply proportional to the ion inertial length, as also emerges from a scaling law derived from dimensional arguments.
Origins of effective resistivity in collisionless magnetic reconnection
Singh, Nagendra
2014-07-15
The mechanisms that provide effective resistivity for supporting collisonless magnetic reconnection have remained unsettled despite numerous studies. Some of these studies demonstrated that the electron pressure nongyrotropy generates the resistivity (η{sub npg}) in the electron diffusion region (EDR). We derive an analytical relation for the effective resistivity (η{sub kin}) by momentum balance in a control volume in the EDR. Both η{sub npg} and η{sub kin} mutually compare well and they also compare well with the resistivity required to support reconnection electric field E{sub rec} in multi-dimensional particle-in-cell simulations as well as in satellite observations when reconnection occurs in an EDR. But they are about an order of magnitude or so smaller than that required when the reconnection occurred in a much wider reconnecting current sheet (RCS) of half width (w) of the order of the ion skin depth (d{sub i}), observed in the Earth magnetosphere. The chaos-induced resistivity reported in the literature is found to be even more deficient. We find that for reconnection in RCS with w ∼ d{sub i}, anomalous diffusion, such as the universal Bhom diffusion and/or that arising from kinetic Alfven waves, could fairly well account for the required resistivity.
Design of a Magnetic Reconnection Experiment in the Collisionless Regime
NASA Astrophysics Data System (ADS)
Egedal, J.; Le, A.; Daughton, W. S.
2012-12-01
A new model for effective heating of electrons during reconnection is now gaining support from spacecraft observations, theoretical considerations and kinetic simulations [1]. The key ingredient in the model is the physics of trapped electrons whose dynamics causes the electron pressure tensor to be strongly anisotropic [2]. The heating mechanism becomes highly efficient for geometries with low upstream electron pressure, conditions relevant to the magnetotail. We propose a new experiment that will be optimized for the study of kinetic reconnection including the dynamics of trapped electrons and associated pressure anisotropy. This requires an experiment that accesses plasmas with much lower collisionality and lower plasma beta than are available in present reconnection experiments. The new experiment will be designed such that a large variety of magnetic configurations can be established and tailored for continuation of our ongoing study of spontaneous 3D reconnection [3]. The flexible design will also allow for configurations suitable for the study of merging magnetic islands, which may be a source of super thermal electrons in naturally occurring plasmas. [1] Egedal J et al., Nature Physics, 8, 321 (2012). [2] Le A et al., Phys. Rev. Lett. 102, 085001 (2009). [3] Katz N et al., Phys. Rev. Lett. 104, 255004 (2010).;
Yoo, Jongsoo; Yamada, Masaaki; Ji, Hantao; Myers, Clayton E.
2012-12-10
The ion dynamics in a collisionless magnetic reconnection layer are studied in a laboratory plasma. The measured in-plane plasma potential profile, which is established by electrons accelerated around the electron diffusion region, shows a saddle-shaped structure that is wider and deeper towards the outflow direction. This potential structure ballistically accelerates ions near the separatrices toward the outflow direction. Ions are heated as they travel into the high pressure downstream region.
Porkolab, Miklos; Egedal, Jan
2007-11-30
The Grant DE-FG-02-00ER54712, ?Experimental Studies of Collisionless Reconnection Processes in Plasmas?, financed within the DoE/NSF, spanned a period from September , 2003 to August, 2007. It partly supported an MIT Research scientist, two graduate students and material expenses. The grant enabled the operation of a basic plasma physics experiment (on magnetic reconnection) at the MIT Plasma Science and Fusion Center and the MIT Physics Department. A strong educational component characterized this work throughout, with the participation of a large number of graduate and undergraduate students and interns to the experimental activities. The study of the collisionless magnetic reconnection constituted the primary work carried out under this grant. The investigations utilized two magnetic configurations with distinct boundary conditions. Both configurations were based upon the Versatile Toroidal Facility (VTF). The first configuration is characterized by open boundary conditions where the magnetic field lines interface directly with the vacuum vessel walls. The reconnection dynamics for this configuration has been methodically characterized and it has been shown that kinetic effects related to trapped electron trajectories are responsible for the high rates of reconnection observed [7]. This type of reconnection has not been investigated before. Nevertheless, the results are directly relevant to observations by the Wind spacecraft of fast reconnection deep in the Earth magnetotail [9]. The second configuration was developed to be specifically relevant to numerical simulations of magnetic reconnection, allowing the magnetic field-lines to be contained inside the device. The configuration is compatible with the presence of large current sheets in the reconnection region and reconnection is observed in fast powerful bursts. These reconnection events facilitate the first experimental investigations of the physics governing the spontaneous onset of fast reconnection [12
Influence of the Hall effect and electron inertia in collisionless magnetic reconnection
NASA Astrophysics Data System (ADS)
Andrés, Nahuel; Dmitruk, Pablo; Gómez, Daniel
2016-02-01
We study the role of the Hall current and electron inertia in collisionless magnetic reconnection within the framework of full two-fluid MHD. At spatial scales smaller than the electron inertial length, a topological change of magnetic field lines exclusively due to the electron inertia becomes possible. Assuming stationary conditions, we derive a theoretical scaling for the reconnection rate, which is simply proportional to the Hall parameter. Using a pseudo-spectral code with no dissipative effects, our numerical results confirm this theoretical scaling. In particular, for a sequence of different Hall parameter values, our numerical results show that the width of the current sheet is independent of the Hall parameter, while its thickness is of the order of the electron inertial range, thus confirming that the stationary reconnection rate is proportional to the Hall parameter.
NASA Astrophysics Data System (ADS)
Jain, Neeraj; Büchner, Jörg; Munoz Sepulveda, Patricio Alejandro
2016-07-01
In collisionless magnetic reconnection, dissipation region, where frozen-in condition of magnetic field breaks down, develops two scale structure, viz., electron current sheets embedded inside ion current sheets. Instabilities of these current sheets lead to the development of electromagnetic turbulence which can cause anomalous dissipation enhancing the reconnection rate. Laboratory experiments, e.g., Magnetic Reconnection Experiment and VINETA-II have measured fluctuations in electron current sheets in the lower hybrid frequency range. We present simulations of the electromagnetic turbulence generated by current sheet instabilities. The characteristic features of the electromagnetic turbulence, which can be used to identify the unstable modes responsible for the turbulence, will be studied. The results will be compared with the laboratory experiments.
Dayton, William S; Roytershteyn, Vadim; Gary, Peter; Yin, L; Albright, B J; Bowers, K J; Karimabadi, H
2009-01-01
The evolution of magnetic reconnection in large-scale systems often gives rise to extended current layers that are unstable to the formation of secondary magnetic islands. The role of these islands in the reconnection process and the conditions under which they form remains a subject of debate. In this work, we benchmark two different kinetic particle-in-cell codes to address the formation of secondary islands for several types of global boundary conditions. The influence on reconnection is examined for a range of conditions and collisionality limits. Although secondary islands are observed in all cases, their influence on reconnection may be different depending on the regime. In the collisional limit, the secondary islands playa key role in breaking away from the Sweet-Parker scaling and enabling faster reconnection. In the collisionless limit, their formation is one mechanism for controlling the length of the diffusion region. In both limits, the onset of secondary islands leads to a time dependent behavior in the reconnection rate. In all cases considered, the number of secondary islands increases for larger systems.
Fast Reconnection of Weak Magnetic Fields
NASA Technical Reports Server (NTRS)
Zweibel, Ellen G.
1998-01-01
Fast magnetic reconnection refers to annihilation or topological rearrangement of magnetic fields on a timescale that is independent (or nearly independent) of the plasma resistivity. The resistivity of astrophysical plasmas is so low that reconnection is of little practical interest unless it is fast. Yet, the theory of fast magnetic reconnection is on uncertain ground, as models must avoid the tendency of magnetic fields to pile up at the reconnection layer, slowing down the flow. In this paper it is shown that these problems can be avoided to some extent if the flow is three dimensional. On the other hand, it is shown that in the limited but important case of incompressible stagnation point flows, every flow will amplify most magnetic fields. Although examples of fast magnetic reconnection abound, a weak, disordered magnetic field embedded in stagnation point flow will in general be amplified, and should eventually modify the flow. These results support recent arguments against the operation of turbulent resistivity in highly conducting fluids.
Graf von der Pahlen, J.; Tsiklauri, D.
2014-01-15
Works of Tsiklauri and Haruki [Phys. Plasmas 15, 102902 (2008); 14, 112905 (2007)] are extended by inclusion of the out-of-plane magnetic (guide) field. In particular, magnetic reconnection during collisionless, stressed X-point collapse for varying out-of-plane guide-fields is studied using a kinetic, 2.5D, fully electromagnetic, relativistic particle-in-cell numerical code. For zero guide-field, cases for both open and closed boundary conditions are investigated, where magnetic flux and particles are lost and conserved, respectively. It is found that reconnection rates, out-of-plane currents and density in the X-point increase more rapidly and peak sooner in the closed boundary case, but higher values are reached in the open boundary case. The normalized reconnection rate is fast: 0.10-0.25. In the open boundary case it is shown that an increase of guide-field yields later onsets in the reconnection peak rates, while in the closed boundary case initial peak rates occur sooner but are suppressed. The reconnection current changes similarly with increasing guide-field; however for low guide-fields the reconnection current increases, giving an optimal value for the guide-field between 0.1 and 0.2 times the in-plane field in both cases. Also, in the open boundary case, it is found that for guide-fields of the order of the in-plane magnetic field, the generation of electron vortices occurs. Possible causes of the vortex generation, based on the flow of decoupled particles in the diffusion region and localized plasma heating, are discussed. Before peak reconnection onset, oscillations in the out-of-plane electric field at the X-point are found, ranging in frequency from approximately 1 to 2 ω{sub pe} and coinciding with oscillatory reconnection. These oscillations are found to be part of a larger wave pattern in the simulation domain. Mapping the out-of-plane electric field along the central lines of the domain over time and applying a 2D Fourier transform reveal that
HYBRID AND HALL-MHD SIMULATIONS OF COLLISIONLESS RECONNECTION: EFFECTS OF PLASMA PRESSURE TENSOR
L. YIN; D. WINSKE; ET AL
2001-05-01
In this study we performed two-dimensional hybrid (particle ions, massless fluid electrons) and Hall-MHD simulations of collisionless reconnection in a thin current sheet. Both calculations include the full electron pressure tensor (instead of a localized resistivity) in the generalized Ohm's law to initiate reconnection, and in both an initial perturbation to the Harris equilibrium is applied. First, electron dynamics from the two calculations are compared, and we find overall agreement between the two calculations in both the reconnection rate and the global configuration. To address the issue of how kinetic treatment for the ions affects the reconnection dynamics, we compared the fluid-ion dynamics from the Hall-MHD calculation to the particle-ion dynamics obtained from the hybrid simulation. The comparison demonstrates that off-diagonal elements of the ion pressure tensor are important in correctly modeling the ion out-of-plane momentum transport from the X point. It is that these effects can be modeled efficiently using a particle Hall-MHD simulation method in which particle ions used in a predictor/corrector to implement the ion gyro-radius corrections. We also investigate the micro- macro-scale coupling in the magnetotail dynamics by using a new integrated approach in which particle Hall-MHD calculations are embedded inside a MHD simulation. Initial results of the simulation concerning current sheet thinning and reconnection dynamics are discussed.
NASA Astrophysics Data System (ADS)
Werner, G. R.; Uzdensky, D. A.; Cerutti, B.; Nalewajko, K.; Begelman, M. C.
2016-01-01
Using two-dimensional particle-in-cell simulations, we characterize the energy spectra of particles accelerated by relativistic magnetic reconnection (without guide field) in collisionless electron-positron plasmas, for a wide range of upstream magnetizations σ and system sizes L. The particle spectra are well-represented by a power law {γ }-α , with a combination of exponential and super-exponential high-energy cutoffs, proportional to σ and L, respectively. For large L and σ, the power-law index α approaches about 1.2.
Werner, G. R.; Uzdensky, D. A.; Cerutti, B.; Nalewajko, K.; Begelman, M. C.
2015-12-30
Using two-dimensional particle-in-cell simulations, we characterize the energy spectra of particles accelerated by relativistic magnetic reconnection (without guide field) in collisionless electron–positron plasmas, for a wide range of upstream magnetizations σ and system sizes L. The particle spectra are well-represented by a power lawmore » $${\\gamma }^{-\\alpha }$$, with a combination of exponential and super-exponential high-energy cutoffs, proportional to σ and L, respectively. As a result, for large L and σ, the power-law index α approaches about 1.2.« less
Werner, G. R.; Uzdensky, D. A.; Cerutti, B.; Nalewajko, K.; Begelman, M. C.
2015-12-30
Using two-dimensional particle-in-cell simulations, we characterize the energy spectra of particles accelerated by relativistic magnetic reconnection (without guide field) in collisionless electron–positron plasmas, for a wide range of upstream magnetizations σ and system sizes L. The particle spectra are well-represented by a power law ${\\gamma }^{-\\alpha }$, with a combination of exponential and super-exponential high-energy cutoffs, proportional to σ and L, respectively. As a result, for large L and σ, the power-law index α approaches about 1.2.
Secondary instabilities and vortex formation in collisionless-fluid magnetic reconnection.
Del Sarto, D; Califano, F; Pegoraro, F
2003-12-01
It is shown that the pattern of current layers formed within a magnetic island in the nonlinear phase of magnetic field line reconnection in a collisionless two-dimensional fluid plasma is subject to the onset of a secondary instability, the effect of which increases with decreasing electron temperature. In the cold electron limit the saturation of the island growth is accompanied by a turbulent redistribution of the current layers and by the development of long lived fluid vortices while, in the opposite limit, the current layer structure remains regular. PMID:14683188
Energy budgets in collisionless magnetic reconnection: Ion heating and bulk acceleration
Aunai, N.; Belmont, G.; Smets, R.
2011-12-15
This paper investigates the energy transfer in the process of collisionless antiparallel magnetic reconnection. Using two-dimensional hybrid simulations, we measure the increase of the bulk and thermal kinetic energies and compare it to the loss of magnetic energy through a contour surrounding the ion decoupling region. It is shown, for both symmetric and asymmetric configurations, that the loss of magnetic energy is not equally partitioned between heating and acceleration. The heating is found to be dominant and the partition ratio depends on the asymptotic parameters, and future investigations will be needed to understand this dependence.
Kinetic turbulence in 3D collisionless magnetic reconnection with a guide magnetic field
NASA Astrophysics Data System (ADS)
Alejandro Munoz Sepulveda, Patricio; Kilian, Patrick; Jain, Neeraj; Büchner, Jörg
2016-04-01
The features of kinetic plasma turbulence developed during non-relativistic 3D collisionless magnetic reconnection are still not fully understood. This is specially true under the influence of a strong magnetic guide field, a scenario common in space plasmas such as in the solar corona and also in laboratory experiments such as MRX or VINETA II. Therefore, we study the mechanisms and micro-instabilities leading to the development of turbulence during 3D magnetic reconnection with a fully kinetic PIC code, emphasizing the role of the guide field with an initial setup suitable for the aforementioned environments. We also clarify the relations between these processes and the generation of non-thermal populations and particle acceleration.
Zenitani, Seiji; Hesse, Michael; Klimas, Alex; Black, Carrie; Kuznetsova, Masha
2011-12-15
It was recently proposed that the electron-frame dissipation measure, the energy transfer from the electromagnetic field to plasmas in the electron's rest frame, identifies the dissipation region of collisionless magnetic reconnection [Zenitani et al., Phys. Rev. Lett. 106, 195003 (2011)]. The measure is further applied to the electron-scale structures of antiparallel reconnection, by using two-dimensional particle-in-cell simulations. The size of the central dissipation region is controlled by the electron-ion mass ratio, suggesting that electron physics is essential. A narrow electron jet extends along the outflow direction until it reaches an electron shock. The jet region appears to be anti-dissipative. At the shock, electron heating is relevant to a magnetic cavity signature. The results are summarized to a unified picture of the single dissipation region in a Hall magnetic geometry.
Del Sarto, D.; Califano, F.; Pegoraro, F.
2005-01-01
The nonlinear phase of a magnetic field line reconnection instability in a collisionless two-dimensional cold plasma is investigated in the Hall dominated regime, described by the electron-magnetohydrodynamic equations, which corresponds to the frequency range of whistler waves. It is found that the regular pattern of current density layers that forms in the initial nonlinear phase of the reconnection instability is destroyed by the onset of a Kelvin-Helmholtz-type instability and the formation of current jets that develop into vortex rings. These processes can be interpreted in terms of a Hasegawa-Mima-type regime inside the magnetic island and lead to the creation of magnetic vortices. It is shown that electron compressibility, which is related to charge separation, tends to stabilize these processes.
In-plane electric fields in magnetic islands during collisionless magnetic reconnection
Chen Lijen; Bhattacharjee, Amitava; Torbert, Roy B.; Bessho, Naoki; Daughton, William; Roytershteyn, Vadim
2012-11-15
Magnetic islands are a common feature in both the onset and nonlinear evolution of magnetic reconnection. In collisionless regimes, the onset typically occurs within ion-scale current layers leading to the formation of magnetic islands when multiple X lines are involved. The nonlinear evolution of reconnection often gives rise to extended electron current layers (ECL) which are also unstable to formation of magnetic islands. Here, we show that the excess negative charge and strong out-of-plane electron velocity in the ECL are passed on to the islands generated therein, and that the corresponding observable distinguishing the islands generated in the ECL is the strongly enhanced in-plane electric fields near the island core. The islands formed in ion-scale current layers do not have these properties of the ECL-generated islands. The above result provides a way to assess the occurrence and importance of extended ECLs that are unstable to island formation in space and laboratory plasmas.
Evidence of Magnetic Field Switch-off in Collisionless Magnetic Reconnection
NASA Astrophysics Data System (ADS)
Innocenti, M. E.; Goldman, M.; Newman, D.; Markidis, S.; Lapenta, G.
2015-09-01
The long-term evolution of large domain particle-in-cell simulations of collisionless magnetic reconnection is investigated following observations that show two possible outcomes for collisionless reconnection: toward a Petschek-like configuration or toward multiple X points. In the present simulation, a mixed scenario develops. At earlier time, plasmoids are emitted, disrupting the formation of Petschek-like structures. Later, an almost stationary monster plasmoid forms, preventing the emission of other plasmoids. A situation reminiscent of Petschek’s switch-off then ensues. Switch-off is obtained through a slow shock/rotational discontinuity compound structure. Two external slow shocks (SS) located at the separatrices reduce the in-plane tangential component of the magnetic field, but not to zero. Two transitions reminiscent of rotational discontinuities (RD) in the internal part of the exhaust then perform the final switch-off. Both the SS and the RD are characterized through analysis of their Rankine–Hugoniot jump conditions. A moderate guide field is used to suppress the development of the firehose instability in the exhaust.
Fast magnetic reconnection with large guide fields
Stanier, A.; Simakov, Andrei N.; Chacón, L.; Daughton, W.
2015-01-09
Here, we demonstrate using two-fluid simulations that low-βmagnetic reconnection remains fast, regardless of the presence of fast dispersive waves, which have been previously suggested to play a critical role. In order to understand these results, a discrete model is constructed that offers scaling relationships for the reconnection rate and dissipation region (DR) thickness in terms of the upstream magnetic field and DR length. We verify these scalings numerically and show how the DR self-adjusts to process magnetic flux at the same rate that it is supplied to a larger region where two-fluid effects become important. Ultimately, the rate is independent of the DR physics and is in good agreement with kinetic results.
Fast magnetic reconnection with large guide fields
Stanier, A.; Simakov, Andrei N.; Chacón, L.; Daughton, W.
2015-01-09
Here, we demonstrate using two-fluid simulations that low-βmagnetic reconnection remains fast, regardless of the presence of fast dispersive waves, which have been previously suggested to play a critical role. In order to understand these results, a discrete model is constructed that offers scaling relationships for the reconnection rate and dissipation region (DR) thickness in terms of the upstream magnetic field and DR length. We verify these scalings numerically and show how the DR self-adjusts to process magnetic flux at the same rate that it is supplied to a larger region where two-fluid effects become important. Ultimately, the rate is independentmore » of the DR physics and is in good agreement with kinetic results.« less
Particle-in-Cell Simulation of Collisionless Driven Reconnection with Open Boundaries
NASA Technical Reports Server (NTRS)
Kimas, Alex; Hesse, Michael; Zenitani, Seiji; Kuznetsova, Maria
2010-01-01
First results are discussed from an ongoing study of driven collisionless reconnection using a 2 1/2-dimensional electromagnetic particle-in-cell simulation model with open inflow and outflow boundaries. An extended electron diffusion region (EEDR) is defined as that region surrounding a reconnecting neutral line in which the out-of-plane nonideal electric field is positive. It is shown that the boundaries of this region in the directions of the outflow jets are at the positions where the electrons make the transition from unfrozen meandering motion in the current sheet to outward drifting with the magnetic field in the outflow jets; a turning length scale is defined to mark these positions, The initial width of the EEDR in the inflow directions is comparable to the electron bounce width. Later. as shoulders develop to form a two-scale structure. thc EEDR width expands to the ion bounce width scale. The inner portion of the EEDR or the electron diffusion region proper remains at the electron bounce width. Two methods are introduced for predicting the reconnection electric field using the dimensions of the EEDR. These results are interpreted as further evidence that the EEDR is the region that is relevant to understanding the electron role in the neutral line vicinity.
Jain, Neeraj; Büchner, Jörg
2014-06-15
In collisionless magnetic reconnection, electron current sheets (ECS) with thickness of the order of an electron inertial length form embedded inside ion current sheets with thickness of the order of an ion inertial length. These ECS's are susceptible to a variety of instabilities which have the potential to affect the reconnection rate and/or the structure of reconnection. We carry out a three dimensional linear eigen mode stability analysis of electron shear flow driven instabilities of an electron scale current sheet using an electron-magnetohydrodynamic plasma model. The linear growth rate of the fastest unstable mode was found to drop with the thickness of the ECS. We show how the nature of the instability depends on the thickness of the ECS. As long as the half-thickness of the ECS is close to the electron inertial length, the fastest instability is that of a translational symmetric two-dimensional (no variations along flow direction) tearing mode. For an ECS half thickness sufficiently larger or smaller than the electron inertial length, the fastest mode is not a tearing mode any more and may have finite variations along the flow direction. Therefore, the generation of plasmoids in a nonlinear evolution of ECS is likely only when the half-thickness is close to an electron inertial length.
Micro-instabilities and anomalous transport effects in collisionless guide field reconnection
NASA Astrophysics Data System (ADS)
Munoz Sepulveda, Patricio Alejandro; Büchner, Jörg; Kilian, Patrick
2016-07-01
It is often the case that magnetic reconnection takes place in collisionless plasmas with a current aligned guide magnetic field, such as in the Solar corona. The general characteristics of this process have been exhaustively analyzed with theory and numerical simulations, under different approximations, since some time ago. However, some consequences and properties of the secondary instabilities arising spontaneously -other than tearing instability-, and their dependence on the guide field strength, have not been completely understood yet. For this sake, we use the results of fully kinetic 2D PIC numerical simulations of guide field reconnection. By using a mean field approach for the Generalized Ohm's law that explains the balance of the reconnected electric field, we find that some of the cross-streaming and gradient driven instabilities -in the guide field case- produce an additional anomalous transport term. The latter can be interpreted as a result of the enhanced correlated electromagnetic fluctuations, leading to a slow down of the current carriers and kinetic scale turbulence. We characterize these processes on dependence on the guide field strength, and explore the causal relation with the source of free energy driving the mentioned instabilities. Finally, we show the main consequences that a fully 3D approach have on all those phenomena in contrast to the reduced 2D description.
Physical processes of driven magnetic reconnection in collisionless plasmas: Zero guide field case
NASA Astrophysics Data System (ADS)
Cheng, C. Z.; Inoue, S.; Ono, Y.; Horiuchi, R.
2015-10-01
The key physical processes of the electron and ion dynamics, the structure of the electric and magnetic fields, and how particles gain energy in the driven magnetic reconnection in collisionless plasmas for the zero guide field case are presented. The key kinetic physics is the decoupling of electron and ion dynamics around the magnetic reconnection region, where the magnetic field is reversed and the electron and ion orbits are meandering, and around the separatrix region, where electrons move mainly along the field line and ions move mainly across the field line. The decoupling of the electron and ion dynamics causes charge separation to produce a pair of in-plane bipolar converging electrostatic electric field ( E→ e s ) pointing toward the neutral sheet in the magnetic field reversal region and the monopolar E→ e s around the separatrix region. A pair of electron jets emanating from the reconnection current layer generate the quadrupole out-of-plane magnetic field, which causes the parallel electric field ( E→ || ) from E→ i n d to accelerate particles along the magnetic field. We explain the electron and ion dynamics and their velocity distributions and flow structures during the time-dependent driven reconnection as they move from the upstream to the downstream. In particular, we address the following key physics issues: (1) the decoupling of electron and ion dynamics due to meandering orbits around the field reversal region and the generation of a pair of converging bipolar electrostatic electric field ( E→ e s ) around the reconnection region; (2) the slowdown of electron and ion inflow velocities due to acceleration/deceleration of electrons and ions by E→ e s as they move across the neutral sheet; (3) how the reconnection current layer is enhanced and how the orbit meandering particles are accelerated inside the reconnection region by E→ i n d ; (4) why the electron outflow velocity from the reconnection region reaches super-Alfvenic speed
NASA Astrophysics Data System (ADS)
Yun, Gunsu; Ji, Jeong-Young; Thatipamula, Shekar; Kstar Team
2015-11-01
Viscous dissipation rate of magnetic field energy due to wave-like fluctuations in collisionless magnetized plasma is obtained analytically using the exact integral closure for electron fluid viscosity [Ji, Phys. Plasmas 21 (2014)]. For typical high-temperature tokamak plasma, the viscous resistivity is several orders larger than the Spitzer (collisional) resistivity. For magnetic reconnection, it is also found that the radiative transport (i.e. Poynting flux) of the field energy of Alfven waves [Bellan, Phys. Plasmas 5, 3081 (1998)] is comparable to the viscous dissipation. The viscous dissipation is more effective for shorter wavelength fluctuation. The importance of viscous dissipation is supported by broadband emission and chirping-down phenomena observed in the ion cyclotron harmonic frequency range at the crash onset of edge-localized mode on the KSTAR tokamak. Work supported by the National Research Foundation of Korea and the Asia-Pacific Center for Theoretical Physics.
NASA Astrophysics Data System (ADS)
Higashimori, K.; Hoshino, M.
2012-01-01
We perform a two-dimensional simulation by using an electromagnetic hybrid code to study the formation of slow-mode shocks in collisionless magnetic reconnection in low beta plasmas, and we focus on the relation between the formation of slow shocks and the ion temperature anisotropy enhanced at the shock downstream region. It is known that as magnetic reconnection develops, the parallel temperature along the magnetic field becomes large in association with the anisotropic plasma sheet boundary layer ion beams, and this temperature anisotropy has a tendency to suppress the formation of slow shocks. On the basis of our simulation result, we found that the slow shock formation is suppressed due to the large temperature anisotropy near the X-type region, but the ion temperature anisotropy relaxes with increasing the distance from the magnetic neutral point. As a result, two pairs of current structures, which are the strong evidence of dissipation of magnetic field in slow shocks, are formed at the distance ∣x∣ ≥ 115 λi from the neutral point.
Lu, San; Lu, Quanming; Huang, Can; Wang, Shui
2013-06-15
By performing two-dimensional particle-in-cell simulations, we investigate the transfer between electron bulk kinetic and electron thermal energy in collisionless magnetic reconnection. In the vicinity of the X line, the electron bulk kinetic energy density is much larger than the electron thermal energy density. The evolution of the electron bulk kinetic energy is mainly determined by the work done by the electric field force and electron pressure gradient force. The work done by the electron gradient pressure force in the vicinity of the X line is changed to the electron enthalpy flux. In the magnetic island, the electron enthalpy flux is transferred to the electron thermal energy due to the compressibility of the plasma in the magnetic island. The compression of the plasma in the magnetic island is the consequence of the electromagnetic force acting on the plasma as the magnetic field lines release their tension after being reconnected. Therefore, we can observe that in the magnetic island the electron thermal energy density is much larger than the electron bulk kinetic energy density.
Plasma waves around separatrix in collisionless magnetic reconnection with weak guide field
NASA Astrophysics Data System (ADS)
Chen, Yangao; Fujimoto, Keizo; Xiao, Chijie; Ji, Hantao
2015-11-01
Electrostatic and electromagnetic waves excited by electron beam around the separatrix region are analyzed in detail during the collisionless magnetic reconnection with a weak guide field by using 2D particle-in-cell simulation with the adaptive mesh refinement. Broadband electrostatic waves are excited both in the inflow and outflow regions around the separatrices due to the electron bump-on-tail, two-stream, and Buneman instabilities. In contrast, the quasi-monochromatic electromagnetic waves are excited only in the inflow side of the separatrices due to a beam-driven Whistler instability. The localization of the Whistler waves is attributed to the non-uniformity of the out-of-plane magnetic field By. The Whistler instability is suppressed in the outflow side where By is too small for the oblique propagation. The electrostatic waves with distinct speeds can explain the in situ spacecraft observations. From the causality point of view, the waves are generated as the consequence of the electron bulk acceleration to thermalize the particles through wave-particle interactions. These simulation results provide guidance to analyze high-resolution wave observations during reconnection in the ongoing and upcoming satellite missions, as well as in dedicated laboratory experiments.
Plasma waves around separatrix in collisionless magnetic reconnection with weak guide field
NASA Astrophysics Data System (ADS)
Chen, Yangao; Fujimoto, Keizo; Xiao, Chijie; Ji, Hantao
2015-08-01
Electrostatic and electromagnetic waves excited by electron beam around the separatrix region are analyzed in detail during the collisionless magnetic reconnection with a weak guide field by using 2-D particle-in-cell simulation with the adaptive mesh refinement. Broadband electrostatic waves are excited both in the inflow and outflow regions around the separatrices due to the electron bump-on-tail, two-stream, and Buneman instabilities. In contrast, the quasi-monochromatic electromagnetic waves are excited only in the inflow side of the separatrices due to a beam-driven whistler instability. The localization of the whistler waves is attributed to the nonuniformity of the out-of-plane magnetic field By. The whistler instability is suppressed in the outflow side where By is too small for the oblique propagation. The electrostatic waves with distinct speeds can explain the in situ spacecraft observations. From the causality point of view, the waves are generated as the consequence of the electron bulk acceleration to thermalize the particles through wave-particle interactions. These simulation results provide guidance to analyze high-resolution wave observations during reconnection in the ongoing and upcoming satellite missions, as well as in dedicated laboratory experiments.
NASA Astrophysics Data System (ADS)
Hirota, Makoto; Hattori, Yuji
2014-10-01
Explosive behavior of collisionless magnetic reconnection is investigated by analyzing a two-fluid model that includes the effects of the electron inertia and the electron temperature (or compressibility). By micrifying both the electron skin depth de and the ion-sound gyroradius ρs such that ρs =de < 0 . 01 L (where L is the system size), a direct numerical simulation is performed to enlarge strongly nonlinear regime of a collisionless tearing instability. The nonlinear evolution is shown to be explosive when the inverse of the tearing index 1 /Δ' is smaller than ρs =de , whereas the maximum reconnection speed at the fully reconnected state does not significantly depend on the size of ρs =de . The singular current-vortex sheets are generated in the form of the X shape. In the explosive phase, the expansion of this X shape as well as the magnetic island occurs locally near the reconnection point. By taking an approach similar to the asymptotic matching, the dynamics of the current-vortex sheets is analyzed and the explosive reconnection speed is estimated theoretically. This work is supported by JSPS Grant-in-Aid for Young Scientists(B) (No. 25800308).
Fast Magnetic Reconnection in the Plasmoid-Dominated Regime
Uzdensky, D. A.; Loureiro, N. F.; Schekochihin, A. A.
2010-12-03
A conceptual model of resistive magnetic reconnection via a stochastic plasmoid chain is proposed. The global reconnection rate is shown to be independent of the Lundquist number. The distribution of fluxes in the plasmoids is shown to be an inverse-square law. It is argued that there is a finite probability of emergence of abnormally large plasmoids, which can disrupt the chain (and may be responsible for observable large abrupt events in solar flares and sawtooth crashes). A criterion for the transition from the resistive magnetohydrodynamic to the collisionless regime is provided.
Fast magnetic reconnection in the plasmoid-dominated regime.
Uzdensky, D A; Loureiro, N F; Schekochihin, A A
2010-12-01
A conceptual model of resistive magnetic reconnection via a stochastic plasmoid chain is proposed. The global reconnection rate is shown to be independent of the Lundquist number. The distribution of fluxes in the plasmoids is shown to be an inverse-square law. It is argued that there is a finite probability of emergence of abnormally large plasmoids, which can disrupt the chain (and may be responsible for observable large abrupt events in solar flares and sawtooth crashes). A criterion for the transition from the resistive magnetohydrodynamic to the collisionless regime is provided. PMID:21231473
Graf von der Pahlen, J.; Tsiklauri, D.
2014-06-15
The out-of-plane magnetic field, generated by fast magnetic reconnection, during collisionless, stressed X-point collapse, was studied with a kinetic, 2.5D, fully electromagnetic, relativistic particle-in-cell numerical code, using both closed (flux conserving) and open boundary conditions on a square grid. It was discovered that the well known quadrupolar structure in the out-of-plane magnetic field gains four additional regions of opposite magnetic polarity, emerging near the corners of the simulation box, moving towards the X-point. The emerging, outer, magnetic field structure has opposite polarity to the inner quadrupolar structure, leading to an overall octupolar structure. Using Ampere's law and integrating electron and ion currents, defined at grid cells, over the simulation domain, contributions to the out-of-plane magnetic field from electron and ion currents were determined. The emerging regions of opposite magnetic polarity were shown to be the result of ion currents. Magnetic octupolar structure is found to be a signature of X-point collapse, rather than tearing mode, and factors relating to potential discoveries in experimental scenarios or space-craft observations are discussed.
Physical conditions for fast reconnection evolution in space plasmas
Ugai, M.
2012-07-15
The present paper studies physical conditions for fast reconnection mechanism involving slow shocks to evolve spontaneously in space (high-temperature) plasmas. This is fundamental for onset mechanisms of geomagnetic substorms and solar flares. It is demonstrated that reconnection evolution strongly depends on effective resistivity available in space plasmas as well as on dimensions of initial current sheet. If a current sheet is sufficiently thin, fast reconnection spontaneously evolves only when resistivity is locally enhanced around X reconnection point. This is because in space plasmas reconnection flows cause vital current concentration locally around X point. For current-driven anomalous resistivity, the resulting resistivity is automatically localized around X point, so fast reconnection mechanism can be realized. On the other hand, for uniform or Spitzer resistivity, any fast reconnection cannot grow; in particular, Spitzer resistivity is reduced around X point because of Joule heating. Regarding reconnection simulations (either fluid or particle), unless numerical resistivities are made negligibly small, they seriously mask the effects of physical resistivity, leading to a misleading conclusion that reconnection evolution is little influenced by plasma resistivity.
NASA Astrophysics Data System (ADS)
Egedal, Jan; Le, Ari; Daughton, William
2013-06-01
From spacecraft data, it is evident that electron pressure anisotropy develops in collisionless plasmas. This is in contrast to the results of theoretical investigations, which suggest this anisotropy should be limited. Common for such theoretical studies is that the effects of electron trapping are not included; simply speaking, electron trapping is a non-linear effect and is, therefore, eliminated when utilizing the standard methods for linearizing the underlying kinetic equations. Here, we review our recent work on the anisotropy that develops when retaining the effects of electron trapping. A general analytic model is derived for the electron guiding center distribution f¯(v∥,v⊥) of an expanding flux tube. The model is consistent with anisotropic distributions observed by spacecraft, and is applied as a fluid closure yielding anisotropic equations of state for the parallel and perpendicular components (relative to the local magnetic field direction) of the electron pressure. In the context of reconnection, the new closure accounts for the strong pressure anisotropy that develops in the reconnection regions. It is shown that for generic reconnection in a collisionless plasma nearly all thermal electrons are trapped, and dominate the properties of the electron fluid. A new numerical code is developed implementing the anisotropic closure within the standard two-fluid framework. The code accurately reproduces the detailed structure of the reconnection region observed in fully kinetic simulations. These results emphasize the important role of pressure anisotropy for the reconnection process. In particular, for reconnection geometries characterized by small values of the normalized upstream electron pressure, βe∞, the pressure anisotropy becomes large with p∥≫p⊥ and strong parallel electric fields develop in conjunction with this anisotropy. The parallel electric fields can be sustained over large spatial scales and, therefore, become important for
Collisionless Magnetic Reconnection and Dynamo Processes in a Spatially Rotating Magnetic Field
NASA Astrophysics Data System (ADS)
Choe, Gwangson; Lee, Junggi
2016-04-01
Spatially rotating magnetic fields have been observed in the solar wind and in the Earth's magnetopause as well as in reversed field pinch (RFP) devices. Such field configurations have a similarity with extended current layers having a spatially varying plasma pressure instead of the spatially varying guide field. It is thus expected that magnetic reconnection may take place in a rotating magnetic field no less than in an extended current layer. We have investigated the spontaneous evolution of a collisionless plasma system embedding a rotating magnetic field with a two-and-a-half-dimensional electromagnetic particle-in-cell (PIC) simulation. It is found that a magnetic-flux-reducing diffusion phase and a magnetic-flux-increasing dynamo phase are alternating with a certain period. The temperature of the system also varies with the same period, showing a similarity to sawtooth oscillations in tokamaks. We have shown that a modified theory of sawtooth oscillations can explain the periodic behavior observed in the simulation. A strong guide field distorts the current layer as was observed in laboratory experiments. This distortion is smoothed out as magnetic islands fade away by the O-line diffusion, but is soon strengthened by the growth of magnetic islands. These processes are all repeating with a fixed period. Our results suggest that a rotating magnetic field configuration continuously undergoes deformation and relaxation in a short time-scale although it might look rather steady in a long-term view.
Particle description of the electron diffusion region in collisionless magnetic reconnection
Fujimoto, Keizo; Sydora, Richard D.
2009-11-15
The present study clarifies the dissipation mechanism of collisionless magnetic reconnection in two-dimensional system based on particle dynamics. The electrons are accelerated without thermalization in the electron diffusion region, carry out the meandering oscillation, and are ejected away from the X-line. This electron behavior not only generates the electron inertia resistivity based on the particle description, but also it can be the origin of the electron viscosity resulting in the off-diagonal pressure tensor term in the generalized Ohm's law near the X-line. We derive an analytical profile for the electron pressure tensor term and confirm that the profile is consistent with the particle-in-cell simulation. The present results demonstrate that the magnetic dissipation due to the electron viscosity in the fluid picture is equivalent to that due to the inertia resistivity in the particle description. It is also suggested that the width of the electron current sheet is on the order of the electron inertia length in the case without electron scattering and thermalization, while it is expected that the width is broadened if the electron scattering occurs in the current sheet.
NASA Astrophysics Data System (ADS)
Del Sarto, Daniele; Pucci, Fulvia; Tenerani, Anna; Velli, Marco
2016-03-01
This paper discusses the transition to fast growth of the tearing instability in thin current sheets in the collisionless limit where electron inertia drives the reconnection process. It has been previously suggested that in resistive MHD there is a natural maximum aspect ratio (ratio of sheet length and breadth to thickness) which may be reached for current sheets with a macroscopic length L, the limit being provided by the fact that the tearing mode growth time becomes of the same order as the Alfvén time calculated on the macroscopic scale. For current sheets with a smaller aspect ratio than critical the normalized growth rate tends to zero with increasing Lundquist number S, while for current sheets with an aspect ratio greater than critical the growth rate diverges with S. Here we carry out a similar analysis but with electron inertia as the term violating magnetic flux conservation: previously found scalings of critical current sheet aspect ratios with the Lundquist number are generalized to include the dependence on the ratio de2/L2, where de is the electron skin depth, and it is shown that there are limiting scalings which, as in the resistive case, result in reconnecting modes growing on ideal time scales. Finite Larmor radius effects are then included, and the rescaling argument at the basis of "ideal" reconnection is proposed to explain secondary fast reconnection regimes naturally appearing in numerical simulations of current sheet evolution.
Fast Magnetic Reconnection: Bridging Laboratory and Space Plasma Physics
Bhattacharjee, Amitava
2012-02-16
Recent developments in experimental and theoretical studies of magnetic reconnection hold promise for providing solutions to outstanding problems in laboratory and space plasma physics. Examples include sawtooth crashes in tokamaks, substorms in the Earth’s Magnetosphere, eruptive solar flares, and more recently, fast reconnection in laser-produced high energy density plasmas. In each of these examples, a common and long-standing challenge has been to explain why fast reconnection proceeds rapidly from a relatively quiescent state. In this talk, we demonstrate the advantages of viewing these problems and their solutions from a common perspective. We focus on some recent, surprising discoveries regarding the role of secondary plasmoid instabilities of thin current sheets. Nonlinearly, these instabilities lead to fast reconnection rates that are very weakly dependent on the Lundquist number of the plasma.
Fast magnetic reconnection in three-dimensional magnetohydrodynamics simulations
Pang Bijia; Pen, U.-L.; Vishniac, Ethan T.
2010-10-15
A constructive numerical example of fast magnetic reconnection in a three-dimensional periodic box is presented. Reconnection is initiated by a strong, localized perturbation to the field lines. The solution is intrinsically three-dimensional and its gross properties do not depend on the details of the simulations. {approx}30% of the magnetic energy is released in an event which lasts about one Alfven time, but only after a delay during which the field lines evolve into a critical configuration. The physical picture of the process is presented. The reconnection regions are dynamical and mutually interacting. In the comoving frame of these regions, reconnection occurs through a x-like point, analogous to Petschek reconnection. The dynamics appear to be driven by global flows, not local processes.
Fingerprints of collisionless reconnection at the separator, I, Ambipolar-Hall signatures
NASA Astrophysics Data System (ADS)
Scudder, J. D.; Mozer, F. S.; Maynard, N. C.; Russell, C. T.
2002-10-01
Plasma, electric, and magnetic field data on the Polar spacecraft have been analyzed for the 29 May 1996 magnetopause traversal searching for evidence of in situ reconnection and traversal of the separator. In this paper we confine our analysis to model-free observations and intrasensor coherence of detection of the environs of the separator. (1) We illustrate the first documented penetration of the separator of collisionless magnetic reconnection in temporal proximity to successful Walén tests with opposite slopes. (2) We present the first direct measurements of E∥ at the magnetopause. (3) We make the first empirical argument that E∥ derives from the electron pressure gradient force. (4) We document the first detection of the electron pressure ridge astride the magnetic depression that extends from the separator. (5) We provide the first empirical detection of the reconnection rate at the magnetopause with the locally sub-Alfvénic ion inflow, MAi ≃ 0.1, and trans-Alfvénic exhaust at high electron pressure of MiA ≃ 1.1-5. (6) We exhibit the first empirical detection of supra-Alfvénic electron flows parallel to B in excess of 5 in narrow sheets. (7) We illustrate the detection of heat flux sheets indicative of separatrices near, but not always in superposition, with the supra-Alfvénic parallel electron bulk flows. (8) We present the first evidence that pressure gradient scales are short enough to explain the electron fluid's measured cross-field drifts not explained by E × B drift but predicted by the measured size of E∥. (9) We illustrate that the size of the observed E∥ is well organized with the limit implied by Vasyliunas's analysis of the generalized Ohm's law of scale length ?, indicative of the intermediate scale of the diffusion region. (10) We document the first detection of departure from electron gyrotropy not only at the separator crossing but also in its vicinity, an effect presaged by [1975]. (11) We make the first reports of very
Fast sawtooth reconnection at realistic Lundquist numbers
NASA Astrophysics Data System (ADS)
Günter, S.; Yu, Q.; Lackner, K.; Bhattacharjee, A.; Huang, Y.-M.
2015-01-01
Magnetic reconnection, a ubiquitous phenomenon in astrophysics, space science and magnetic confinement research, frequently proceeds much faster than predicted by simple resistive MHD theory. Acceleration can result from the break-up of the thin Sweet-Parker current sheet into plasmoids, or from two-fluid effects decoupling mass and magnetic flux transport over the ion inertial length {{v}A}/{ωci} or the drift scale \\sqrt{{{T}e}/{{m}i}}/{ωci}, depending on the absence or presence of a strong magnetic guide field. We describe new results on the modelling of sawtooth reconnection in a simple tokamak geometry (circular cylindrical equilibrium) pushed to realistic Lundquist numbers for present day tokamaks. For the resistive MHD case, the onset criteria and the influence of plasmoids on the reconnection process agree well with earlier results found in the case of vanishing magnetic guide fields. While plasmoids are also observed in two-fluid calculations, they do not dominate the reconnection process for the range of plasma parameters considered in this study. In the two-fluid case they form as a transient phenomenon only. The reconnection times become weakly dependent on the S-value and for the most complete model—including two-fluid effects and equilibrium temperature and density gradients—agree well with those experimentally found on ASDEX Upgrade ≤ft(≤slant 100 μ s\\right).
Wang, Liang Germaschewski, K.; Hakim, Ammar H.; Bhattacharjee, A.
2015-01-15
We introduce an extensible multi-fluid moment model in the context of collisionless magnetic reconnection. This model evolves full Maxwell equations and simultaneously moments of the Vlasov-Maxwell equation for each species in the plasma. Effects like electron inertia and pressure gradient are self-consistently embedded in the resulting multi-fluid moment equations, without the need to explicitly solving a generalized Ohm's law. Two limits of the multi-fluid moment model are discussed, namely, the five-moment limit that evolves a scalar pressures for each species and the ten-moment limit that evolves the full anisotropic, non-gyrotropic pressure tensor for each species. We first demonstrate analytically and numerically that the five-moment model reduces to the widely used Hall magnetohydrodynamics (Hall MHD) model under the assumptions of vanishing electron inertia, infinite speed of light, and quasi-neutrality. Then, we compare ten-moment and fully kinetic particle-in-cell (PIC) simulations of a large scale Harris sheet reconnection problem, where the ten-moment equations are closed with a local linear collisionless approximation for the heat flux. The ten-moment simulation gives reasonable agreement with the PIC results regarding the structures and magnitudes of the electron flows, the polarities and magnitudes of elements of the electron pressure tensor, and the decomposition of the generalized Ohm's law. Possible ways to improve the simple local closure towards a nonlocal fully three-dimensional closure are also discussed.
NASA Astrophysics Data System (ADS)
Wang, Liang; Hakim, Ammar H.; Bhattacharjee, A.; Germaschewski, K.
2015-01-01
We introduce an extensible multi-fluid moment model in the context of collisionless magnetic reconnection. This model evolves full Maxwell equations and simultaneously moments of the Vlasov-Maxwell equation for each species in the plasma. Effects like electron inertia and pressure gradient are self-consistently embedded in the resulting multi-fluid moment equations, without the need to explicitly solving a generalized Ohm's law. Two limits of the multi-fluid moment model are discussed, namely, the five-moment limit that evolves a scalar pressures for each species and the ten-moment limit that evolves the full anisotropic, non-gyrotropic pressure tensor for each species. We first demonstrate analytically and numerically that the five-moment model reduces to the widely used Hall magnetohydrodynamics (Hall MHD) model under the assumptions of vanishing electron inertia, infinite speed of light, and quasi-neutrality. Then, we compare ten-moment and fully kinetic particle-in-cell (PIC) simulations of a large scale Harris sheet reconnection problem, where the ten-moment equations are closed with a local linear collisionless approximation for the heat flux. The ten-moment simulation gives reasonable agreement with the PIC results regarding the structures and magnitudes of the electron flows, the polarities and magnitudes of elements of the electron pressure tensor, and the decomposition of the generalized Ohm's law. Possible ways to improve the simple local closure towards a nonlocal fully three-dimensional closure are also discussed.
Trigger of Fast Reconnection via Collapsing Current Sheets
NASA Astrophysics Data System (ADS)
Tenerani, A.; Velli, M.; Rappazzo, A. F.; Pucci, F.
2015-12-01
It has been widely believed that reconnection is the underlying mechanism of many explosive processes observed both in astrophysical and laboratory plasmas. However, both the questions of how magnetic reconnection is triggered in high Lundquist (S) and Reynolds (R) number plasmas, and how it can then occur on fast, ideal, time-scales remain open. Indeed, it has been argued that fast reconnection rates could be achieved once kinetic scales are reached, or, alternatively, by the onset of the so-called plasmoid instability within Sweet-Parker current sheets. However, it has been shown recently that a tearing mode instability (the "ideal tearing") can grow on an ideal, i.e., S-independent, timescale once the width a of a current sheet becomes thin enough with respect to its macroscopic length L, a/L ~ S-1/3. This suggests that current sheet thinning down to such a threshold aspect ratio —much larger, for S>>1, than the Sweet-Parker one that scales as a/L ~ S-1/2— might provide the trigger for fast reconnection even within the fluid plasma framework. Here we discuss the transition to fast reconnection by studying with visco-resistive MHD simulations the onset and evolution of the tearing instability within a single collapsing current sheet. We indeed show that the transition to a fast tearing mode instability takes place when an inverse aspect ratio of the order of the threshold a/L ~ S-1/3 is reached, and that the secondary current sheets forming nonlinearly become the source of a succession of recursive tearing instabilities. The latter is reminiscent of the fractal reconnection model of flares, which we modify in the light of the "ideal tearing" scenario.
NASA Astrophysics Data System (ADS)
Innocenti, M. E.; Goldman, M. V.; Newman, D. L.; Markidis, S.; Lapenta, G.
2015-12-01
The long term evolution of large domain Particle In Cell simulations of collisionless magnetic reconnection is investigated following observations that show two possible outcomes for collisionless reconnection: towards a Petschek-like configuration (Gosling 2007) or towards multiple X points (Eriksson et al. 2014). In the simulations presented here and described in [Innocenti2015*], a mixed scenario develops. At earlier time, plasmoids are emitted, disrupting the formation of Petschek-like structures. Later, an almost stationary monster plasmoid forms, preventing the emission of other plasmoids. A situation reminding of Petschek's switch-off then ensues. Switch-off is obtained through a slow shock / rotational discontinuity (SS/RD) compound structure, with the rotation discontinuity downstreamthe slow shock. Two external slow shocks located in correspondence of the separatrices reduce the in plane tangential component of the magnetic field, but not to zero. Two transitions reminding of rotational discontinuities in the internal part of the exhausts then perform the final switch-off. Both the slow shocks and the rotational discontinuities are characterized as such through the analysis of their Rankine-Hugoniot jump conditions. A moderate guide field is used to suppress the development of the firehose instability in the exhaust that prevented switch off in [Liu2012]. Compound SS/RD structures, with the RD located downstream the SS, have been observed in both the solar wind and the magnetosphere in Wind and Geotail data respectively [Whang1998, Whang2004]. Ion trajectiories across the SS/RD structure are followed and the kinetic origin of the SS/RD structure is investigated. * Innocenti, Goldman, Newman, Markidis, Lapenta, Evidence of magnetic field switch-off in collisionless magnetic reconnection, accepted in Astrophysical Journal Letters, 2015 Acknowledgements: NERSC, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of
A simple, analytical model of collisionless magnetic reconnection in a pair plasma
Hesse, Michael; Zenitani, Seiji; Kuznetsova, Masha; Klimas, Alex
2009-10-15
A set of conservation equations is utilized to derive balance equations in the reconnection diffusion region of a symmetric pair plasma. The reconnection electric field is assumed to have the function to maintain the current density in the diffusion region and to impart thermal energy to the plasma by means of quasiviscous dissipation. Using these assumptions it is possible to derive a simple set of equations for diffusion region parameters in dependence on inflow conditions and on plasma compressibility. These equations are solved by means of a simple, iterative procedure. The solutions show expected features such as dominance of enthalpy flux in the reconnection outflow, as well as combination of adiabatic and quasiviscous heating. Furthermore, the model predicts a maximum reconnection electric field of E{sup *}=0.4, normalized to the parameters at the inflow edge of the diffusion region.
A Simple, Analytical Model of Collisionless Magnetic Reconnection in a Pair Plasma
NASA Technical Reports Server (NTRS)
Hesse, Michael; Zenitani, Seiji; Kuznetova, Masha; Klimas, Alex
2011-01-01
A set of conservation equations is utilized to derive balance equations in the reconnection diffusion region of a symmetric pair plasma. The reconnection electric field is assumed to have the function to maintain the current density in the diffusion region, and to impart thermal energy to the plasma by means of quasi-viscous dissipation. Using these assumptions it is possible to derive a simple set of equations for diffusion region parameters in dependence on inflow conditions and on plasma compressibility. These equations are solved by means of a simple, iterative, procedure. The solutions show expected features such as dominance of enthalpy flux in the reconnection outflow, as well as combination of adiabatic and quasi-viscous heating. Furthermore, the model predicts a maximum reconnection electric field of E(sup *)=0.4, normalized to the parameters at the inflow edge of the diffusion region.
Analysis of Fast Reconnection in MRX
NASA Astrophysics Data System (ADS)
Yamada, M.; Ji, H.; Kulsrud, R.; Kuritsyn, A.; Terry, S.
2002-11-01
In MRX (Magnetic Reconnection Experiment), the detailed magnetic field structure and the plasma profiles of the neutral sheet have been measured and evidence for non-MHD physics mechanisms has been obtained(M. Yamada et al., Phys. Plasmas, 7), 1781 (2000). As the electron-ion collision frequency is reduced, the reconnection rate has been observed to be significantly enhanced over the classical value. Both electrostatic and magnetic turbulence of lower hybrid frequency range has been observed in the low collisionality regime. The amplitude of magnetic fluctuation is the largest at the center of neutral sheet while electropotential fluctuations peak at the edge of the sheath. To describe the physics of this region, much thought has been given to the `` generalized" Ohm's law where electrons and ions are treated separately. In this `` E-MHD" region electrons but not ions are considered to be magnetized and the lower hybrid wave can be connected continuously to an obliquely propagating Whistler wave(R.L. Stenzel and J.M. Urrutia, Phys. Plasmas, 7), 4450 (2000). In this paper we examine the entire spectrum of waves from the electron cyclotron frequency down to the ion cyclotron frequency, focusing on comparisons with the recent experimental data from the MRX neutral sheet plasmas(H. Ji, et al., this meeting). The physical mechanisms of the observed enhanced resistivity due to these waves will be discussed. This work is supported by DoE, NASA and NSF.
NASA Astrophysics Data System (ADS)
Liu, Yi-Hsin; Drake, J. F.; Swisdak, M.
2011-09-01
Simulations of collisionless oblique propagating slow shocks have revealed the existence of a transition associated with a critical temperature anisotropy ɛ = 1 - μ0(P|| - P⊥)/B2 = 0.25 (Y.-H. Liu, J. F. Drake, and M. Swisdak, Phys. Plasmas 18, 062110 (2011)). An explanation for this phenomenon is proposed here based on anisotropic fluid theory, in particular, the anisotropic derivative nonlinear-Schrödinger-Burgers equation, with an intuitive model of the energy closure for the downstream counter-streaming ions. The anisotropy value of 0.25 is significant because it is closely related to the degeneracy point of the slow and intermediate modes and corresponds to the lower bound of the coplanar to non-coplanar transition that occurs inside a compound slow shock (SS)/rotational discontinuity (RD) wave. This work implies that it is a pair of compound SS/RD waves that bound the outflows in magnetic reconnection, instead of a pair of switch-off slow shocks as in Petschek's model. This fact might explain the rareness of in-situ observations of Petschek-reconnection-associated switch-off slow shocks.
Liu, Yi-Hsin; Drake, J. F.; Swisdak, M.
2011-09-15
Simulations of collisionless oblique propagating slow shocks have revealed the existence of a transition associated with a critical temperature anisotropy {epsilon} = 1 - {mu}{sub 0}(P{sub ||} - P{sub perpendicular})/B{sup 2} = 0.25 (Y.-H. Liu, J. F. Drake, and M. Swisdak, Phys. Plasmas 18, 062110 (2011)). An explanation for this phenomenon is proposed here based on anisotropic fluid theory, in particular, the anisotropic derivative nonlinear-Schroedinger-Burgers equation, with an intuitive model of the energy closure for the downstream counter-streaming ions. The anisotropy value of 0.25 is significant because it is closely related to the degeneracy point of the slow and intermediate modes and corresponds to the lower bound of the coplanar to non-coplanar transition that occurs inside a compound slow shock (SS)/rotational discontinuity (RD) wave. This work implies that it is a pair of compound SS/RD waves that bound the outflows in magnetic reconnection, instead of a pair of switch-off slow shocks as in Petschek's model. This fact might explain the rareness of in-situ observations of Petschek-reconnection-associated switch-off slow shocks.
Faganello, M; Califano, F; Pegoraro, F
2008-09-01
We give evidence for the first time of the onset of undriven fast, collisionless magnetic reconnection during the evolution of an initially homogeneous magnetic field advected in a sheared velocity field. We consider the interaction of the solar wind with the magnetospheric plasma at low latitude and show that reconnection takes place in the layer between adjacent vortices generated by the Kelvin-Helmholtz instability. This process generates coherent magnetic structures with a size comparable to the ion inertial scale, much smaller than the system dimensions but much larger than the electron inertial scale. These magnetic structures are further advected in the plasma in a complex pattern but remain stable over a time interval much longer than their formation time. These results can be crucial for the interpretation of satellite data showing coherent magnetic structures in the Earth's magnetosheath or the magnetotail. PMID:18851219
Fast magnetic reconnection due to anisotropic electron pressure
NASA Astrophysics Data System (ADS)
Cassak, Paul; Baylor, Robert; Fermo, Raymond; Beidler, Matthew; Shay, Michael; Swisdak, Marc; Drake, James; Karimabadi, Homa
2015-11-01
A new regime of fast magnetic reconnection with an out-of-plane (guide) magnetic field is reported in which the key role is played by an electron pressure anisotropy described by the Chew-Goldberger-Low gyrotropic equations of state in the generalized Ohm's law, which even dominates the Hall term. A description of the physical cause of this behavior is provided and two-dimensional fluid simulations are used to confirm the results. The electron pressure anisotropy causes the out-of-plane magnetic field to develop a quadrupole structure of opposite polarity to the Hall magnetic field and gives rise to dispersive waves. In addition to being important for understanding what causes reconnection to be fast, this mechanism should dominate in plasmas with low plasma beta and a high in-plane plasma beta with electron temperature comparable to or larger than ion temperature, so it could be relevant in the solar wind and some tokamaks.
Fast magnetic reconnection due to anisotropic electron pressure
Cassak, P. A.; Baylor, R. N.; Fermo, R. L.; Beidler, M. T.; Shay, M. A.; Swisdak, M.; Drake, J. F.; Karimabadi, H.
2015-02-15
A new regime of fast magnetic reconnection with an out-of-plane (guide) magnetic field is reported in which the key role is played by an electron pressure anisotropy described by the Chew-Goldberger-Low gyrotropic equations of state in the generalized Ohm's law, which even dominates the Hall term. A description of the physical cause of this behavior is provided and two-dimensional fluid simulations are used to confirm the results. The electron pressure anisotropy causes the out-of-plane magnetic field to develop a quadrupole structure of opposite polarity to the Hall magnetic field and gives rise to dispersive waves. In addition to being important for understanding what causes reconnection to be fast, this mechanism should dominate in plasmas with low plasma beta and a high in-plane plasma beta with electron temperature comparable to or larger than ion temperature, so it could be relevant in the solar wind and some tokamaks.
Fast magnetic reconnection due to anisotropic electron pressure
NASA Astrophysics Data System (ADS)
Cassak, P. A.; Baylor, R. N.; Fermo, R. L.; Beidler, M. T.; Shay, M. A.; Swisdak, M.; Drake, J. F.; Karimabadi, H.
2015-02-01
A new regime of fast magnetic reconnection with an out-of-plane (guide) magnetic field is reported in which the key role is played by an electron pressure anisotropy described by the Chew-Goldberger-Low gyrotropic equations of state in the generalized Ohm's law, which even dominates the Hall term. A description of the physical cause of this behavior is provided and two-dimensional fluid simulations are used to confirm the results. The electron pressure anisotropy causes the out-of-plane magnetic field to develop a quadrupole structure of opposite polarity to the Hall magnetic field and gives rise to dispersive waves. In addition to being important for understanding what causes reconnection to be fast, this mechanism should dominate in plasmas with low plasma beta and a high in-plane plasma beta with electron temperature comparable to or larger than ion temperature, so it could be relevant in the solar wind and some tokamaks.
NASA Astrophysics Data System (ADS)
Melzani, Mickaël; Walder, Rolf; Folini, Doris; Winisdoerffer, Christophe; Favre, Jean M.
2014-10-01
Magnetic reconnection is a leading mechanism for magnetic energy conversion and high-energy non-thermal particle production in a variety of high-energy astrophysical objects, including ones with relativistic ion-electron plasmas (e.g., microquasars or AGNs), a regime where first principle studies are scarce. We present 2D particle-in-cell (PIC) simulations of low β ion-electron plasmas under relativistic conditions, i.e., with inflow magnetic energy exceeding the plasma restmass energy. We identify outstanding properties: (i) For relativistic inflow magnetizations (here 10 ≤ σe ≤ 360), the reconnection outflows are dominated by thermal agitation instead of bulk kinetic energy. (ii) At high inflow electron magnetization (σe ≥ 80), the reconnection electric field is sustained more by bulk inertia than by thermal inertia. It challenges the thermal-inertia paradigm and its implications. (iii) The inflows feature sharp transitions at the entrance of the diffusion zones. These are not shocks but results from particle ballistic motions, all bouncing at the same location, provided that the thermal velocity in the inflow is far lower than the inflow E × B bulk velocity. (iv) Island centers are magnetically isolated from the rest of the flow and can present a density depletion at their center. (v) The reconnection rates are slightly higher than in non-relativistic studies. They are best normalized by the inflow relativistic Alfvén speed projected in the outflow direction, which then leads to rates in a close range (0.14-0.25), thus allowing for an easy estimation of the reconnection electric field.
NASA Astrophysics Data System (ADS)
Zelenyi, Lev M.; Artemyev, Anton V.
2016-02-01
In this paper we revisit the paradigm of space science turbulent dissipation traditionally considered as myth (Coroniti, Space Sci. Rev., vol. 42, 1985, pp. 399-410). We demonstrate that due to approach introduced by Pitaevskii (Sov. J. Expl Theor. Phys., vol. 44, 1963, pp. 969-979 (in Russian)) (the effect of a finite Larmor radius on a classical collision integral) dissipation induced by effective interaction with microturbulence produces a significant effect on plasma dynamics, especially in the vicinity of the reconnection region. We estimate the multiplication factor of collision frequency in the collision integral for short wavelength perturbations. For waves propagating transverse to the background magnetic field, this factor is approximately ρekx)2 an electron gyroradius and where kx a transverse wavenumber. We consider recent spacecraft observations in the Earth's magnetotail reconnection region to the estimate possible impact of this multiplication factor. For small-scale reconnection regions this factor can significantly increase the effective collision frequency produced both by lower-hybrid drift turbulence and by kinetic Alfvén waves. We discuss the possibility that the Pitaevskii's effect may be responsible for the excitation of a resistive electron tearing mode in thin current sheets formed in the outflow region of the primary X-line.
How Does Collisionless Magnetic Reconnection Work in the Presence of a Guide Magnetic Field?
NASA Technical Reports Server (NTRS)
Hesse, Michael
2006-01-01
The dissipation mechanism of guide field magnetic reconnection remains a subject of intense scientific interest. On one hand, one set of recent studies have shown that particle inertia-based processes, which include thermal and bulk inertial effects, provide the reconnection electric field in the diffusion region. On the other hand, a second set of studies emphasizes the role of wave-particle interactions in providing anomalous resistivity in the diffusion region. In this presentation, we present analytical theory results, as well as 2.5 and three-dimensional PIC simulations of guide-field magnetic reconnection. We will show that diffusion region scale sizes in moderate and large guide field cases are determined by electron Larmor radii, and that analytical estimates of diffusion region dimensions need to include description of the heat flux tensor. The dominant electron dissipation process appears to be based on thermal electron inertia, expressed through nongyrotropic electron pressure tensors. We will argue that this process remains viable in three dimensions by means of a detailed comparison of high resolution particle-in-cell simulations.
Fast magnetic reconnection with plasmoid / current sheet ejection events in laboratory experiments
NASA Astrophysics Data System (ADS)
Inomoto, Michiaki; Ono, Yasushi; Hayashi, Yoshinori
2012-07-01
Non-steady and fast magnetic reconnections due to plasmoid or current sheet ejection events have been investigated in laboratory experiments using TS-3, TS-4 and UTST plasma merging devices in the University of Tokyo. In these devices, magnetic reconnection is induced by two different schemes, a) push reconnection driven by flux injection from the upstream region, b) pull reconnection driven by flux extraction to the downstream region. Current sheet or plasmoid ejection events are observed in these reconnection experiments particularly with strong guide magnetic field parallel to the reconnection electric field. In push reconnection experiments, anomalous resistivity is induced by the ion's kinetic effect (meandering motion) when the current sheet width is compressed shorter than the ion gyroradius by the strongly injected inflow flux. This fast reconnection regime does not involve plasmoid / current sheet ejection events. On the other hand, the guide field reduces the ion gyroradius and suppresses the onset of the anomalous resistivity, providing slow and steady magnetic reconnection. Impulsive fast reconnection with strong guide field develops, nevertheless, due to plasmoid / current sheet ejection events in pull and push reconnection experiments with extremely large external driving forces. In such a situation, the inflow flux is forcedly pushed into the reconnection region even faster than the maximal reconnection rate, resulting in flux pile up in front of the diffusion region. This piled flux induces large current density inside the current sheet in which plasmoid structure with closed flux surface is formed in pull reconnection case. The induced large current density or plasmoid is then ejected from the diffusion region with significant increase of reconnection electric field. As a result, magnetic reconnection condition with even larger reconnection rate than that obtained by anomalous resistivity was achieved under strong guide field and large external
Models of coronal heating, turbulence and fast reconnection.
Velli, M; Pucci, F; Rappazzo, F; Tenerani, A
2015-05-28
Coronal heating is at the origin of the EUV and X-ray emission and mass loss from the sun and many other stars. While different scenarios have been proposed to explain the heating of magnetically confined and open regions of the corona, they must all rely on the transfer, storage and dissipation of the abundant energy present in photospheric motions, which, coupled to magnetic fields, give rise to the complex phenomenology seen at the chromosphere and transition region (i.e. spicules, jets, 'tornadoes'). Here we discuss models and numerical simulations which rely on magnetic fields and electric currents both for energy transfer and for storage in the corona. We will revisit the sources and frequency spectrum of kinetic and electromagnetic energies, the role of boundary conditions, and the routes to small scales required for effective dissipation. Because reconnection in current sheets has been, and still is, one of the most important processes for coronal heating, we will also discuss recent aspects concerning the triggering of reconnection instabilities and the transition to fast reconnection. PMID:25897086
Models of coronal heating, turbulence and fast reconnection
NASA Astrophysics Data System (ADS)
Velli, M.; Pucci, F.; Rappazzo, F.; Tenerani, A.
2015-04-01
Coronal heating is at the origin of the EUV and X-ray emission and mass loss from the sun and many other stars. While different scenarios have been proposed to explain the heating of magnetically confined and open regions of the corona, they must all rely on the transfer, storage and dissipation of the abundant energy present in photospheric motions, which, coupled to magnetic fields, give rise to the complex phenomenology seen at the chromosphere and transition region (i.e. spicules, jets, 'tornadoes'). Here we discuss models and numerical simulations which rely on magnetic fields and electric currents both for energy transfer and for storage in the corona. We will revisit the sources and frequency spectrum of kinetic and electromagnetic energies, the role of boundary conditions, and the routes to small scales required for effective dissipation. Because reconnection in current sheets has been, and still is, one of the most important processes for coronal heating, we will also discuss recent aspects concerning the triggering of reconnection instabilities and the transition to fast reconnection.
NASA Astrophysics Data System (ADS)
Innocenti, Maria Elena; Lapenta, Giovanni; Goldman, Martin; Newman, David; Markidis, Stefano
2015-04-01
In Petschek's model for magnetic reconnection, switch-off (SO) condition is achieved through back-to-back slow mode shocks (SS). No rotational discontinuity (RD) is needed, unless in specific cases detailed in [Vasyliunas 1975]. Decades of simulations with different models (MHD, Hall MHD, hybrid, PIC) have yielded contradictory results regarding the achievement of the SO condition during magnetic reconnection events. It has been recently argued that the formation of Petschek's SO-SS is inhibited by the development of the firehose instability, which provokes the flapping of the magnetic field in the reconnection exhausts (Liu et al., 2012). We report here on the formation of localized switch-off areas in simulations of collisionless magnetic reconnection in extremely large domains (hundreds of ion skin depths) for extremely long times (hundreds of inverse ion cyclotron frequency). A guide field (a magnetic field in the direction perpendicular to the reconnection plane) prevents the development of the firehose instability. The switch-off areas are marked by magnetic field line bending (in a way closely resembling the textbook description of RDs), by the formation of a nozzle-like structure in the in-plane projection of the ion and electron velocities perpendicular to the magnetic field direction and by a reduced rate of plasmoid formation. We use Rankine-Hugoniot conditions to characterize the transitions as Rotational Discontinuities and we comment on their origin. Priest, E. and Forbes, T. (2007). Magnetic reconnection. Magnetic Reconnection, by Eric Priest, Terry Forbes, Cambridge, UK: Cambridge University Press, 2007, 1. Vasyliunas V. (1975). Theoretical models of magnetic field line merging. Review of Geophysics and Space Phsyics, 1975. Liu, Y.-H., Drake, J. F., and Swisdak, M. (2012). The structure of the magnetic reconnection exhaust boundary. Physics of Plasmas (1994-present), 19(2):-.
Fast reconnection in relativistic plasmas: the magnetohydrodynamics tearing instability revisited
NASA Astrophysics Data System (ADS)
Del Zanna, L.; Papini, E.; Landi, S.; Bugli, M.; Bucciantini, N.
2016-08-01
Fast reconnection operating in magnetically dominated plasmas is often invoked in models for magnetar giant flares, for magnetic dissipation in pulsar winds, or to explain the gamma-ray flares observed in the Crab nebula, hence its investigation is of paramount importance in high-energy astrophysics. Here we study, by means of two dimensional numerical simulations, the linear phase and the subsequent nonlinear evolution of the tearing instability within the framework of relativistic resistive magnetohydrodynamics, as appropriate in situations where the Alfven velocity approaches the speed of light. It is found that the linear phase of the instability closely matches the analysis in classical MHD, where the growth rate scales with the Lundquist number S as S^-1/2, with the only exception of an enhanced inertial term due to the thermal and magnetic energy contributions. In addition, when thin current sheets of inverse aspect ratio scaling as S^-1/3 are considered, the so-called "ideal" tearing regime is retrieved, with modes growing independently on S and extremely fast, on only a few light crossing times of the sheet length. The overall growth of fluctuations is seen to solely depend on the value of the background Alfven velocity. In the fully nonlinear stage we observe an inverse cascade towards the fundamental mode, with Petschek-type supersonic jets propagating at the external Alfven speed from the X-point, and a fast reconnection rate at the predicted value R~(ln S)^-1.
NASA Astrophysics Data System (ADS)
Scudder, J. D.; Daughton, W.; Karimabadi, H.
2006-12-01
Many researchers are "looking" for "the" electron diffusion region layers with in situ data. Some publish papers about the statistics of such regions operationally "defined" to be electron diffusion regions, pending contradiction. Theorists also use a variety of recipes to identify diffusion regions in their numerical simulations. For example, some would say that the electron diffusion region is wherever E ≠ -U_e×B where U_e is the electron fluid velocity. Others in the "diffusion region hunt" identify the (ion) "diffusion region" as a proxy for the detection of the "electron diffusion region", in spite of the fact that any current carrying layer in space is an "ion diffusion region", whether it is attached to the reconnection site or not! Some observationalists insist that E∥ \
Characteristics of a current sheet shear mode in collisionless magnetic reconnection
NASA Astrophysics Data System (ADS)
Fujimoto, Keizo
2016-05-01
The current study shows the characteristics of the kink-type electromagnetic mode excited in the thin current layer formed around the x-line during the quasi-steady phase of magnetic reconnection. The linear wave analyses are carried out for the realistic current sheet profile which differs significantly from the Harris current sheet. It is found that the peak growth rate is very sensitive to the current sheet width even though the relative drift velocity at the center of the current sheet is fixed. This indicates that the mode is excited by the velocity shear rather than the relative drift velocity. Thus, the mode is termed here a current sheet shear mode. It is also shown that the wavenumber ky has a clear mass ratio dependency as ky λi ∝ (mi /me )1/4, implying the coupling of the ion and electron dynamics, where λi is the ion inertia length.
Global Extended MHD Studies of Fast Magnetic Reconnection
Breslau J.A.; Jardin, S.C.
2002-09-18
Recent experimental and theoretical results have led to two lines of thought regarding the physical processes underlying fast magnetic reconnection. One is based on the traditional Sweet-Parker model but replaces the Spitzer resistivity with an enhanced resistivity caused by electron scattering by ion acoustic turbulence. The other includes the finite gyroradius effects that enter Ohm's law through the Hall and electron pressure gradient terms. A 2-D numerical study, conducted with a new implicit parallel two-fluid code, has helped to clarify the similarities and differences in predictions between these two models and provides some insight into their respective ranges of validity.
Fast reconnection in relativistic plasmas: the magnetohydrodynamics tearing instability revisited
NASA Astrophysics Data System (ADS)
Del Zanna, L.; Papini, E.; Landi, S.; Bugli, M.; Bucciantini, N.
2016-08-01
Fast reconnection operating in magnetically dominated plasmas is often invoked in models for magnetar giant flares, for magnetic dissipation in pulsar winds, or to explain the gamma-ray flares observed in the Crab nebula; hence, its investigation is of paramount importance in high-energy astrophysics. Here we study, by means of two-dimensional numerical simulations, the linear phase and the subsequent non-linear evolution of the tearing instability within the framework of relativistic resistive magnetohydrodynamics (MHD), as appropriate in situations where the Alfvén velocity approaches the speed of light. It is found that the linear phase of the instability closely matches the analysis in classical MHD, where the growth rate scales with the Lundquist number S as S-1/2, with the only exception of an enhanced inertial term due to the thermal and magnetic energy contributions. In addition, when thin current sheets of inverse aspect ratio scaling as S-1/3 are considered, the so-called ideal tearing regime is retrieved, with modes growing independently of S and extremely fast, on only a few light crossing times of the sheet length. The overall growth of fluctuations is seen to solely depend on the value of the background Alfvén velocity. In the fully non-linear stage, we observe an inverse cascade towards the fundamental mode, with Petschek-type supersonic jets propagating at the external Alfvén speed from the X-point, and a fast reconnection rate at the predicted value {R}˜ (ln S)^{-1}.
Three-dimensional fast magnetic reconnection driven by relativistic ultraintense femtosecond lasers.
Ping, Y L; Zhong, J Y; Sheng, Z M; Wang, X G; Liu, B; Li, Y T; Yan, X Q; He, X T; Zhang, J; Zhao, G
2014-03-01
Three-dimensional fast magnetic reconnection driven by two ultraintense femtosecond laser pulses is investigated by relativistic particle-in-cell simulation, where the two paralleled incident laser beams are shot into a near-critical plasma layer to form a magnetic reconnection configuration in self-generated magnetic fields. A reconnection X point and out-of-plane quadrupole field structures associated with magnetic reconnection are formed. The reconnection rate is found to be faster than that found in previous two-dimensional Hall magnetohydrodynamic simulations and electrostatic turbulence contribution to the reconnection electric field plays an essential role. Both in-plane and out-of-plane electron and ion accelerations up to a few MeV due to the magnetic reconnection process are also obtained. PMID:24730781
Measurements of Fast Magnetic Reconnection Driven by Relativistic Electrons
NASA Astrophysics Data System (ADS)
Raymond, Anthony; McKelvey, Andrew; Zulick, Calvin; Chuanfei, Dong; Maksimchuk, Anatoly; Thomas, Alexander; Yanovsky, Victor; Krushelnick, Karl; Willingale, Louise; Chykov, Vladimir; Nilson, Phil; Chen, Hui; Williams, Gerald; Bhattacharjee, Amitava; Fox, Will
2015-11-01
Magnetic reconnection is a process whereby opposing magnetic field lines are forced together and topologically rearrange, resulting in lower magnetic potential energy and in corresponding plasma heating. Such occurrences are ubiquitous in astrophysics as well as appearing in laboratory plasmas such as in ICF in the form of instabilities. We report measurements in the domain of ultra-fast, ultra-intense lasers, in which the mechanism responsible follows from radially expanding surface electrons with v ~ c . Results are compared from two laser facilities (HERCULES and Omega EP), both of which produced two relativistic intensity pulses focused within close proximity onto copper foils. A spherical X-ray crystal was used to image the Kα radiation induced by electron currents, revealing the midplane diffusion region wherein electrons are accelerated into the target by the electric field generated during reconnection. The characteristics of this signal are studied as a function of the focal spot separation, laser energy, and pulse duration. The results are then compared to 3D PIC simulations.
Fast magnetic reconnection supported by sporadic small-scale Petschek-type shocks
Shibayama, Takuya Nakabou, Takashi; Kusano, Kanya; Miyoshi, Takahiro; Vekstein, Grigory
2015-10-15
Standard magnetohydrodynamic (MHD) theory predicts reconnection rate that is far too slow to account for a wide variety of reconnection events observed in space and laboratory plasmas. Therefore, it was commonly accepted that some non-MHD (kinetic) effects play a crucial role in fast reconnection. A recently renewed interest in simple MHD models is associated with the so-called plasmoid instability of reconnecting current sheets. Although it is now evident that this effect can significantly enhance the rate of reconnection, many details of the underlying multiple-plasmoid process still remain controversial. Here, we report results of a high-resolution computer simulation which demonstrate that fast albeit intermittent magnetic reconnection is sustained by numerous small-scale Petschek-type shocks spontaneously formed in the current sheet due to its plasmoid instability.
PARTICLE ACCELERATION DURING MAGNETOROTATIONAL INSTABILITY IN A COLLISIONLESS ACCRETION DISK
Hoshino, Masahiro
2013-08-20
Particle acceleration during the magnetorotational instability (MRI) in a collisionless accretion disk was investigated by using a particle-in-cell simulation. We discuss the important role that magnetic reconnection plays not only on the saturation of MRI but also on the relativistic particle generation. The plasma pressure anisotropy of p > p{sub ||} induced by the action of MRI dynamo leads to rapid growth in magnetic reconnection, resulting in the fast generation of nonthermal particles with a hard power-law spectrum. This efficient particle acceleration mechanism involved in a collisionless accretion disk may be a possible model to explain the origin of high-energy particles observed around massive black holes.
Fast magnetic reconnection in low-density electron-positron plasmas
Bessho, Naoki; Bhattacharjee, A.
2010-10-15
Two-dimensional particle-in-cell simulations have been performed to study magnetic reconnection in low-density electron-positron plasmas without a guide magnetic field. Impulsive reconnection rates become of the order of unity when the background density is much smaller than 10% of the density in the initial current layer. It is demonstrated that the outflow speed is less than the upstream Alfven speed, and that the time derivative of the density must be taken into account in the definition of the reconnection rate. The reconnection electric fields in the low-density regime become much larger than the ones in the high-density regime, and it is possible to accelerate the particles to high energies more efficiently. The inertial term in the generalized Ohm's law is the most dominant term that supports a large reconnection electric field. An effective collisionless resistivity is produced and tracks the extension of the diffusion region in the late stage of the reconnection dynamics, and significant broadening of the diffusion region is observed. Because of the broadening of the diffusion region, no secondary islands, which have been considered to play a role to limit the diffusion region, are generated during the extension of the diffusion region in the outflow direction.
Comparison of Secondary Islands in Collisional Reconnection to Hall Reconnection
Shepherd, L. S.; Cassak, P. A.
2010-07-02
Large-scale resistive Hall-magnetohydrodynamic simulations of the transition from Sweet-Parker (collisional) to Hall (collisionless) magnetic reconnection are presented; the first to separate secondary islands from collisionless effects. Three main results are described. There exists a regime with secondary islands but without collisionless effects, and the reconnection rate is faster than Sweet-Parker, but significantly slower than Hall reconnection. This implies that secondary islands do not cause the fastest reconnection rates. The onset of Hall reconnection ejects secondary islands from the vicinity of the X line, implying that energy is released more rapidly during Hall reconnection. Coronal applications are discussed.
Direct evidence for kinetic effects associated with solar wind reconnection.
Xu, Xiaojun; Wang, Yi; Wei, Fengsi; Feng, Xueshang; Deng, Xiaohua; Ma, Yonghui; Zhou, Meng; Pang, Ye; Wong, Hon-Cheng
2015-01-01
Kinetic effects resulting from the two-fluid physics play a crucial role in the fast collisionless reconnection, which is a process to explosively release massive energy stored in magnetic fields in space and astrophysical plasmas. In-situ observations in the Earth's magnetosphere provide solid consistence with theoretical models on the point that kinetic effects are required in the collisionless reconnection. However, all the observations associated with solar wind reconnection have been analyzed in the context of magnetohydrodynamics (MHD) although a lot of solar wind reconnection exhausts have been reported. Because of the absence of kinetic effects and substantial heating, whether the reconnections are still ongoing when they are detected in the solar wind remains unknown. Here, by dual-spacecraft observations, we report a solar wind reconnection with clear Hall magnetic fields. Its corresponding Alfvenic electron outflow jet, derived from the decouple between ions and electrons, is identified, showing direct evidence for kinetic effects that dominate the collisionless reconnection. The turbulence associated with the exhaust is a kind of background solar wind turbulence, implying that the reconnection generated turbulence has not much developed. PMID:25628139
Direct evidence for kinetic effects associated with solar wind reconnection
Xu, Xiaojun; Wang, Yi; Wei, Fengsi; Feng, Xueshang; Deng, Xiaohua; Ma, Yonghui; Zhou, Meng; Pang, Ye; Wong, Hon-Cheng
2015-01-01
Kinetic effects resulting from the two-fluid physics play a crucial role in the fast collisionless reconnection, which is a process to explosively release massive energy stored in magnetic fields in space and astrophysical plasmas. In-situ observations in the Earth's magnetosphere provide solid consistence with theoretical models on the point that kinetic effects are required in the collisionless reconnection. However, all the observations associated with solar wind reconnection have been analyzed in the context of magnetohydrodynamics (MHD) although a lot of solar wind reconnection exhausts have been reported. Because of the absence of kinetic effects and substantial heating, whether the reconnections are still ongoing when they are detected in the solar wind remains unknown. Here, by dual-spacecraft observations, we report a solar wind reconnection with clear Hall magnetic fields. Its corresponding Alfvenic electron outflow jet, derived from the decouple between ions and electrons, is identified, showing direct evidence for kinetic effects that dominate the collisionless reconnection. The turbulence associated with the exhaust is a kind of background solar wind turbulence, implying that the reconnection generated turbulence has not much developed. PMID:25628139
Zhou, Fushun; Huang, Can Lu, Quanming; Wang, Shui; Xie, Jinlin
2015-09-15
Two-dimensional particle-in-cell simulation is performed to investigate magnetic reconnection in a force-free current sheet. The results show that the evolution of the ion diffusion region has two different phases. In the first phase, the electrons flow toward the X line along one pair of separatrices and away from the X line along the other pair of separatrices. Therefore, in the ion diffusion region, a distorted quadrupole structure of the out-of-plane magnetic field is formed, which is similar to that of a typical guide field reconnection in the Harris current sheet. In the second phase, the electrons move toward the X line along the separatrices and then flow away from the X line at the inner side of the separatrices. In the ion diffusion region, the out-of-plane magnetic field exhibits a characteristic quadrupole pattern with a good symmetry, which is similar to that of antiparallel reconnection in the Harris current sheet.
On the noncoplanarity of the magnetic field within a fast collisionless shock
NASA Technical Reports Server (NTRS)
Thomsen, M. F.; Gosling, J. T.; Bame, S. J.; Quest, K. B.; Winske, D.
1987-01-01
Within the magnetic ramp of fast collisionless plasma shocks observed with spacecraft instruments and simulated numerically, the magnetic field undergoes an excursion out of the plane of coplanarity. This rotation is consistently in the direction such that the electrostatic potential jump across the shock, as measured in the de Hoffman-Teller frame of the reference (HTF), is about 2-6 times smaller than the electrostatic potential jump measured in the normal incidence frame. The preferred direction is consistent with a basic whistler mode transition between the upstream and downstream orientations. The potential jump in the HTF is considerably smaller than the change in bulk flow energy across the shock, confirming the recent suggestion that magnetic forces contribute importantly to the slowing of the plasma in that frame. A further consequence is that suprathermal particles leaking back into the upstream region across the shock do not gain much energy from the cross-shock electric field.
Scudder, Jack
2011-02-04
The final years of this grant have been dedicated to diagnosing the observable properties of Collisionless Magnetic Reconnection (CMR) as disclosed by the open boundary condition PIC simulations developed under this grant. Particular attention has focussed on identifying the Electron Diffusion Region (EDR), the short scale domain where the process is thought to be enabled. The critical issue has been the need for experimental closure for CMR that is widely invoked in astrophysics, but has actually rather little direct, incontrovertible evidence for its involvement. This difficulty arises because CMR is about topology change of the magnetic field - a concept that is not conducive to single, or even few point correlations as are beginning to be possible with spacecraft armada, like Cluster or the planned Magnetospheric Multi-Scale (MMS) mission to be launched in 2014. Alternate formulations about the time rate of magnetic flux inventoried by a moving observer, reformulate the needed evidence in terms of the curl of various weak vector fields, such as E+UexB, that is zero in ideal MHD. To sense E+UexB from space measurements is already a heroic task. The curl of such a small vector field is outside the domain of the possible.
ON THE ROLE OF FAST MAGNETIC RECONNECTION IN ACCRETING BLACK HOLE SOURCES
Singh, C. B.; De Gouveia Dal Pino, E. M.; Kadowaki, L. H. S. E-mail: dalpino@iag.usp.br
2015-01-30
We attempt to explain the observed radio and gamma-ray emission produced in the surroundings of black holes by employing a magnetically dominated accretion flow model and fast magnetic reconnection triggered by turbulence. In earlier work, a standard disk model was used and we refine the model by focusing on the sub-Eddington regime to address the fundamental plane of black hole activity. The results do not change substantially with regard to previous work, ensuring that the details of accretion physics are not relevant in the magnetic reconnection process occurring in the corona. Rather, our work puts fast magnetic reconnection events as a powerful mechanism operating in the core region near the jet base of black hole sources on more solid ground. For microquasars and low-luminosity active galactic nuclei, the observed correlation between radio emission and the mass of the sources can be explained by this process. The corresponding gamma-ray emission also seems to be produced in the same core region. On the other hand, emission from blazars and gamma-ray bursts cannot be correlated to core emission based on fast reconnection.
NASA Astrophysics Data System (ADS)
Shimizu, Tohru; Torii, Hiroyuki; Kondoh, Koji
2016-05-01
The 3D instability of spontaneous fast magnetic reconnection process is studied with magnetohydrodynamic simulations, where 2D model of the spontaneous fast magnetic reconnection process is destabilized in three dimensions. In this 3D instability, the spontaneous fast magnetic reconnection process is intermittently and randomly caused in 3D. In this paper, as a typical event study, a single 3D fast magnetic reconnection process often observed in the 3D instability is studied in detail. As a remarkable feature, it is reported that, when the 3D fast magnetic reconnection process starts, plasma inflows along the magnetic neutral line are observed, which are driven by plasma static pressure gradient along the neutral line. The plasma inflow speed reaches about 15 in the upstream field region. The unmagnetized inflow tends to prevent the 3D reconnection process; nevertheless, the 3D reconnection process is intermittently maintained. Such high-speed plasma inflows along the neutral line may be observed as dawn-dusk flows in space satellite observations of magnetotail's bursty bulk flows.
Fast Reconnection Rates Based on Group Velocity Cones: Whistler Regime and Pair Plasmas
NASA Astrophysics Data System (ADS)
Singh, N.
2009-05-01
Based on the group velocity vector of the whistler mode, we predict the range of whistler-regime reconnection rate depending on the half width (w) of the current sheet (CS. During the reconnection process electromagnetic perturbations (EMPs) are generated in the localized diffusion region (DR, which acts like an antenna and radiates whistler waves for certain range of CS widths. The reconnection structure (exhaust) is approximately the radiation pattern of the DR antenna and it is determined by the group velocity directions. Since the whistler waves originate from the electromagnetic perturbations (EMPs) localized in the DR, we calculate R over a range of the discrete values of the perpendicular wave number (k'') contained in the Fourier spectrum of the EMPs. We have used such calculations to determine the reconnection rates
Ugai, M.
2009-11-15
It is well known that magnetic pulsations of long periods impulsively occur in accordance with the sudden onset of geomagnetic substorms and drastic enhancement of electrojets in the ionosphere. On the basis of the spontaneous fast reconnection model, the present paper examines the physical mechanism by which both magnetic pulsations and strong electrojets are impulsively driven by the fast (Alfvenic) reconnection jet. When a large-scale plasmoid [or traveling compression region (TCR)], directly caused by the fast reconnection jet, collides with the magnetic loop footpoint, strong electrojets are impulsively driven in a finite extent in the loop footpoint in accordance with the evolution of the current wedge and the generator current circuit. Simultaneously, magnetohydrodynamic (Alfven) waves, accompanied by the TCR, are reflected from the electrojet layer, leading to impulsive magnetic pulsations ahead of the loop footpoint because of the interaction (or resonance) between the reflected waves and the waves traveling toward the footpoint. The pulsations propagate outward in all directions from the source region of the wave reflection, and the pulsation periods are typically estimated to be of several tens of seconds.
Fast-shock formation in line-tied magnetic reconnection models of solar flares
NASA Technical Reports Server (NTRS)
Forbes, T. G.
1986-01-01
In a previous study by the author, an approximately stationary fast shock was tentatively identified in a numerical experiment designed to study line-tied magnetic reconnection. Here the evidence for the occurrence of a stationary fast shock is reexamined, and the previous identification is confirmed. In the numerical experiment, line-tied reconnection is modeled by a configuration which produces two supermagnetosonic outflow jets - one directed upward, away from the photosphere, and one directed downward, toward an arcade of closed magnetic loops tied to the photosphere. The fast shock occurs when the downward-directed jet encounters the obstacle formed by the closed loops. Although the existence of a stationary, or nearly stationary, fast shock is confirmed, the transition from the supermagnetosonic flow region upstream of the shock to the nearly static region downstream of the shock is more complicated than was previously thought. Immediately downstream of the shock, there exists a deflection sheath in which the submagnetosonic flow coming out of the shock is diverted around the region of static closed loops. The MHD jump conditions are used to investigate the characteristics of the fast shock and to show that a stationary shock cannot exist unless accompanied by a deflection sheath. Analysis of the shock's location and dimensions suggests that such fast shocks may contribute to particle acceleration and to thermal condensation in flares.
FAST MAGNETIC RECONNECTION IN THE SOLAR CHROMOSPHERE MEDIATED BY THE PLASMOID INSTABILITY
Ni, Lei; Kliem, Bernhard; Lin, Jun; Wu, Ning
2015-01-20
Magnetic reconnection in the partially ionized solar chromosphere is studied in 2.5 dimensional magnetohydrodynamic simulations including radiative cooling and ambipolar diffusion. A Harris current sheet with and without a guide field is considered. Characteristic values of the parameters in the middle chromosphere imply a high magnetic Reynolds number of ∼10{sup 6}-10{sup 7} in the present simulations. Fast magnetic reconnection then develops as a consequence of the plasmoid instability without the need to invoke anomalous resistivity enhancements. Multiple levels of the instability are followed as it cascades to smaller scales, which approach the ion inertial length. The reconnection rate, normalized to the asymptotic values of magnetic field and Alfvén velocity in the inflow region, reaches values in the range ∼0.01-0.03 throughout the cascading plasmoid formation and for zero as well as for strong guide field. The outflow velocity reaches ≈40 km s{sup –1}. Slow-mode shocks extend from the X-points, heating the plasmoids up to ∼8 × 10{sup 4} K. In the case of zero guide field, the inclusion of both ambipolar diffusion and radiative cooling causes a rapid thinning of the current sheet (down to ∼30 m) and early formation of secondary islands. Both of these processes have very little effect on the plasmoid instability for a strong guide field. The reconnection rates, temperature enhancements, and upward outflow velocities from the vertical current sheet correspond well to their characteristic values in chromospheric jets.
Bessho, Naoki; Bhattacharjee, A.
2012-05-10
Magnetic reconnection and particle acceleration in relativistic Harris sheets in low-density electron-positron plasmas with no guide field have been studied by means of two-dimensional particle-in-cell simulations. Reconnection rates are of the order of one when the background density in a Harris sheet is of the order of 1% of the density in the current sheet, which is consistent with previous results in the non-relativistic regime. It has been demonstrated that the increase of the Lorentz factors of accelerated particles significantly enhances the collisionless resistivity needed to sustain a large reconnection electric field. It is shown analytically and numerically that the energy spectrum of accelerated particles near the X-line is the product of a power law and an exponential function of energy, {gamma}{sup -1/4}exp (- a{gamma}{sup 1/2}), where {gamma} is the Lorentz factor and a is a constant. However, in the low-density regime, while the most energetic particles are produced near X-lines, many more particles are energized within magnetic islands. Particles are energized in contracting islands by multiple reflection, but the mechanism is different from Fermi acceleration in magnetic islands for magnetized particles in the presence of a guide field. In magnetic islands, strong core fields are generated and plasma beta values are reduced. As a consequence, the fire-hose instability condition is not satisfied in most of the island region, and island contraction and particle acceleration can continue. In island coalescence, reconnection between two islands can accelerate some particles, however, many particles are decelerated and cooled, which is contrary to what has been discussed in the literature on particle acceleration due to reconnection in non-relativistic hydrogen plasmas.
Experimental Study of Lower-hybrid Drift Turbulence in a Reconnecting Current Sheet
Carter, T. A.; Yamada, M.; Ji, H.; Kulsrud, R. M.; Trintchouck, F.
2002-06-18
The role of turbulence in the process of magnetic reconnection has been the subject of a great deal of study and debate in the theoretical literature. At issue in this debate is whether turbulence is essential for fast magnetic reconnection to occur in collisionless current sheets. Some theories claim it is necessary in order to provide anomalous resistivity, while others present a laminar fast reconnection mechanism based on the Hall term in the generalized Ohm's law. In this work, a thorough study of electrostatic potential fluctuations in the current sheet of the Magnetic Reconnection Experiment (MRX) [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)] was performed in order to ascertain the importance of turbulence in a laboratory reconnection experiment. Using amplified floating Langmuir probes, broadband fluctuations in the lower hybrid frequency range (fLH approximately 5-15 MHz) were measured which arise with the formation of the current sheet in MRX. The frequency spectrum, spatial amplitude profile, and spatial correlation characteristics of the measured turbulence were examined carefully, finding consistency with theories of the lower-hybrid drift instability (LHDI). The LHDI and its role in magnetic reconnection has been studied theoretically for decades, but this work represents the first detection and detailed study of the LHDI in a laboratory current sheet. The observation of the LHDI in MRX has provided the unique opportunity to uncover the role of this instability in collisionless reconnection. It was found that: (1) the LHDI fluctuations are confined to the low-beta edge of current sheets in MRX; (2) the LHDI amplitude does not correlate well in time or space with the reconnection electric field, which is directly related to the rate of reconnection; and (3) significant LHDI amplitude persists in high collisionality current sheets where the reconnection rate is classical. These findings suggest that the measured LHDI fluctuations do not play an
Dissipation mechanism in 3D magnetic reconnection
Fujimoto, Keizo
2011-11-15
Dissipation processes responsible for fast magnetic reconnection in collisionless plasmas are investigated using 3D electromagnetic particle-in-cell simulations. The present study revisits the two simulation runs performed in the previous study (Fujimoto, Phys. Plasmas 16, 042103 (2009)); one with small system size in the current density direction, and the other with larger system size. In the case with small system size, the reconnection processes are almost the same as those in 2D reconnection, while in the other case a kink mode evolves along the current density and deforms the current sheet structure drastically. Although fast reconnection is achieved in both the cases, the dissipation mechanism is very different between them. In the case without kink mode, the electrons transit the electron diffusion region without thermalization, so that the magnetic dissipation is supported by the inertia resistivity alone. On the other hand, in the kinked current sheet, the electrons are not only accelerated in bulk, but they are also partly scattered and thermalized by the kink mode, which results in the anomalous resistivity in addition to the inertia resistivity. It is demonstrated that in 3D reconnection the thickness of the electron current sheet becomes larger than the local electron inertia length, consistent with the theoretical prediction in Fujimoto and Sydora (Phys. Plasmas 16, 112309 (2009)).
NASA Astrophysics Data System (ADS)
Chai, Kil-Byoung; Zhai, Xiang; Bellan, Paul M.
2016-03-01
A spatially localized energetic extreme ultra-violet (EUV) burst is imaged at the presumed position of fast magnetic reconnection in a plasma jet produced by a coaxial helicity injection source; this EUV burst indicates strong localized electron heating. A circularly polarized high frequency magnetic field perturbation is simultaneously observed at some distance from the reconnection region indicating that the reconnection emits whistler waves and that Hall dynamics likely governs the reconnection. Spectroscopic measurement shows simultaneous fast ion heating. The electron heating is consistent with Ohmic dissipation, while the ion heating is consistent with ion trajectories becoming stochastic.
Does the resistive tearing instability nonlinearly evolve to a fast reconnection mode
NASA Technical Reports Server (NTRS)
Steinolfson, R. S.
1988-01-01
A fundamental problem in applying linear tearing instability theory to the rapid processes (particle acceleration, heating) in flares was the characteristically slow rate of reconnection. This problem can be at least partially overcome if the tearin mode nonlinearly evolves to a regime in which the reconnection rate is substantially enhanced, such as that for the Petschek configuration. This possibility was often suggested, and some numerical simulations appear to provide support for such a view. Numerical simulation are used to study the nonlinear evolution of the tearing stability and show that a fast Petschek-like regime may not be achieved. This conclusion follows when there are sufficient grid points within the diffusion region to completely resolve the nonlinear dynamic interactions in the diffusion layer. When the numerical resolution is not adequate, the solution does appear to approach a Petschek configuration. The resolved solution contains reverse flow vortices and current sheets, terminated with a current reversal, similar to those obtained by Syrovatsky (JEPT, 33, 933, 1971).
NASA Astrophysics Data System (ADS)
Chai, Kil-Byoung; Zhai, Xiang; Bellan, Paul
2015-11-01
The Caltech astrophysical jet experiment provides a highly resolved demonstration of the interaction between single fluid and 2-fluid scales and possibly kinetic scales as well. The jet evolves through the following sequence: (i) a current-carrying MHD-driven plasma jet self-forms, (ii) the jet undergoes a kink instability, (iii) the kink provides the environment for a secondary, Rayleigh-Taylor (RT) instability, (iv) the RT instability erodes the current channel radius to a scale smaller than ion skin depth to cause fast magnetic reconnection, (v) the reconnection emits broadband obliquely-propagating, right-hand circularly polarized whistler waves, and (vi) the reconnection energizes electrons and ions. The observation of the whistler waves confirms that the reconnection is in the Hall MHD regime (i.e., 2-fluid). An energetic extreme ultraviolet burst is observed at the location of reconnection indicating strong, localized electron heating. Spectroscopic measurement shows simultaneous fast ion heating. The analysis shows that electrons are plausibly heated by Ohmic dissipation, and that ions are plausibly heated stochastically, i.e., the guiding center approximation fails, a kinetic effect.
Magnetic reconnection in the presence of externally driven and self-generated turbulence
Karimabadi, H.; Lazarian, A.
2013-11-15
Magnetic reconnection is an important process that violates flux freezing and induces change of magnetic field topology in conducting fluids and, as a consequence, converts magnetic field energy into particle energy. It is thought to be operative in laboratory, heliophysical, and astrophysical plasmas. These environments exhibit wide variations in collisionality, ranging from collisionless in the Earth's magnetosphere to highly collisional in molecular clouds. A common feature among these plasmas is, however, the presence of turbulence. We review the present understanding of the effects of turbulence on the reconnection rate, discussing both how strong pre-existing turbulence modifies Sweet-Parker reconnection and how turbulence may develop as a result of reconnection itself. In steady state, reconnection rate is proportional to the aspect ratio of the diffusion region. Thus, two general MHD classes of models for fast reconnection have been proposed, differing on whether they keep the aspect ratio finite by increasing the width due to turbulent broadening or shortening the length of the diffusion layer due to plasmoid instability. One of the consequences of the plasmoid instability model is the possibility that the current sheet thins down to collisionless scales where kinetic effects become dominant. As a result, kinetic effects may be of importance for many astrophysical applications which were considered to be in the realm of MHD. Whether pre-existing turbulence can significantly modify the transition to the kinetic regime is not currently known. Although most studies of turbulent reconnection have been based on MHD, recent advances in kinetic simulations are enabling 3D studies of turbulence and reconnection in the collisionless regime. A summary of these recent works, highlighting similarities and differences with the MHD models of turbulent reconnection, as well as comparison with in situ observations in the magnetosphere and in the solar wind, are presented
Explosive turbulent magnetic reconnection.
Higashimori, K; Yokoi, N; Hoshino, M
2013-06-21
We report simulation results for turbulent magnetic reconnection obtained using a newly developed Reynolds-averaged magnetohydrodynamics model. We find that the initial Harris current sheet develops in three ways, depending on the strength of turbulence: laminar reconnection, turbulent reconnection, and turbulent diffusion. The turbulent reconnection explosively converts the magnetic field energy into both kinetic and thermal energy of plasmas, and generates open fast reconnection jets. This fast turbulent reconnection is achieved by the localization of turbulent diffusion. Additionally, localized structure forms through the interaction of the mean field and turbulence. PMID:23829741
NASA Astrophysics Data System (ADS)
Graf von der Pahlen, J.; Tsiklauri, D.
2015-12-01
Magnetic X-point collapse is investigated using a 2.5D fully relativistic particle-in-cell simulation, with varying strengths of guide-field as well as open and closed boundary conditions. In the zero guide-field case we discover a new signature of Hall-reconnection in the out-of-plane magnetic field, namely an octupolar pattern, as opposed to the well-studied quadrupolar out-of-plane field of reconnection. The emergence of the octupolar components was found to be caused by ion currents and is a general feature of X-point collapse. In a comparative study of tearing-mode reconnection, signatures of octupolar components are found only in the out-flow region. It is argued that space-craft observations of magnetic fields at reconnection sites may be used accordingly to identify the type of reconnection [1][2]. Further, initial oscillatory reconnection is observed, prior to reconnection onset, generating electro-magnetic waves at the upper-hybrid frequency, matching solar flare progenitor emission. When applying a guide-field, in both open and closed boundary conditions, thinner dissipation regions are obtained and the onset of reconnection is increasingly delayed. Investigations with open boundary conditions show that, for guide-fields close to the strength of the in-plane field, shear flows emerge, leading to the formation of electron flow vortices and magnetic islands [3]. Asymmetries in the components of the generalised Ohm's law across the dissipation region are observed. Extended in 3D geometry, it is shown that locations of magnetic islands and vortices are not constant along the height of the current-sheet. Vortices formed on opposite sites of the current-sheet travel in opposite directions along it, leading to a criss-cross vortex pattern. Possible instabilities resulting from this specific structure formation are to be investigated [4].[1] J. Graf von der Pahlen and D. Tsiklauri, Phys. Plasmas 21, 060705 (2014), [2] J. Graf von der Pahlen and D. Tsiklauri
NASA Astrophysics Data System (ADS)
Cheng, Chio Z.; Ono, Yasushi; Yang, Ya-Hui; Choe, Gwangson
2012-10-01
Impulsively fast magnetic reconnection has been shown to be the major mechanism responsible for explosive flare non-thermal energy release and acceleration of coronal mass ejection (CME) motion. It has been observed that for most large solar flares non-thermal emissions in hard X-rays (HXR) and millimeter/submillimeter waves impulsively rise and decade during the soft X-ray (SXR) emission rise phase. Moreover, the filament/CME upward motion is accelerated temporally in correlation with the impulsive enhancement of flare non-thermal emission and reconnection electric field in the current sheet in both simulations and observations. The peak reconnection electric field during flare impulsive phase is on the order of a few kV/m for X-class flares. Here, we demonstrated for the first time in laboratory plasma merging experiments the correlation of the magnetic reconnection rate with the acceleration of plasmoid ejected from the current sheet using the TS-4 device of the Tokyo University. Moreover, we have also found that the electron heating occurs in the current sheet and the ion heating occurs in the down-stream outflow region. Thus, we conclude that the plasmoid/CME acceleration is a key mechanism for the impulsive enhancement of magnetic reconnection rate (electric field).
NASA Astrophysics Data System (ADS)
Pucci, Fulvia; Del Sarto, Daniele; Tenerani, Anna; Velli, Marco
2015-04-01
By examining sheets with thicknesses scaling as different powers of the Lundquist number S, we previously showed (Pucci and Velli, 2014) that the growth rate of the tearing mode increases as current sheets thin and, once the inverse aspect ratio reaches a scaling a/L = S-1/3, the time-scale for the instability to develop becomes of the order of the Alfvén time. That means that a fast instability sets in well before Sweet-Parker type current sheets can form. In addition, such an instability produces many islands in the sheet, leading to fast nonlinear evolution and most probably a turbulent disruption of the sheet itself. This has fundamental implications for magnetically driven reconnection throughout the corona, and in particular for coronal heating and the triggering of coronal mass ejections. Here we extend the study of reconnection instabilities to magnetic fields of grater complexity, displaying different current structures such as, for example, multiple or asymmetric current layers. We also consider the possibility of a Δ' dependence on wave-number k-p for different values of p, studying analogies and variations of the trigger scaling relation a/L ~ S-1/3 with respect to the Harris current sheet equilibrium. At large Lundquist numbers in typical Heliospheric plasmas kinetic effects become more important in Ohm's law: we consider the effects of electron skin depth reconnection, showing that we can define a trigger relation similar to the resistive case. The results are important to the transition to fast reconnection in the solar corona, solar wind, magnetosphere as well as laboratory plasmas. F. Pucci and M. Velli, "Reconnection of quasi-singular current sheets: the 'ideal" tearing mode" ApJ 780:L19, 2014.
Jan Egedal-Pedersen
2010-01-29
The study of the collisionless magnetic reconnection constituted the primary work carried out under this grant. The investigations utilized two magnetic configurations with distinct boundary conditions. Both configurations were based upon the Versatile Toroidal Facility (VTF). The first configuration is characterized by open boundary conditions where the magnetic field lines interface directly with the vacuum vessel walls. The reconnection dynamics for this configuration has been methodically characterized and it has been shown that kinetic effects related to trapped electron trajectories are responsible for the high rates of reconnection observed. This type of reconnection has not been investigated before. Nevertheless, the results are directly relevant to observations by the Wind spacecraft of fast reconnection deep in the Earth magnetotail. The second configuration was developed to be specifically relevant to numerical simulations of magnetic reconnection, allowing the magnetic field-lines to be contained inside the device. The configuration is compatible with the presence of large current sheets in the reconnection region and reconnection is observed in fast powerful bursts. These reconnection events facilitate the first experimental investigations of the physics governing the spontaneous onset of fast reconnection. In this Report we review the general motivation of this work, the experimental set-up, and the main physics results.
Multi-Scale Modeling of Magnetospheric Reconnection
NASA Technical Reports Server (NTRS)
Kuznetsova, M. M.; Hesse, M.; Rastatter, L.; Toth, G.; Dezeeuw, D.; Gomobosi, T.
2007-01-01
One of the major challenges in modeling the magnetospheric magnetic reconnection is to quantify the interaction between large-scale global magnetospheric dynamics and microphysical processes in diffusion regions near reconnection sites. There is still considerable debate as to what degree microphysical processes on kinetic scales affect the global evolution and how important it is to substitute numerical dissipation and/or ad hoc anomalous resistivity by a physically motivated model of dissipation. Comparative studies of magnetic reconnection in small scale geometries demonstrated that MHD simulations that included non-ideal processes in terms of a resistive term $\\eta J$ did not produce the fast reconnection rates observed in kinetic simulations. For a broad range of physical parameters in collisionless magnetospheric plasma, the primary mechanism controlling the dissipation in the vicinity of the reconnection site is non-gyrotropic effects with spatial scales comparable with the particle Larmor radius. We utilize the global MHD code BATSRUS and incorporate nongyrotropic effects in diffusion regions in terms of corrections to the induction equation. We developed an algorithm to search for magnetotail reconnection sites, specifically where the magnetic field components perpendicular to the local current direction approaches zero and form an X-type configuration. Spatial scales of the diffusion region and magnitude of the reconnection electric field are calculated selfconsistently using MHD plasma and field parameters in the vicinity of the reconnection site. The location of the reconnection sites is updated during the simulations. To clarify the role of nongyrotropic effects in diffusion region on the global magnetospheric dynamic we perform simulations with steady southward IMF driving of the magnetosphere. Ideal MHD simulations with magnetic reconnection supported by numerical resistivity produce steady configuration with almost stationary near-earth neutral
Final Report: Laboratory Studies of Spontaneous Reconnection and Intermittent Plasma Objects
Egedal-Pedersen, Jan; Porkolab, Miklos
2011-05-31
The study of the collisionless magnetic reconnection constituted the primary work carried out under this grant. The investigations utilized two magnetic configurations with distinct boundary conditions. Both configurations were based upon the Versatile Toroidal Facility (VTF) at the MIT Plasma Science and Fusion Center and the MIT Physics Department. The NSF/DOE award No. 0613734, supported two graduate students (now Drs. W. Fox and N. Katz) and material expenses. The grant enabled these students to operate the VTF basic plasma physics experiment on magnetic reconnection. The first configuration was characterized by open boundary conditions where the magnetic field lines interface directly with the vacuum vessel walls. The reconnection dynamics for this configuration has been methodically characterized and it has been shown that kinetic effects related to trapped electron trajectories are responsible for the high rates of reconnection observed. This type of reconnection has not been investigated before. Nevertheless, the results are directly relevant to observations by the Wind spacecraft of fast reconnection deep in the Earth magnetotail. The second configuration was developed to be relevant to specifically to numerical simulations of magnetic reconnection, allowing the magnetic field-lines to be contained inside the device. The configuration is compatible with the presence of large current sheets in the reconnection region and reconnection is observed in fast powerful bursts. These reconnection events facilitate the first experimental investigations of the physics governing the spontaneous onset of fast reconnection. In the Report we review the general motivation of this work and provide an overview of our experimental and theoretical results enabled by the support through the awards.
Masaaki Yamada, Russell Kulsrud and Hantao Ji
2009-09-17
We review the fundamental physics of magnetic reconnection in laboratory and space plasmas, by discussing results from theory, numerical simulations, observations from space satellites, and the recent results from laboratory plasma experiments. After a brief review of the well-known early work, we discuss representative recent experimental and theoretical work and attempt to interpret the essence of significant modern findings. In the area of local reconnection physics, many significant findings have been made with regard to two- uid physics and are related to the cause of fast reconnection. Profiles of the neutral sheet, Hall currents, and the effects of guide field, collisions, and micro-turbulence are discussed to understand the fundamental processes in a local reconnection layer both in space and laboratory plasmas. While the understanding of the global reconnection dynamics is less developed, notable findings have been made on this issue through detailed documentation of magnetic self-organization phenomena in fusion plasmas. Application of magnetic reconnection physics to astrophysical plasmas is also brie y discussed.
NASA Astrophysics Data System (ADS)
Kichigin, G. N.
2016-01-01
Solutions describing solitary fast magnetosonic (FMS) waves (FMS solitons) in cold magnetized plasma are obtained by numerically solving two-fluid hydrodynamic equations. The parameter domain within which steady-state solitary waves can propagate is determined. It is established that the Mach number for rarefaction FMS solitons is always less than unity. The restriction on the propagation velocity leads to the limitation on the amplitudes of the magnetic field components of rarefaction solitons. It is shown that, as the soliton propagates in plasma, the transverse component of its magnetic field rotates and makes a complete turn around the axis along which the soliton propagates.
Frontiers for Laboratory Research of Magnetic Reconnection
Ji, Hantao; Guo, Fan
2015-07-16
Magnetic reconnection occcurs throughout heliophysical and astrophysical plasmas as well as in laboratory fusion plasmas. Two broad categories of reconnection models exist: collisional MHD and collisionless kinetic. Eight major questions with respect to magnetic connection are set down, and past and future devices for studying them in the laboratory are described. Results of some computerized simulations are compared with experiments.
Magnetic reconnection physics in the solar wind with Voyager 2
NASA Astrophysics Data System (ADS)
Stevens, Michael L.
2009-08-01
Magnetic reconnection is the process by which the magnetic topology evolves in collisionless plasmas. This phenomenon is fundamental to a broad range of astrophysical processes such as stellar flares, magnetospheric substorms, and plasma accretion, yet it is poorly understood and difficult to observe in situ . In this thesis, the solar wind plasma permeating interplanetary space is treated as a laboratory for reconnection physics. I present an exhaustive statistical approach to the identification of reconnection outflow jets in turbulent plasma and magnetic field time series data. This approach has been automated and characterized so that the resulting reconnection survey can be put in context with other related studies. The algorithm is shown to perform similarly to ad hoc studies in the inner heliosphere. Based on this technique, I present a survey of 138 outflow jets for the Voyager 2 spacecraft mission, including the most distant in situ evidence of reconnection discovered to date. Reconnection in the solar wind is shown to be strongly correlated with stream interactions and with solar activity. The solar wind magnetic field is found to be reconnecting via large, quasi-steady slow- mode magnetohydrodynamic structures as far out as the orbit of Neptune. The role of slow-mode shocks is explored and, in one instance, a well-developed reconnection structure is shown to be in good agreement with the Petschek theory for fast reconnection. This is the first reported example of a reconnection exhaust that satisfies the full jump conditions for a stationary slow-mode shock pair. A complete investigation into corotating stream interactions over the Voyager 2 mission has revealed that detectable reconnection structure occurs in about 23% of forced, global-scale current sheets. Contrary to previous studies, I find that signatures of this kind are most likely to be observed for current sheets where the magnetic field shear and the plasma-b are high. Evidence has been found
Rosenberg, M. J.; Li, C. K.; Fox, W.; Zylstra, A. B.; Stoeckl, C.; Séguin, F. H.; Frenje, J. A.; Petrasso, R. D.
2015-05-20
An evolution of magnetic reconnection behavior, from fast jets to the slowing of reconnection and the establishment of a stable current sheet, has been observed in strongly-driven, β ≲ 20 laser-produced plasma experiments. This process has been inferred to occur alongside a slowing of plasma inflows carrying the oppositely-directed magnetic fields as well as the evolution of plasma conditions from collisionless to collisional. High-resolution proton radiography has revealed unprecedented detail of the forced interaction of magnetic fields and super-Alfvénic electron jets (Vjet~ 20VA) ejected from the reconnection region, indicating that two-fluid or collisionless magnetic reconnection occurs early in time. Themore » absence of jets and the persistence of strong, stable magnetic fields at late times indicates that the reconnection process slows down, while plasma flows stagnate and plasma conditions evolve to a cooler, denser, more collisional state. These results demonstrate that powerful initial plasma flows are not sufficient to force a complete reconnection of magnetic fields, even in the strongly-driven regime.« less
Rosenberg, M. J.; Li, C. K.; Fox, W.; Zylstra, A. B.; Stoeckl, C.; Séguin, F. H.; Frenje, J. A.; Petrasso, R. D.
2015-05-20
An evolution of magnetic reconnection behavior, from fast jets to the slowing of reconnection and the establishment of a stable current sheet, has been observed in strongly-driven, β ≲ 20 laser-produced plasma experiments. This process has been inferred to occur alongside a slowing of plasma inflows carrying the oppositely-directed magnetic fields as well as the evolution of plasma conditions from collisionless to collisional. High-resolution proton radiography has revealed unprecedented detail of the forced interaction of magnetic fields and super-Alfvénic electron jets (V_{jet}~ 20V_{A}) ejected from the reconnection region, indicating that two-fluid or collisionless magnetic reconnection occurs early in time. The absence of jets and the persistence of strong, stable magnetic fields at late times indicates that the reconnection process slows down, while plasma flows stagnate and plasma conditions evolve to a cooler, denser, more collisional state. These results demonstrate that powerful initial plasma flows are not sufficient to force a complete reconnection of magnetic fields, even in the strongly-driven regime.
NASA Astrophysics Data System (ADS)
Rosenberg, M. J.; Li, C. K.; Fox, W.; Zylstra, A. B.; Stoeckl, C.; Séguin, F. H.; Frenje, J. A.; Petrasso, R. D.
2015-05-01
An evolution of magnetic reconnection behavior, from fast jets to the slowing of reconnection and the establishment of a stable current sheet, has been observed in strongly driven, β ≲20 laser-produced plasma experiments. This process has been inferred to occur alongside a slowing of plasma inflows carrying the oppositely directed magnetic fields as well as the evolution of plasma conditions from collisionless to collisional. High-resolution proton radiography has revealed unprecedented detail of the forced interaction of magnetic fields and super-Alfvénic electron jets (Vjet˜20 VA ) ejected from the reconnection region, indicating that two-fluid or collisionless magnetic reconnection occurs early in time. The absence of jets and the persistence of strong, stable magnetic fields at late times indicates that the reconnection process slows down, while plasma flows stagnate and plasma conditions evolve to a cooler, denser, more collisional state. These results demonstrate that powerful initial plasma flows are not sufficient to force a complete reconnection of magnetic fields, even in the strongly driven regime.
Rosenberg, M J; Li, C K; Fox, W; Zylstra, A B; Stoeckl, C; Séguin, F H; Frenje, J A; Petrasso, R D
2015-05-22
An evolution of magnetic reconnection behavior, from fast jets to the slowing of reconnection and the establishment of a stable current sheet, has been observed in strongly driven, β≲20 laser-produced plasma experiments. This process has been inferred to occur alongside a slowing of plasma inflows carrying the oppositely directed magnetic fields as well as the evolution of plasma conditions from collisionless to collisional. High-resolution proton radiography has revealed unprecedented detail of the forced interaction of magnetic fields and super-Alfvénic electron jets (V_{jet}∼20V_{A}) ejected from the reconnection region, indicating that two-fluid or collisionless magnetic reconnection occurs early in time. The absence of jets and the persistence of strong, stable magnetic fields at late times indicates that the reconnection process slows down, while plasma flows stagnate and plasma conditions evolve to a cooler, denser, more collisional state. These results demonstrate that powerful initial plasma flows are not sufficient to force a complete reconnection of magnetic fields, even in the strongly driven regime. PMID:26047236
Laboratory Experiment of Magnetic Reconnection between Merging Flux Tubes with Strong Guide FIeld
NASA Astrophysics Data System (ADS)
Inomoto, M.; Kamio, S.; Kuwahata, A.; Ono, Y.
2013-12-01
Magnetic reconnection governs variety of energy release events in the universe, such as solar flares, geomagnetic substorms, and sawtooth crash in laboratory nuclear fusion experiments. Differently from the classical steady reconnection models, non-steady behavior of magnetic reconnection is often observed. In solar flares, intermittent enhancement of HXR emission is observed synchronously with multiple ejection of plammoids [1]. In laboratory reconnection experiments, the existence of the guide field, that is perpendicular to the reconnection field, makes significant changes on reconnection process. Generally the guide field will slow down the reconnection rate due to the increased magnetic pressure inside the current sheet. It also brings about asymmetric structure of the separatrices or effective particle acceleration in collisionless conditions. We have conducted laboratory experiments to study the behavior of the guide-field magnetic reconnection using plasma merging technique (push reconnection). Under substantial guide field even larger than the reconnection field, the reconnection generally exhibits non-steady feature which involves intermittent detachment of X-point and reconnection current center[2]. Transient enhancement of reconnection rate is observed simultaneously with the X-point motion[3]. We found two distinct phenomena associated with the guide-field non-steady reconnection. The one is the temporal and localized He II emission from X-point region, suggesting the production of energetic electrons which could excite the He ions in the vicinity of the X-point. The other is the excitation of large-amplitude electromagnetic waves which have similar properties with kinetic Alfven waves, whose amplitude show positive correlation with the enhancement of the reconnection electric field[4]. Electron beam instability caused by the energetic electrons accelerated to more than twice of the electron thermal velocity could be a potential driver of the
Magnetic reconnection in space
Boozer, Allen H.
2012-09-15
Models of magnetic reconnection in space plasmas generally consider only a segment of the magnetic field lines. The consideration of only a segment of the lines is shown to lead to paradoxical results in which reconnection can be impossible even in a magnetic field constrained to be curl free or can be at an Alfven rate even when the plasma is a perfect conductor. A model of reconnecting magnetic fields is developed which shows the smallness of the interdiffusion distance {delta}{sub d} of magnetic field lines does not limit the speed of reconnection but does provide a reconnection trigger. When the reconnection region has a natural length L{sub r}, the spatial scale of the gradient of magnetic field across the magnetic field lines must reach L{sub g} Almost-Equal-To 0.3L{sub r}/ln(L{sub r}/{delta}{sub d}) for fast reconnection to be triggered, which implies a current density j Almost-Equal-To B/{mu}{sub 0}L{sub g} that is far lower than that usually thought required for fast reconnection. The relation between magnetic reconnection in space and in toroidal laboratory plasmas is also discussed.
Gu, Y J; Klimo, O; Kumar, D; Liu, Y; Singh, S K; Esirkepov, T Zh; Bulanov, S V; Weber, S; Korn, G
2016-01-01
The magnetic quadrupole structure formation during the interaction of two ultrashort high power laser pulses with a collisionless plasma is demonstrated with 2.5-dimensional particle-in-cell simulations. The subsequent expansion of the quadrupole is accompanied by magnetic-field annihilation in the ultrarelativistic regime, when the magnetic field cannot be sustained by the plasma current. This results in a dominant contribution of the displacement current exciting a strong large scale electric field. This field leads to the conversion of magnetic energy into kinetic energy of accelerated electrons inside the thin current sheet. PMID:26871179
NASA Technical Reports Server (NTRS)
Kuznetsova, M. M.; Hesse, M.; Rastaetter, L.; Toth, G.; DeZeeuw, D. L.; Gombosi, T. I.
2008-01-01
Magnetotail current sheet thinning and magnetic reconnection are key elements of magnetospheric substorms. We utilized the global MHD model BATS-R-US with Adaptive Mesh Refinement developed at the University of Michigan to investigate the formation and dynamic evolution of the magnetotail thin current sheet. The BATSRUS adaptive grid structure allows resolving magnetotail regions with increased current density up to ion kinetic scales. We investigated dynamics of magnetotail current sheet thinning in response to southwards IMF turning. Gradual slow current sheet thinning during the early growth phase become exponentially fast during the last few minutes prior to nightside reconnection onset. The later stage of current sheet thinning is accompanied by earthward flows and rapid suppression of normal magnetic field component $B-z$. Current sheet thinning set the stage for near-earth magnetic reconnection. In collisionless magnetospheric plasma, the primary mechanism controlling the dissipation in the vicinity of the reconnection site is non-gyrotropic effects with spatial scales comparable with the particle Larmor radius. One of the major challenges in global MHD modeling of the magnetotail magnetic reconnection is to reproduce fast reconnection rates typically observed in smallscale kinetic simulations. Bursts of fast reconnection cause fast magnetic field reconfiguration typical for magnetospheric substorms. To incorporate nongyritropic effects in diffusion regions we developed an algorithm to search for magnetotail reconnection sites, specifically where the magnetic field components perpendicular to the local current direction approaches zero and form an X-type configuration. Spatial scales of the diffusion region and magnitude of the reconnection electric field are calculated self-consistently using MHD plasma and field parameters in the vicinity of the reconnection site. The location of the reconnection sites and spatial scales of the diffusion region are updated
Sub-grid-scale description of turbulent magnetic reconnection in magnetohydrodynamics
NASA Astrophysics Data System (ADS)
Widmer, F.; Büchner, J.; Yokoi, N.
2016-04-01
Magnetic reconnection requires, at least locally, a non-ideal plasma response. In collisionless space and astrophysical plasmas, turbulence could transport energy from large to small scales where binary particle collisions are rare. We have investigated the influence of small scale magnetohydrodynamics (MHD) turbulence on the reconnection rate in the framework of a compressible MHD approach including sub-grid-scale (SGS) turbulence. For this sake, we considered Harris-type and force-free current sheets with finite guide magnetic fields directed out of the reconnection plane. The goal is to find out whether unresolved by conventional simulations MHD turbulence can enhance the reconnection process in high-Reynolds-number astrophysical plasmas. Together with the MHD equations, we solve evolution equations for the SGS energy and cross-helicity due to turbulence according to a Reynolds-averaged turbulence model. The SGS turbulence is self-generated and -sustained through the inhomogeneities of the mean fields. By this way, the feedback of the unresolved turbulence into the MHD reconnection process is taken into account. It is shown that the turbulence controls the regimes of reconnection by its characteristic timescale τt. The dependence on resistivity was investigated for large-Reynolds-number plasmas for Harris-type as well as force-free current sheets with guide field. We found that magnetic reconnection depends on the relation between the molecular and apparent effective turbulent resistivity. We found that the turbulence timescale τt decides whether fast reconnection takes place or whether the stored energy is just diffused away to small scale turbulence. If the amount of energy transferred from large to small scales is enhanced, fast reconnection can take place. Energy spectra allowed us to characterize the different regimes of reconnection. It was found that reconnection is even faster for larger Reynolds numbers controlled by the molecular resistivity η, as
NASA Astrophysics Data System (ADS)
McLaughlin, J. A.; De Moortel, I.; Hood, A. W.; Brady, C. S.
2009-01-01
Context: This paper extends the models of Craig & McClymont (1991, ApJ, 371, L41) and McLaughlin & Hood (2004, A&A, 420, 1129) to include finite β and nonlinear effects. Aims: We investigate the nature of nonlinear fast magnetoacoustic waves about a 2D magnetic X-point. Methods: We solve the compressible and resistive MHD equations using a Lagrangian remap, shock capturing code (Arber et al. 2001, J. Comp. Phys., 171, 151) and consider an initial condition in {v}×{B} \\cdot {hat{z}} (a natural variable of the system). Results: We observe the formation of both fast and slow oblique magnetic shocks. The nonlinear wave deforms the X-point into a “cusp-like” point which in turn collapses to a current sheet. The system then evolves through a series of horizontal and vertical current sheets, with associated changes in connectivity, i.e. the system exhibits oscillatory reconnection. Our final state is non-potential (but in force balance) due to asymmetric heating from the shocks. Larger amplitudes in our initial condition correspond to larger values of the final current density left in the system. Conclusions: The inclusion of nonlinear terms introduces several new features to the system that were absent from the linear regime. A movie is available in electronic form at http://www.aanda.org
Radiative tearing - Magnetic reconnection on a fast thermal-instability time scale
NASA Technical Reports Server (NTRS)
Steinolfson, R. S.; Van Hoven, G.
1984-01-01
Two energy modification mechanisms which are known to occur in sheared magnetic fields are the tearing and thermal instabilities. These processes can be studied separately with formalisms incorporating just the effective driving mechanism of interest (finite resistivity for the tearing mode and unstable radiation for the thermal mode). A model which includes both effects, and a temperature-dependent resistivity, indicates that modified forms of these two instabilities may coexist for identical physical conditions. When they are isolated computationally, one can show that their limiting growth rates are approximately those of the uncoupled instabilities. The spatial structure and energy content of these two new hybrid processes are then individually examined and are found to differ considerably from those obtained from separate treatments of the driving mechanisms. The faster radiative instability, which has a hydromagnetically scaled growth rate like the condensation mode of the thermal instability, is shown to involve a substantial amount of magnetic field reconnection. This can be partially explained by a large temperature drop (or resistivity rise) at the X-point. The island width of the Coulomb-coupled radiative mode is 30 percent of that produced by a comparable level of the slower tearing instability. In addition, the perturbed magnetic energy in the radiative instability is 5 times that of the perturbed thermal energy, indicating an appreciable modification of the initial magnetic structure.
Collisional Behaviors of Astrophysical Collisionless Plasmas
NASA Astrophysics Data System (ADS)
Bret, A.
2015-12-01
In collisional fluids, a number of key processes rely on the frequency of binary collisions. Collisions seem necessary to generate a shock wave when two fluids collide fast enough, to fulfill the Rankine-Hugoniot (RH) relations, to establish an equation of state or a Maxwellian distribution. Yet, these seemingly collisional features are routinely either observed or assumed, in relation with collisionless astrophysical plasmas. This article will review our current answers to the following questions: How do colliding collisionless plasmas end-up generating a shock as if they were fluids? To which extent are the RH relations fulfilled in this case? Do collisionless shocks propagate like fluid ones? Can we use an equation of state to describe collisionless plasmas, like MHD codes for astrophysics do? Why are Maxwellian distributions ubiquitous in particle-in-cell simulations of collisionless shocks? Time and length scales defining the border between the collisional and the collisionless behavior will be given when relevant. In general, when the time and length scales involved in the collisionless processes responsible for the fluid-like behavior may be neglected, the system may be treated like a fluid.
NASA Astrophysics Data System (ADS)
Eastwood, J. P.; Goldman, M. V.; Zhang, X.; Hietala, H.; Krupar, V.; Newman, D. L.; Angelopoulos, V.; Lapenta, G.
2015-12-01
Whistler waves are a ubiquitous plasma phenomenon, observed in a variety of space and laboratory plasma environments. They play a key role in many important and diverse processes, such as particle acceleration in the radiation belts and auroral acceleration region, the dissipation of plasma turbulence at small scales below the inertial range, collisionless shock physics, and magnetic reconnection. In reconnection they may modify the reconnection rate and also whistler physics is crucial to enabling fast reconnection in the Hall reconnection model. Consequently, understanding how whistler waves are generated and how they subsequently interact with the plasma is a problem of wide importance and application. It is well known that whistlers can arise as a result of kinetic instabilities, which grow exponentially from noise as a consequence of unstable electron distributions, for example temperature anisotropy. This is used ubiquitously to predict where and when whistler waves are likely to exist and therefore be of importance in many plasma phenomena. Recently it has been demonstrated theoretically and via computer simulations that whistler waves may also arise via Cerenkov emission from electron hole quasi-particles [Goldman et al., PRL, 2014]. Such wave emission can arise even when the temperature anisotropy leads to damping; in this case the system is analogous to a damped forced oscillator. Here we present novel experimental analysis from THEMIS showing for the first time evidence consistent with the generation of whistlers via Cerenkov emission during magnetotail reconnection. By considering the electromagnetic properties of the electron holes, the amplitude, phase speed and frequency of the associated whistlers, and also the available sub-spin observations of the electron distribution function, we find that the data are best explained by the Cerenkov emission theory rather than by kinetic instabilities due to the electron temperature anisotropy. Whilst the
SELF-ORGANIZATION OF RECONNECTING PLASMAS TO MARGINAL COLLISIONALITY IN THE SOLAR CORONA
Imada, S.; Zweibel, E. G.
2012-08-20
We explore the suggestions by Uzdensky and Cassak et al. that coronal loops heated by magnetic reconnection should self-organize to a state of marginal collisionality. We discuss their model of coronal loop dynamics with a one-dimensional hydrodynamic calculation. We assume that many current sheets are present, with a distribution of thicknesses, but that only current sheets thinner than the ion skin depth can rapidly reconnect. This assumption naturally causes a density-dependent heating rate which is actively regulated by the plasma. We report nine numerical simulation results of coronal loop hydrodynamics in which the absolute values of the heating rates are different but their density dependences are the same. We find two regimes of behavior, depending on the amplitude of the heating rate. In the case that the amplitude of heating is below a threshold value, the loop is in stable equilibrium. Typically, the upper and less dense part of a coronal loop is collisionlessly heated and conductively cooled. When the amplitude of heating is above the threshold, the conductive flux to the lower atmosphere required to balance collisionless heating drives an evaporative flow which quenches fast reconnection, ultimately cooling and draining the loop until the cycle begins again. The key elements of this cycle are gravity and the density dependence of the heating function. Some additional factors are present, including pressure-driven flows from the loop top, which carry a large enthalpy flux and play an important role in reducing the density. We find that on average the density of the system is close to the marginally collisionless value.
Study of the effects of guide field on Hall reconnection
Tharp, T. D.; Yamada, M.; Ji, H.; Lawrence, E.; Dorfman, S.; Myers, C.; Yoo, J.; Huang, Y.-M.; Bhattacharjee, A.
2013-05-15
The results from guide field studies on the Magnetic Reconnection Experiment (MRX) are compared with results from Hall magnetohydrodynamic (HMHD) reconnection simulation with guide field. The quadrupole field, a signature of two-fluid reconnection at zero guide field, is modified by the presence of a finite guide field in a manner consistent with HMHD simulation. The modified Hall current profile contains reduced electron flows in the reconnection plane, which quantitatively explains the observed reduction of the reconnection rate. The present results are consistent with the hypothesis that the local reconnection dynamics is dominated by Hall effects in the collisionless regime of the MRX plasmas. While very good agreement is seen between experiment and simulations, we note that an important global feature of the experiments, a compression of the guide field by the reconnecting plasma, is not represented in the simulations.
BEAMING AND RAPID VARIABILITY OF HIGH-ENERGY RADIATION FROM RELATIVISTIC PAIR PLASMA RECONNECTION
Cerutti, B.; Werner, G. R.; Uzdensky, D. A.; Begelman, M. C. E-mail: greg.werner@colorado.edu E-mail: mitch@jila.colorado.edu
2012-08-01
We report on the first study of the angular distribution of energetic particles and radiation generated in relativistic collisionless electron-positron pair plasma reconnection using two-dimensional particle-in-cell simulations. We discover a strong anisotropy of the particles accelerated by reconnection and the associated strong beaming of their radiation. The focusing of particles and radiation increases with their energy; in this sense, this 'kinetic beaming' effect differs fundamentally from the relativistic Doppler beaming usually invoked in high-energy astrophysics, in which all photons are focused and boosted achromatically. We also present, for the first time, the modeling of the synchrotron emission as seen by an external observer during the reconnection process. The expected light curves comprise several bright symmetric sub-flares emitted by the energetic beam of particles sweeping across the line of sight intermittently, and exhibit super-fast time variability as short as about one-tenth of the system light-crossing time. The concentration of the energetic particles into compact regions inside magnetic islands and particle anisotropy explain the rapid variability. This radiative signature of reconnection can account for the brightness and variability of the gamma-ray flares in the Crab Nebula and in blazars.
Application of PDSLin to the magnetic reconnection problem
NASA Astrophysics Data System (ADS)
Yuan, Xuefei; Li, Xiaoye S.; Yamazaki, Ichitaro; Jardin, Stephen C.; Koniges, Alice E.; Keyes, David E.
2013-01-01
Magnetic reconnection is a fundamental process in a magnetized plasma at both low and high magnetic Lundquist numbers (the ratio of the resistive diffusion time to the Alfvén wave transit time), which occurs in a wide variety of laboratory and space plasmas, e.g. magnetic fusion experiments, the solar corona and the Earth's magnetotail. An implicit time advance for the two-fluid magnetic reconnection problem is known to be difficult because of the large condition number of the associated matrix. This is especially troublesome when the collisionless ion skin depth is large so that the Whistler waves, which cause the fast reconnection, dominate the physics (Yuan et al 2012 J. Comput. Phys. 231 5822-53). For small system sizes, a direct solver such as SuperLU can be employed to obtain an accurate solution as long as the condition number is bounded by the reciprocal of the floating-point machine precision. However, SuperLU scales effectively only to hundreds of processors or less. For larger system sizes, it has been shown that physics-based (Chacón and Knoll 2003 J. Comput. Phys. 188 573-92) or other preconditioners can be applied to provide adequate solver performance. In recent years, we have been developing a new algebraic hybrid linear solver, PDSLin (Parallel Domain decomposition Schur complement-based Linear solver) (Yamazaki and Li 2010 Proc. VECPAR pp 421-34 and Yamazaki et al 2011 Technical Report). In this work, we compare numerical results from a direct solver and the proposed hybrid solver for the magnetic reconnection problem and demonstrate that the new hybrid solver is scalable to thousands of processors while maintaining the same robustness as a direct solver.
Magnetohydrodynamic simulations of turbulent magnetic reconnection
Fan Quanlin; Feng Xueshang; Xiang Changqing
2004-12-01
Turbulent reconnection process in a one-dimensional current sheet is investigated by means of a two-dimensional compressible one-fluid magnetohydrodynamic simulation with spatially uniform, fixed resistivity. Turbulence is set up by adding to the sheet pinch small but finite level of broadband random-phased magnetic field components. To clarify the nonlinear spatial-temporal nature of the turbulent reconnection process the reconnection system is treated as an unforced initial value problem without any anomalous resistivity model adopted. Numerical results demonstrate the duality of turbulent reconnection, i.e., a transition from Sweet-Parker-like slow reconnection to Petschek-like fast reconnection in its nonlinear evolutionary process. The initial slow reconnection phase is characterized by many independent microreconnection events confined within the sheet region and a global reconnection rate mainly dependent on the initially added turbulence and insensitive to variations of the plasma {beta} and resistivity. The formation and amplification of the major plasmoid leads the following reconnection process to a rapid reconnection stage with a fast reconnection rate of the order of 0.1 or even larger, drastically changing the topology of the global magnetic field. That is, the presence of magnetohydrodynamic turbulence in large-scale current sheets can raise the reconnection rate from small values on the order of the Sweet-Parker rate to high values on the order of the Petscheck rate through triggering an evolution toward fast magnetic reconnection. Meanwhile, the backward coupling between the small- and large-scale magnetic field dynamics has been properly represented through the present high resolution simulation. The undriven turbulent reconnection model established here expresses a solid numerical basis for the previous schematic two-step magnetic reconnection models and a possible explanation of two-stage energy release process of solar explosives.
A Reconnection Switch to Trigger gamma-Ray Burst Jet Dissipation
McKinney, Jonathan C.; Uzdensky, Dmitri A.
2012-03-14
Prompt gamma-ray burst (GRB) emission requires some mechanism to dissipate an ultrarelativistic jet. Internal shocks or some form of electromagnetic dissipation are candidate mechanisms. Any mechanism needs to answer basic questions, such as what is the origin of variability, what radius does dissipation occur at, and how does efficient prompt emission occur. These mechanisms also need to be consistent with how ultrarelativistic jets form and stay baryon pure despite turbulence and electromagnetic reconnection near the compact object and despite stellar entrainment within the collapsar model. We use the latest magnetohydrodynamical models of ultrarelativistic jets to explore some of these questions in the context of electromagnetic dissipation due to the slow collisional and fast collisionless reconnection mechanisms, as often associated with Sweet-Parker and Petschek reconnection, respectively. For a highly magnetized ultrarelativistic jet and typical collapsar parameters, we find that significant electromagnetic dissipation may be avoided until it proceeds catastrophically near the jet photosphere at large radii (r {approx} 10{sup 13}-10{sup 14}cm), by which the jet obtains a high Lorentz factor ({gamma} {approx} 100-1000), has a luminosity of L{sub j} {approx} 10{sup 50}-10{sup 51} erg s{sup -1}, has observer variability timescales of order 1s (ranging from 0.001-10s), achieves {gamma}{theta}{sub j} {approx} 10-20 (for opening half-angle {theta}{sub j}) and so is able to produce jet breaks, and has comparable energy available for both prompt and afterglow emission. A range of model parameters are investigated and simplified scaling laws are derived. This reconnection switch mechanism allows for highly efficient conversion of electromagnetic energy into prompt emission and associates the observed prompt GRB pulse temporal structure with dissipation timescales of some number of reconnecting current sheets embedded in the jet. We hope this work helps motivate the
A Rosetta Stone for in situ Observations of Magnetic Reconnection
NASA Astrophysics Data System (ADS)
Scudder, J. D.; Daughton, W. S.; Karimabadi, H.; Roytershteyn, V.
2015-12-01
Local conditions that constrain the physics of magnetic reconnection in space in 3D will be discussed, including those observable conditions presently used and new ones that enhance experimental closure. Three classes of tests will be discussed: i) proxies for unmeasurable theoretical properties II) observable properties satisfied by all layers that pass mass flux, including those of the reconnection layer, and (iii) observable kinetic tests that are increasingly peculiar to collisionless magnetic reconnection. A Rosetta Stone of state of the art observables will be proposed, including proxies for unmeasurable theoretical local rate of frozen flux violation and measures of the significance of frozen flux encountered. A suite of kinetic observables involving properties peculiar to electrons will also be demonstrated as promising litmus tests for certifying sites of collisionless magnetic reconnection.
Goldman, M V; Newman, D L; Lapenta, G; Andersson, L; Gosling, J T; Eriksson, S; Markidis, S; Eastwood, J P; Ergun, R
2014-04-11
Kinetic simulations of magnetotail reconnection have revealed electromagnetic whistlers originating near the exhaust boundary and propagating into the inflow region. The whistler production mechanism is not a linear instability, but rather is Čerenkov emission of almost parallel whistlers from localized moving clumps of charge (finite-size quasiparticles) associated with nonlinear coherent electron phase space holes. Whistlers are strongly excited by holes without ever growing exponentially. In the simulation the whistlers are emitted in the source region from holes that accelerate down the magnetic separatrix towards the x line. The phase velocity of the whistlers vφ in the source region is everywhere well matched to the hole velocity vH as required by the Čerenkov condition. The simulation shows emission is most efficient near the theoretical maximum vφ=half the electron Alfven speed, consistent with the new theoretical prediction that faster holes radiate more efficiently. While transferring energy to whistlers the holes lose coherence and dissipate over a few local ion inertial lengths. The whistlers, however, propagate to the x line and out over many 10's of ion inertial lengths into the inflow region of reconnection. As the whistlers pass near the x line they modulate the rate at which magnetic field lines reconnect. PMID:24765977
Magnetic reconnection under anisotropic magnetohydrodynamic approximation
Hirabayashi, K.; Hoshino, M.
2013-11-15
We study the formation of slow-mode shocks in collisionless magnetic reconnection by using one- and two-dimensional collisionless MHD codes based on the double adiabatic approximation and the Landau closure model. We bridge the gap between the Petschek-type MHD reconnection model accompanied by a pair of slow shocks and the observational evidence of the rare occasion of in-situ slow shock observations. Our results showed that once magnetic reconnection takes place, a firehose-sense (p{sub ∥}>p{sub ⊥}) pressure anisotropy arises in the downstream region, and the generated slow shocks are quite weak comparing with those in an isotropic MHD. In spite of the weakness of the shocks, however, the resultant reconnection rate is 10%–30% higher than that in an isotropic case. This result implies that the slow shock does not necessarily play an important role in the energy conversion in the reconnection system and is consistent with the satellite observation in the Earth's magnetosphere.
Dynamical plasma response during driven magnetic reconnection.
Egedal, J; Fasoli, A; Nazemi, J
2003-04-01
Direct measurements of a collisionless current channel during driven magnetic reconnection are obtained for the first time on the Versatile Toroidal Facility. The size of the diffusion region is found to scale with the electron drift orbit width, independent of the ion mass and plasma density. Based on experimental observations, analytic expressions governing the dynamical evolution of the current profile and the formation of the electrostatic potential that develops in response to the externally imposed reconnection drive are established. This time response is closely linked to the presence of ion polarization currents. PMID:12689297
Kinetic theory of collisionless tearing at the magnetopause
NASA Astrophysics Data System (ADS)
Daughton, William; Karimabadi, H.
2005-03-01
electron gyroscale in the guide field. During the onset phase, these structures represent a distinct signature of the collisionless tearing mode that is significantly different than the typical ion-scale quadrupole pattern from fast reconnection. Finally, we note that the tearing mode maintains a significant growth rate over a large range of guide field so that component merging cannot be ruled out based on linear theory.
This science visualization shows a magnetospheric substorm, during which, magnetic reconnection causes energy to be rapidly released along the field lines in the magnetotail, that part of the magne...
Evolutions of nonsteady state magnetic reconnection
Wan, Weigang; Lapenta, Giovanni
2008-01-01
The full evolutions of collisionless non-steady-state magnetic reconnection are studied with full kinetic particle-in-cell simulations. There are different stages of reconnection: the onset or early growing stage when the out-of-plane electric field (Ey) structure is a monopole at the X-point, the bipolar stage when the Ey structure is bipolar and the outer electron diffusion region (EDR) is being elongated over time, and the possible final steady-state stage when E{sub y} is uniform in the reconnection plane. We find the change of reconnection rate is not empowered or dependent on the length of the EDR. During the early growing stage, the EDR is elongated while the reconnection rate is growing. During the later stage, the reconnection rate may significantly decrease but the length of the inner EDR is largely stable. The results indicate that reconnection is not controlled by the downstream physics, but rather by the availability of plasma inflows from upstream. The physical mechanism of the EDR elongation is studied. The Hall current induced by the quadrupole magnetic field (B{sub y}) is discovered to play an important role in this process. The condition of forming an extended electron super-Alfvenic outflow jet structure in nature is discussed. The jet structure could be elongated during the bipolar stage, and remains stable during steady state. The sufficiency of the electron inflow is crucial for the elongation. Open boundary conditions are applied in the outflow direction.
Zenitani, Seiji
2015-03-15
The shock structure of a plasmoid in magnetic reconnection in low-beta plasmas is investigated by two-dimensional magnetohydrodynamic simulations. Using a high-accuracy code with unprecedented resolution, shocks, discontinuities, and their intersections are resolved and clarified. Contact discontinuities emanate from triple-shock intersection points, separating fluids of different origins. Shock-diamonds inside the plasmoid appear to decelerate a supersonic flow. New shock-diamonds and a slow expansion fan are found inside the Petschek outflow. A sufficient condition for the new shock-diamonds and the relevance to astrophysical jets are discussed.
Chromospheric anemone jets and magnetic reconnection in partially ionized solar atmosphere
NASA Astrophysics Data System (ADS)
Singh, K. A. P.; Shibata, K.; Nishizuka, N.; Isobe, H.
2011-11-01
The solar optical telescope onboard Hinode with temporal resolution of less than 5 s and spatial resolution of 150 km has observed the lower solar atmosphere with an unprecedented detail. This has led to many important findings, one of them is the discovery of chromospheric anemone jets in the solar chromosphere. The chromospheric anemone jets are ubiquitous in solar chromosphere and statistical studies show that the typical length, life time and energy of the chromospheric anemone jets are much smaller than the coronal events (e.g., jets/flares/CMEs). Among various observational parameters, the apparent length and maximum velocity shows good correlation. The velocity of chromospheric anemone jets is comparable to the local Alfvén speed in the lower solar chromosphere. Since the discovery of chromospheric anemone jets by Hinode, several evidences of magnetic reconnection in chromospheric anemone jets have been found and these observations are summarized in this paper. These observations clearly suggest that reconnection occurs quite rapidly as well as intermittently in the solar chromosphere. In the solar corona (λi > δSP), anomalous resistivity arises due to various collisionless processes. Previous MHD simulations show that reconnection becomes fast as well as strongly time-dependent due to anomalous resistivity. Such processes would not arise in the solar chromosphere which is fully collisional and partially-ionized. So, it is unclear how the rapid and strongly time-dependent reconnection would occur in the solar chromosphere. It is quite likely that the Hall and ambipolar diffusion are present in the solar chromosphere and they could play an important role in driving such rapid, strongly time-dependent reconnection in the solar chromosphere.
Chromospheric anemone jets and magnetic reconnection in partially ionized solar atmosphere
Singh, K. A. P.; Shibata, K.; Nishizuka, N.; Isobe, H.
2011-11-15
The solar optical telescope onboard Hinode with temporal resolution of less than 5 s and spatial resolution of 150 km has observed the lower solar atmosphere with an unprecedented detail. This has led to many important findings, one of them is the discovery of chromospheric anemone jets in the solar chromosphere. The chromospheric anemone jets are ubiquitous in solar chromosphere and statistical studies show that the typical length, life time and energy of the chromospheric anemone jets are much smaller than the coronal events (e.g., jets/flares/CMEs). Among various observational parameters, the apparent length and maximum velocity shows good correlation. The velocity of chromospheric anemone jets is comparable to the local Alfven speed in the lower solar chromosphere. Since the discovery of chromospheric anemone jets by Hinode, several evidences of magnetic reconnection in chromospheric anemone jets have been found and these observations are summarized in this paper. These observations clearly suggest that reconnection occurs quite rapidly as well as intermittently in the solar chromosphere. In the solar corona ({lambda}{sub i} > {delta}{sub SP}), anomalous resistivity arises due to various collisionless processes. Previous MHD simulations show that reconnection becomes fast as well as strongly time-dependent due to anomalous resistivity. Such processes would not arise in the solar chromosphere which is fully collisional and partially-ionized. So, it is unclear how the rapid and strongly time-dependent reconnection would occur in the solar chromosphere. It is quite likely that the Hall and ambipolar diffusion are present in the solar chromosphere and they could play an important role in driving such rapid, strongly time-dependent reconnection in the solar chromosphere.
Intuitive approach to magnetic reconnection
Kulsrud, Russell M.
2011-11-15
Two reconnection problems are considered. The first problem concerns global physics. The plasma in the global reconnection region is in magnetostatic equilibrium. It is shown that this equilibrium can be uniquely characterized by a set of constraints. During reconnection and independently of the local reconnection physics, these constraints can be uniquely evolved from any initial state. The second problem concerns Petschek reconnection. Petschek's model for fast reconnection, which is governed by resistive MHD equations with constant resistivity is not validated by numerical simulations. Malyshkin et al.[Phys. Plasmas 12, 102920 (2005)], showed that the reason for the discrepancy is that Petschek did not employ Ohm's law throughout the local diffusion region, but only at the X-point. A derivation of Petschek reconnection, including Ohm's law throughout the entire diffusion region, removes the discrepancy. This derivation is based largely on Petschek's original 1964 calculation [in AAS-NASA Symposium on Solar Flares (National Aeronautics and Space Administration, Washington, D.C., 1964), NASA SP50, p. 425]. A useful physical interpretation of the role which Ohm's law plays in the diffusion region is presented.
NASA Astrophysics Data System (ADS)
Sitnov, M. I.; Runov, A. V.; Ohtani, S.
2007-12-01
The physics of fast earthward flows or BBFs, a major mechanism of bursty transfer of the plasma and magnetic flux in the terrestrial magnetotail, remains uncertain and controversial. A part of observations can be explained as signatures of earthward moving flux ropes or secondary plasmoids dragged by the earthward part a larger-scale reconnection region [Slavin et al., 2003]. The statistics of variations of the z-component of the magnetospheric magnetic field in the central plasma sheet [Ohtani et al., 2004] suggest no changes of the magnetic field topology for another group of BBFs. These observations can be explained as signatures of either unsteady reconnection, which remains located tailward of the spacecraft, or other phenomena that are connected but not identical to reconnection in its active phase. These are the plasma bubbles, flux tubes with the reduced specific entropy that may move earthward faster than the neighboring flux tubes due to the buoyancy force. However, the original model of bubbles arising from local reductions of the plasma pressure [Pontius and Wolf, 1990] also explains only a part of observations. Another part [Angelopoulos et al., 1992] reveals no reduction of the plasma pressure in BBFs. One more model, which explains both missing magnetic topology changes and no reduction of the plasma pressure [Sitnov et al., 2005] describes the bubble as a seam in the body of the tail plasma, which appears after the formation and tailward retreat of a small plasmoid, and which is composed of atypical, embedded and bifurcated thin current sheets. Signatures of such atypical current sheets have been convincingly demonstrated recently in CLUSTER observations [Runov et al., 2003]. In this presentation we elaborate the BBF models and compare them with 2001 and 2002 tail CLUSTER observations in the central plasma sheet. These include full-particle simulations of the secondary plasmoid formation in tail-like systems, two- and three- dimensional features and
Drift Wave Turbulence and Magnetic Reconnection
NASA Astrophysics Data System (ADS)
Price, L.; Drake, J. F.; Swisdak, M.
2015-12-01
An important feature in collisionless magnetic reconnection is the development of sharp discontinuities along the separatrices bounding the Alfvenic outflow. The typical scale length of these features is ρs (the Larmor radius based on the sound speed) for guide field reconnection. Temperature gradients in the inflowing plasma (as might be found in the magnetopause and the magnetotail) can lead to instabilities at these separatrices, specifically drift wave turbulence. We present standalone 2D and 3D PIC simulations of drift wave turbulence to investigate scaling properties and growth rates. We specifically consider stabilization of the lower hybrid drift instability (LHDI) and the development of this instability in the presence of a sheared magnetic field. Further investigations of the relative importance of drift wave turbulence in the development of reconnection will also be considered.
Force balance near an X line in a collisionless plasma
NASA Technical Reports Server (NTRS)
Lyons, L. R.; Pridmore-Brown, D. C.
1990-01-01
The suggestion by Dungey (1988) that the gyroviscosity associated with gradients of the off-diagonal elements of the electron pressure tensor can balance a reconnection electric field along a magnetic X line in a collisionless plasma is investigated. The detailed balance of forces in the vicinity of an X line is evaluated using a two-dimensional magnetic field model and a simple model for particle motion. The results show that the gyroviscosity can indeed provide the force required to balance a reconnection electric field in that region, so that neither collisions nor wave turbulence are necessary for reconnection. The results also show that there should not be a significant increase in current from electron acceleration very near an X line. Reasonable numerical estimates are obtained for conditions expected in the vicinity of the distant X line in the geomagnetic tail.
The Time-Dependent Structure of the Electron Reconnection Layer
NASA Technical Reports Server (NTRS)
Hesse, Michael; Zenitani, Seiji; Kuznetsova, Masha; Klimas, Alex
2009-01-01
Collisionless magnetic reconnection is often associated with time-dependent behavior. Specifically, current layers in the diffusion region can become unstable to tearing-type instabilities on one hand, or to instabilities with current-aligned wave vectors on the other. In the former case, the growth of tearing instabilities typically leads to the production of magnetic islands, which potentially provide feedback on the reconnection process itself, as well as on the rate of reconnection. The second class of instabilities tend to modulate the current layer along the direction of the current flow, for instance generating kink-type perturbations, or smaller-scale turbulence with the potential to broaden the current layer. All of these processes contribute to rendering magnetic reconnection time-dependent. In this presentation, we will provide a summary of these effects, and a discussion of how much they contribute to the overall magnetic reconnection rate.
Slipping magnetic reconnection in coronal loops.
Aulanier, Guillaume; Golub, Leon; Deluca, Edward E; Cirtain, Jonathan W; Kano, Ryouhei; Lundquist, Loraine L; Narukage, Noriyuki; Sakao, Taro; Weber, Mark A
2007-12-01
Magnetic reconnection of solar coronal loops is the main process that causes solar flares and possibly coronal heating. In the standard model, magnetic field lines break and reconnect instantaneously at places where the field mapping is discontinuous. However, another mode may operate where the magnetic field mapping is continuous but shows steep gradients: The field lines may slip across each other. Soft x-ray observations of fast bidirectional motions of coronal loops, observed by the Hinode spacecraft, support the existence of this slipping magnetic reconnection regime in the Sun's corona. This basic process should be considered when interpreting reconnection, both on the Sun and in laboratory-based plasma experiments. PMID:18063789
Global scale-invariant dissipation in collisionless plasma turbulence.
Kiyani, K H; Chapman, S C; Khotyaintsev, Yu V; Dunlop, M W; Sahraoui, F
2009-08-14
A higher-order multiscale analysis of the dissipation range of collisionless plasma turbulence is presented using in situ high-frequency magnetic field measurements from the Cluster spacecraft in a stationary interval of fast ambient solar wind. The observations, spanning five decades in temporal scales, show a crossover from multifractal intermittent turbulence in the inertial range to non-Gaussian monoscaling in the dissipation range. This presents a strong observational constraint on theories of dissipation mechanisms in turbulent collisionless plasmas. PMID:19792654
Laboratory Reconnection Experiments - heating and particle acceleration
NASA Astrophysics Data System (ADS)
Ono, Yasushi
Recent laboratory merging/ reconnection experiments have solved a number of key physics of magnetic reconnection: 1) reconnection heating/ acceleration, 2) fast reconnection mechanisms, 3) plasmoid reconnection, 4) non-steady reconnection and 5) non-thermal particle acceleration using new kinetic interpretations. Especially, significant ion temperatures 1.2keV were documented in the world-largest tokamak merging experiment: MAST after detailed 2D elucidation of ion and electron heating characteristics in TS-3 and 4 merging experiments. The measured 2D contours of ion and electron temperatures in TS-3, 4 and MAST reveal ion heating in the downstream by reconnection outflow and electron heating around the X-point by ohmic heating of current sheet. Their detailed heating mechanisms were further investigated by comparing those results with particle simulations developed by NIFS. The ion acceleration mechanism is mostly parallel acceleration by reconnection electric field and partly perpendicular acceleration by electrostatic potential. The fast shock and ion viscosity are the major dumping (heating) mechanisms for the accelerated ions. We successfully applied the reconnection heating - typically 10-50MW to the high-beta spherical tokamak formation and heating. This paper will review major progresses in those international and interdisciplinary merging tokamak experiments.
Indeterminacy and instability in Petschek reconnection
Forbes, Terry G.; Priest, Eric R.; Seaton, Daniel B.; Litvinenko, Yuri E.
2013-05-15
We explain two puzzling aspects of Petschek's model for fast reconnection. One is its failure to occur in plasma simulations with uniform resistivity. The other is its inability to provide anything more than an upper limit for the reconnection rate. We have found that previously published analytical solutions based on Petschek's model are structurally unstable if the electrical resistivity is uniform. The structural instability is associated with the presence of an essential singularity at the X-line that is unphysical. By requiring that such a singularity does not exist, we obtain a formula that predicts a specific rate of reconnection. For uniform resistivity, reconnection can only occur at the slow, Sweet-Parker rate. For nonuniform resistivity, reconnection can occur at a much faster rate provided that the resistivity profile is not too flat near the X-line. If this condition is satisfied, then the scale length of the nonuniformity determines the reconnection rate.
Multiscale modeling of magnetospheric reconnection
NASA Astrophysics Data System (ADS)
Kuznetsova, M. M.; Hesse, M.; RastäTter, L.; Taktakishvili, A.; Toth, G.; de Zeeuw, D. L.; Ridley, A.; Gombosi, T. I.
2007-10-01
In our efforts to bridge the gap between small-scale kinetic modeling and global simulations, we introduced an approach that allows to quantify the interaction between large-scale global magnetospheric dynamics and microphysical processes in diffusion regions near reconnection sites. We use the global MHD code BATS-R-US and replace an ad hoc anomalous resistivity often employed by global MHD models with a physically motivated dissipation model. The primary kinetic mechanism controlling the dissipation in the diffusion region in the vicinity of the reconnection site is incorporated into the MHD description in terms of nongyrotropic corrections to the induction equation. We developed an algorithm to search for reconnection sites in north-south symmetric magnetotail. Spatial scales of the diffusion region and magnitude of the reconnection electric field are calculated consistently using local MHD plasma and field parameters. The locations of the reconnection sites are constantly updated during the simulations. To clarify the role of nongyrotropic effects in the diffusion region on the global magnetospheric dynamics, we perform simulations with steady southward interplanetary magnetic field driving of the magnetosphere. Ideal MHD simulations with magnetic reconnection supported by numerical resistivity often produce quasi-steady configuration with almost stationary near-Earth neutral line (NENL). Simulations with nongyrotropic corrections demonstrate dynamic quasi-periodic response to the steady driving conditions. Fast magnetotail reconnection supported by nongyrotropic effects results in tailward retreat of the reconnection site with average speed of the order of 100 km/s followed by a formation of a new NENL in the near-Earth thin current sheet. This approach allowed to model for the first time loading/unloading cycle frequently observed during extended periods of steady low-mach-number solar wind with southward interplanetary magnetic field.
Magnetic Reconnection: A Fundamental Process in Space Plasmas
NASA Technical Reports Server (NTRS)
Hesse, Michael
2010-01-01
For many years, collisionless magnetic reconnect ion has been recognized as a fundamental process, which facilitates plasma transport and energy release in systems ranging from the astrophysical plasmas to magnetospheres and even laboratory plasma. Beginning with work addressing solar dynamics, it has been understood that reconnection is essential to explain solar eruptions, the interaction of the solar wind with the magnetosphere, and the dynamics of the magnetosphere. Accordingly, the process of magnetic reconnection has been and remains a prime target for space-based and laboratory studies, as well as for theoretical research. Much progress has been made throughout the years, beginning with indirect verifications by studies of processes enabled by reconnection, such as Coronal Mass Ejections, Flux Transfer Events, and Plasmoids. Theoretical advances have accompanied these observations, moving knowledge beyond the Sweet-Parker theory to the recognition that other, collisionless, effects are available and likely to support much faster reconnect ion rates. At the present time we are therefore near a break-through in our understanding of how collisionless reconnect ion works. Theory and modeling have advanced to the point that two competing theories are considered leading candidates for explaining the microphysics of this process. Both theories predict very small spatial and temporal scales. which are. to date, inaccessible to space-based or laboratory measurements. The need to understand magnetic reconnect ion has led NASA to begin the implementation of a tailored mission, Magnetospheric MultiScale (MMS), a four spacecraft cluster equipped to resolve all relevant spatial and temporal scales. In this presentation, we present an overview of current knowledge as well as an outlook towards measurements provided by MMS.
Global Simulations of Magnetotail Reconnection
NASA Technical Reports Server (NTRS)
Kuznetsova, M. M.; Hesse, M.; Rastatter, L.; Toth, G.; Gombosi, T.
2007-01-01
There is a growing number of observational evidences of dynamic quasi-periodical magnetosphere response to continuously southward interplan etary magnetic field (IMF). However, traditional global MHD simulatio ns with magnetic reconnection supported by numerical dissipation and ad hoc anomalous resistivity driven by steady southward IMF often prod uce only quasi-steady configurations with almost stationary near-eart h neutral line. This discrepancy can be explained by the assumption that global MHD simulations significantly underestimate the reconnectio n rate in the magnetotail during substorm expansion phase. Indeed, co mparative studies of magnetic reconnection in small scale geometries demonstrated that traditional resistive MHD did not produce the fast r econnection rates observed in kinetic simulations. The major approxim ation of the traditional MHD approach is an isotropic fluid assumption) with zero off-diagonal pressure tensor components. The approximatio n, however, becomes invalid in the diffusion region around the reconn ection site where ions become unmagnetized and experience nongyrotropic behaviour. Deviation from gyrotropy in particle distribution functi on caused by kinetic effects manifests itself in nongyrotropic pressu re tensor with nonzero off-diagonal components. We use the global MHD code BATS-R-US and replace ad hoc parameters such as "critical curren t density" and "anomalous resistivity" with a physically motivated di ssipation model. The key element of the approach is to identify diffusion regions where the isotropic fluid MHD approximation is not applic able. We developed an algorithm that searches for locations of magnet otail reconnection sites. The algorithm takes advantage of block-based domain-decomposition technique employed by the BATS-R-US. Boundaries of the diffusion region around each reconnection site are estimated from the gyrotropic orbit threshold condition, where the ion gyroradius is equal to the distance to the
Turbulent Reconnection Rates from Cluster Observations in the Magneto sheath
NASA Technical Reports Server (NTRS)
Wendel, Deirdre
2011-01-01
The role of turbulence in producing fast reconnection rates is an important unresolved question. Scant in situ analyses exist. We apply multiple spacecraft techniques to a case of nonlinear turbulent reconnection in the magnetosheath to test various theoretical results for turbulent reconnection rates. To date, in situ estimates of the contribution of turbulence to reconnection rates have been calculated from an effective electric field derived through linear wave theory. However, estimates of reconnection rates based on fully nonlinear turbulence theories and simulations exist that are amenable to multiple spacecraft analyses. Here we present the linear and nonlinear theories and apply some of the nonlinear rates to Cluster observations of reconnecting, turbulent current sheets in the magnetos heath. We compare the results to the net reconnection rate found from the inflow speed. Ultimately, we intend to test and compare linear and nonlinear estimates of the turbulent contribution to reconnection rates and to measure the relative contributions of turbulence and the Hall effect.
Turbulent Reconnection Rates from Cluster Observations in the Magnetosheath
NASA Technical Reports Server (NTRS)
Wendel, Deirdre
2011-01-01
The role of turbulence in producing fast reconnection rates is an important unresolved question. Scant in situ analyses exist. We apply multiple spacecraft techniques to a case of nonlinear turbulent reconnection in the magnetosheath to test various theoretical results for turbulent reconnection rates. To date, in situ estimates of the contribution of turbulence to reconnection rates have been calculated from an effective electric field derived through linear wave theory. However, estimates of reconnection rates based on fully nonlinear turbulence theories and simulations exist that are amenable to multiple spacecraft analyses. Here we present the linear and nonlinear theories and apply some of the nonlinear rates to Cluster observations of reconnecting, turbulent current sheets in the magnetosheath. We compare the results to the net reconnection rate found from the inflow speed. Ultimately, we intend to test and compare linear and nonlinear estimates of the turbulent contribution to reconnection rates and to measure the relative contributions of turbulence and the Hall effect.
Supermagnetosonic Jets behind a Collisionless Quasiparallel Shock
Hietala, H.; Vainio, R.; Laitinen, T. V.; Vaivads, A.; Andreeova, K.; Palmroth, M.; Pulkkinen, T. I.; Koskinen, H. E. J.; Lucek, E. A.; Reme, H.
2009-12-11
The downstream region of a collisionless quasiparallel shock is structured containing bulk flows with high kinetic energy density from a previously unidentified source. We present Cluster multispacecraft measurements of this type of supermagnetosonic jet as well as of a weak secondary shock front within the sheath, that allow us to propose the following generation mechanism for the jets: The local curvature variations inherent to quasiparallel shocks can create fast, deflected jets accompanied by density variations in the downstream region. If the speed of the jet is super(magneto)sonic in the reference frame of the obstacle, a second shock front forms in the sheath closer to the obstacle. Our results can be applied to collisionless quasiparallel shocks in many plasma environments.
Matsumoto, Y; Amano, T; Kato, T N; Hoshino, M
2015-02-27
Explosive phenomena such as supernova remnant shocks and solar flares have demonstrated evidence for the production of relativistic particles. Interest has therefore been renewed in collisionless shock waves and magnetic reconnection as a means to achieve such energies. Although ions can be energized during such phenomena, the relativistic energy of the electrons remains a puzzle for theory. We present supercomputer simulations showing that efficient electron energization can occur during turbulent magnetic reconnection arising from a strong collisionless shock. Upstream electrons undergo first-order Fermi acceleration by colliding with reconnection jets and magnetic islands, giving rise to a nonthermal relativistic population downstream. These results shed new light on magnetic reconnection as an agent of energy dissipation and particle acceleration in strong shock waves. PMID:25722406
Stochastic electron acceleration during turbulent reconnection in strong shock waves
NASA Astrophysics Data System (ADS)
Matsumoto, Yosuke
2016-04-01
Acceleration of charged particles is a fundamental topic in astrophysical, space and laboratory plasmas. Very high energy particles are commonly found in the astrophysical and planetary shocks, and in the energy releases of solar flares and terrestrial substorms. Evidence for relativistic particle production during such phenomena has attracted much attention concerning collisionless shock waves and magnetic reconnection, respectively, as ultimate plasma energization mechanisms. While the energy conversion proceeds macroscopically, and therefore the energy mostly flows to ions, plasma kinetic instabilities excited in a localized region have been considered to be the main electron heating and acceleration mechanisms. We present that efficient electron energization can occur in a much larger area during turbulent magnetic reconnection from the intrinsic nature of a strong collisionless shock wave. Supercomputer simulations have revealed a multiscale shock structure comprising current sheets created via an ion-scale Weibel instability and resulting energy dissipation through magnetic reconnection. A part of the upstream electrons undergoes first-order Fermi acceleration by colliding with reconnection jets and magnetic islands, giving rise to a nonthermal relativistic population downstream. The dynamics has shed new light on magnetic reconnection as an agent of energy dissipation and particle acceleration in strong shock waves.
Reconnection in thin current sheets
NASA Astrophysics Data System (ADS)
Tenerani, Anna; Velli, Marco; Pucci, Fulvia; Rappazzo, A. F.
2016-05-01
It has been widely believed that reconnection is the underlying mechanism of many explosive processes observed both in nature and laboratory, but the question of reconnection speed and initial trigger have remained mysterious. How is fast magnetic energy release triggered in high Lundquist (S) and Reynolds (R) number plasmas?It has been shown that a tearing mode instability can grow on an ideal timescale, i.e., independent from the the Lundquist number, once the current sheet thickness becomes thin enough, or rather the inverse aspect ratio a/L reaches a scale a/L~S-1/3. As such, the latter provides a natural, critical threshold for current sheets that can be formed in nature before they disrupt in a few Alfvén time units. Here we discuss the transition to fast reconnection extended to simple viscous and kinetic models and we propose a possible scenario for the transition to explosive reconnection in high-Lundquist number plasmas, that we support with fully nonlinear numerical MHD simulations of a collapsing current sheet.
Transition from antiparallel to component magnetic reconnection
NASA Astrophysics Data System (ADS)
Swisdak, M.; Drake, J. F.; Shay, M. A.; McIlhargey, J. G.
2005-05-01
We study the transition between antiparallel and component collisionless magnetic reconnection with two-dimensional particle-in-cell simulations. The primary finding is that a guide field ≈0.1 times as strong as the asymptotic reconnecting field (roughly the field strength at which the electron Larmor radius is comparable to the width of the electron current layer) is sufficient to magnetize the electrons in the vicinity of the X line, thus causing significant changes to the structure of the electron dissipation region. This implies that great care should be exercised before concluding that magnetospheric reconnection is antiparallel. We also find that even for such weak guide fields, strong inward flowing electron beams form in the vicinity of the magnetic separatrices and Buneman unstable distribution functions arise at the X line itself. As in the calculations of Hesse et al. (2002) and Yin and Winske (2003) the nongyrotropic elements of the electron pressure tensor play the dominant role in decoupling the electrons from the magnetic field at the X line, regardless of the magnitude of the guide field and the associated strong variations in the pressure tensor's spatial structure. Despite these changes, and consistent with previous work, the reconnection rate does not vary appreciably with the strength of the guide field as it changes between 0 and a value equal to the asymptotic reversed field.
Gyro-induced acceleration of magnetic reconnection
Comisso, L.; Grasso, D.; Waelbroeck, F. L.; Borgogno, D.
2013-09-15
The linear and nonlinear evolution of magnetic reconnection in collisionless high-temperature plasmas with a strong guide field is analyzed on the basis of a two-dimensional gyrofluid model. The linear growth rate of the reconnecting instability is compared to analytical calculations over the whole spectrum of linearly unstable wave numbers. In the strongly unstable regime (large Δ′), the nonlinear evolution of the reconnecting instability is found to undergo two distinctive acceleration phases separated by a stall phase in which the instantaneous growth rate decreases. The first acceleration phase is caused by the formation of strong electric fields close to the X-point due to ion gyration, while the second acceleration phase is driven by the development of an open Petschek-like configuration due to both ion and electron temperature effects. Furthermore, the maximum instantaneous growth rate is found to increase dramatically over its linear value for decreasing diffusion layers. This is a consequence of the fact that the peak instantaneous growth rate becomes weakly dependent on the microscopic plasma parameters if the diffusion region thickness is sufficiently smaller than the equilibrium magnetic field scale length. When this condition is satisfied, the peak reconnection rate asymptotes to a constant value.
VINETA II: a linear magnetic reconnection experiment.
Bohlin, H; Von Stechow, A; Rahbarnia, K; Grulke, O; Klinger, T
2014-02-01
A linear experiment dedicated to the study of driven magnetic reconnection is presented. The new device (VINETA II) is suitable for investigating both collisional and near collisionless reconnection. Reconnection is achieved by externally driving magnetic field lines towards an X-point, inducing a current in the background plasma which consequently modifies the magnetic field topology. Owing to the open field line configuration of the experiment, the current is limited by the axial sheath boundary conditions. A plasma gun is used as an additional electron source in order to counterbalance the charge separation effects and supply the required current. Two drive methods are used in the device. First, an oscillating current through two parallel conductors drive the reconnection. Second, a stationary X-point topology is formed by the parallel conductors, and the drive is achieved by an oscillating current through a third conductor. In the first setup, the magnetic field of the axial plasma current dominates the field topology near the X-point throughout most of the drive. The second setup allows for the amplitude of the plasma current as well as the motion of the flux to be set independently of the X-point topology of the parallel conductors. PMID:24593355
VINETA II: A linear magnetic reconnection experiment
Bohlin, H. Von Stechow, A.; Rahbarnia, K.; Grulke, O.; Klinger, T.
2014-02-15
A linear experiment dedicated to the study of driven magnetic reconnection is presented. The new device (VINETA II) is suitable for investigating both collisional and near collisionless reconnection. Reconnection is achieved by externally driving magnetic field lines towards an X-point, inducing a current in the background plasma which consequently modifies the magnetic field topology. Owing to the open field line configuration of the experiment, the current is limited by the axial sheath boundary conditions. A plasma gun is used as an additional electron source in order to counterbalance the charge separation effects and supply the required current. Two drive methods are used in the device. First, an oscillating current through two parallel conductors drive the reconnection. Second, a stationary X-point topology is formed by the parallel conductors, and the drive is achieved by an oscillating current through a third conductor. In the first setup, the magnetic field of the axial plasma current dominates the field topology near the X-point throughout most of the drive. The second setup allows for the amplitude of the plasma current as well as the motion of the flux to be set independently of the X-point topology of the parallel conductors.
Reconnection of Magnetic Fields
NASA Technical Reports Server (NTRS)
1984-01-01
Spacecraft observations of steady and nonsteady reconnection at the magnetopause are reviewed. Computer simulations of three-dimensional reconnection in the geomagnetic tail are discussed. Theoretical aspects of the energization of particles in current sheets and of the microprocesses in the diffusion region are presented. Terrella experiments in which magnetospheric reconnection is simulated at both the magnetopause and in the tail are described. The possible role of reconnection in the evolution of solar magnetic fields and solar flares is discussed. A two-dimensional magnetohydrodynamic computer simulation of turbulent reconnection is examined. Results concerning reconnection in Tokamak devices are also presented.
A Parallel Two-fluid Code for Global Magnetic Reconnection Studies
J.A. Breslau; S.C. Jardin
2001-08-09
This paper describes a new algorithm for the computation of two-dimensional resistive magnetohydrodynamic (MHD) and two-fluid studies of magnetic reconnection in plasmas. It has been implemented on several parallel platforms and shows good scalability up to 32 CPUs for reasonable problem sizes. A fixed, nonuniform rectangular mesh is used to resolve the different spatial scales in the reconnection problem. The resistive MHD version of the code uses an implicit/explicit hybrid method, while the two-fluid version uses an alternating-direction implicit (ADI) method. The technique has proven useful for comparing several different theories of collisional and collisionless reconnection.
Studies of Magnetic Reconnection in Colliding Laser-Produced Plasmas
NASA Astrophysics Data System (ADS)
Rosenberg, Michael
2013-10-01
Novel images of magnetic fields and measurements of electron and ion temperatures have been obtained in the magnetic reconnection region of high- β, laser-produced plasmas. Experiments using laser-irradiated foils produce expanding, hemispherical plasma plumes carrying MG Biermann-battery magnetic fields, which can be driven to interact and reconnect. Thomson-scattering measurements of electron and ion temperatures in the interaction region of two colliding, magnetized plasmas show no thermal enhancement due to reconnection, as expected for β ~ 8 plasmas. Two different proton radiography techniques used to image the magnetic field structures show deformation, pileup, and annihilation of magnetic flux. High-resolution images reveal unambiguously reconnection-induced jets emerging from the interaction region and show instabilities in the expanding plasma plumes and supersonic, hydrodynamic jets due to the plasma collision. Quantitative magnetic flux data show that reconnection in experiments with asymmetry in the scale size, density, temperature, and plasma flow across the reconnection region occurs less efficiently than in similar, symmetric experiments. This result is attributed to disruption of the Hall mechanism mediating collisionless reconnection. The collision of plasmas carrying parallel magnetic fields has also been probed, illustrating the deformation of magnetic field structures in high-energy-density plasmas in the absence of reconnection. These experiments are particularly relevant to high- β reconnection environments, such as the magnetopause. This work was performed in collaboration with C. Li, F. Séguin, A. Zylstra, H. Rinderknecht, H. Sio, J. Frenje, and R. Petrasso (MIT), I. Igumenshchev, V. Glebov, C. Stoeckl, and D. Froula (LLE), J. Ross and R. Town (LLNL), W. Fox (UNH), and A. Nikroo (GA), and was supported in part by the NLUF, FSC/UR, U.S. DOE, LLNL, and LLE.
Resistive Magnetohydrodynamic Simulations of Relativistic Magnetic Reconnection
NASA Technical Reports Server (NTRS)
Zenitani, Seiji; Hesse, Michael; Klimas, Alex
2010-01-01
Resistive relativistic magnetohydrodynamic (RRMHD) simulations are applied to investigate the system evolution of relativistic magnetic reconnection. A time-split Harten-Lan-van Leer method is employed. Under a localized resistivity, the system exhibits a fast reconnection jet with an Alfv enic Lorentz factor inside a narrow Petschek-type exhaust. Various shock structures are resolved in and around the plasmoid such as the post-plasmoid vertical shocks and the "diamond-chain" structure due to multiple shock reflections. Under a uniform resistivity, Sweet-Parker-type reconnection slowly evolves. Under a current-dependent resistivity, plasmoids are repeatedly formed in an elongated current sheet. It is concluded that the resistivity model is of critical importance for RRMHD modeling of relativistic magnetic reconnection.
Dynamic Response of Magnetic Reconnection Due to Current Sheet Variability
NASA Astrophysics Data System (ADS)
George, D. E.; Jahn, J. M.; Burch, J. L.; Hesse, M.; Pollock, C. J.
2014-12-01
Magnetic reconnection is a process which regulates the interaction between regions of magnetized plasma. While many factors have an impact on the evolution of this process, there still remains a lack of understanding of the key behaviors involved in the triggering of fast reconnection. Despite an abundance of in-situ measurements, indicating the high degree of variability in the thickness, density and composition along the current sheet, no simulation studies exist which account for such current sheet variations. 2D and 3D simulations have a periodic boundary in the dimension along the current sheet and so tend to neglect these variations in the current sheet originating external to the modeled reconnection region. Here we focus on the effects on reconnection due to the variability in the thickness and density of the current sheet. Using 2.5D kinetic simulations of 2-species plasma, we isolate and explore the dynamic effects on reconnection associated with variations in the current sheet originating externally to the reconnection region. While periodic boundary conditions are still used, in the direction along the current sheet, a step-change perturbation in thickness or density of the current sheet is introduced once a stable reconnection rate is reached. The dynamic response of the overall system, after introducing the perturbation, is then evaluated, with a focus on the reconnection rate. When the reconnection rate is slowed significantly over time, loading of the inflow region occurs (a build-up of plasma and magnetic energy/pressure. This state is indicated by an asymptotic behavior in the reconnection rate over time. If a sudden variation in the current sheet is introduced under these conditions, a resultant triggering of fast reconnection may occur, which could lead to an episode of fast reconnection, saw-tooth-crash condition or even act as a trigger for sub-storms.
Magnetopause reconnection diffusion regions resolved by the NASA Magnetospheric Multiscale mission
NASA Astrophysics Data System (ADS)
Chen, Li-Jen
2016-07-01
Our understanding of how magnetic reconnection occurs in collisionless plasmas depends highly on our ability to resolve structures of the diffusion region. Unraveling the physical processes in the diffusion region is the primary goal of the NASA mission Magnetospheric Multiscale (MMS). With its first science phase began in September, 2015, the four MMS satellites have encountered both ion and electron diffusion regions during magnetopause reconnection. We will discuss a few diffusion region events including cases with negligible and finite guide fields, and compare the results with particle-in-cell (PIC) simulations. In particular, a close comparison between particle distribution functions observed by MMS and those predicted by PIC will be made to highlight how the unprecedented high-resolution MMS measurements advance the current state-of-knowledge on collisionless reconnection.
Debye scale turbulence within the electron diffusion layer during magnetic reconnection
Jara-Almonte, J.; Ji, H.
2014-03-15
During collisionless, anti-parallel magnetic reconnection, the electron diffusion layer is the region of both fieldline breaking and plasma mixing. Due to the in-plane electrostatic fields associated with collisionless reconnection, the inflowing plasmas are accelerated towards the X-line and form counter-streaming beams within the unmagnetized diffusion layer. This configuration is inherently unstable to in-plane electrostatic streaming instabilities provided that there is sufficient scale separation between the Debye length λ{sub D} and the electron skin depth c/ω{sub pe}. This scale separation has hitherto not been well resolved in kinetic simulations. Using both 2D fully kinetic simulations and a simple linear model, we demonstrate that these in-plane streaming instabilities generate Debye scale turbulence within the electron diffusion layer at electron temperatures relevant to magnetic reconnection both in the magnetosphere and in laboratory experiments.
Intimate connection of turbulence and reconnection: theory, testing and consequences
NASA Astrophysics Data System (ADS)
Lazarian, Alex
2016-07-01
I shall show that magnetic reconnection and turbulence are intrinsically connected: in the presence of turbulence magnetic reconnection gets fast while magnetic turbulence depends on reconnection for its cascading. I shall present the basics of the theory of turbulent magnetic reconnection in non-relativistic and relativistic plasmas, discuss numerical and observational tests of the theory and outline the consequences of the theory from diffusion of magnetic fields in Parker spiral and in the process of star formation to violent flares accelerating energetic particles in solar flares and gamma ray bursts.
Experimental study of ion heating and acceleration during magnetic reconnection
Hsu, S.C.
2000-01-28
This dissertation reports an experimental study of ion heating and acceleration during magnetic reconnection, which is the annihilation and topological rearrangement of magnetic flux in a conductive plasma. Reconnection is invoked often to explain particle heating and acceleration in both laboratory and naturally occurring plasmas. However, a simultaneous account of reconnection and its associated energy conversion has been elusive due to the extreme inaccessibility of reconnection events, e.g. in the solar corona, the Earth's magnetosphere, or in fusion research plasmas. Experiments for this work were conducted on MRX (Magnetic Reconnection Experiment), which creates a plasma environment allowing the reconnection process to be isolated, reproduced, and diagnosed in detail. Key findings of this work are the identification of local ion heating during magnetic reconnection and the determination that non-classical effects must provide the heating mechanism. Measured ion flows are sub-Alfvenic and can provide only slight viscous heating, and classical ion-electron interactions can be neglected due to the very long energy equipartition time. The plasma resistivity in the reconnection layer is seen to be enhanced over the classical value, and the ion heating is observed to scale with the enhancement factor, suggesting a relationship between the magnetic energy dissipation mechanism and the ion heating mechanism. The observation of non-classical ion heating during reconnection has significant implications for understanding the role played by non-classical dissipation mechanisms in generating fast reconnection. The findings are relevant for many areas of space and laboratory plasma research, a prime example being the currently unsolved problem of solar coronal heating. In the process of performing this work, local measurements of ion temperature and flows in a well-characterized reconnection layer were obtained for the first time in either laboratory or observational
Observations and models of magnetic reconnection
NASA Astrophysics Data System (ADS)
Barta, Miroslav
2015-08-01
Magnetic reconnection is now almost unanimously considered to be a key plasma process for energy release in solar and stellar flares. Recent decade have seen rapid development in the theory, simulations and searching for observational evidences of magnetic reconnection being in action in the core of flares. Modern modeling approach involves many realistic aspects of magnetic reconnection such as intrinsically 3D nature of the process and, namely, its highly dynamic character connected with violent formation of plasmoids at many scales. The cascade of plasmoid formation represents natural process of fast, turbulent energy transfer to the kinetic dissipation scale. This concept, revealed by numerical simulations, has found its ground in the theory of (ideal) plasmoid instability in current layers with high aspect ratio. The plasmoid dominated reconnection regime is capable to account for many puzzling dilemmas in the flare physics ranging from the observation-demanded energy release rate vs. standard reconnection-regime timescales, observed organized large-scale structures vs. signatures of fragmented energy release etc. The talk aims at reviewing recent theoretical and simulation development in this direction and observational support for the concept of plasmoid-driven reconnection cascade namely in solar flares.
Self-generated Turbulence in Magnetic Reconnection
NASA Astrophysics Data System (ADS)
Oishi, Jeffrey S.; Mac Low, Mordecai-Mark; Collins, David C.; Tamura, Moeko
2015-06-01
Classical Sweet-Parker models of reconnection predict that reconnection rates depend inversely on the resistivity, usually parameterized using the dimensionless Lundquist number (S). We describe magnetohydrodynamic (MHD) simulations using a static, nested grid that show the development of a three-dimensional (3D) instability in the plane of a current sheet between reversing field lines without a guide field. The instability leads to rapid reconnection of magnetic field lines at a rate independent of S over at least the range 3.2× {{10}3}≲ S≲ 3.2× {{10}5} resolved by the simulations. We find that this instability occurs even for cases with S≲ {{10}4} that in our models appear stable to the recently described, two-dimensional, plasmoid instability. Our results suggest that 3D, MHD processes alone produce fast (resistivity independent) reconnection without recourse to kinetic effects or external turbulence. The unstable reconnection layers provide a self-consistent environment in which the extensively studied turbulent reconnection process can occur.
Reconnection in Planetary Magnetospheres
NASA Technical Reports Server (NTRS)
Russell, C. T.
2000-01-01
Current sheets in planetary magnetospheres that lie between regions of "oppositely-directed" magnetic field are either magnetopause-like, separating plasmas with different properties, or tail-like, separating plasmas of rather similar properties. The magnetopause current sheets generally have a nearly limitless supply of magnetized plasma that can reconnect, possibly setting up steady-state reconnection. In contrast, the plasma on either side of a tail current sheet is stratified so that, as reconnection occurs, the plasma properties, in particular the Alfven velocity, change. If the density drops and the magnetic field increases markedly perpendicular to the sheet, explosive reconnection can occur. Even though steady state reconnection can take place at magnetopause current sheets, the process often appears to be periodic as if a certain low average rate was demanded by the conditions but only a rapid rate was available. Reconnection of sheared fields has been postulated to create magnetic ropes in the solar corona, at the Earth's magnetopause, and in the magnetotail. However, this is not the only way to produce magnetic ropes as the Venus ionosphere shows. The geometry of the reconnecting regions and the plasma conditions both can affect the rate of reconnection. Sorting out the various controlling factors can be assisted through the examination of reconnection in planetary settings. In particular we observe similar small-scale tearing in the magnetopause current layers of the Earth, Saturn. Uranus and Neptune and the magnetodisk current sheet at Jupiter. These sites may be seeds for rapid reconnection if the reconnection site reaches a high Alfven velocity region. In the Jupiter magnetosphere this appears to be achieved with resultant substorm activity. Similar seeds may be present in the Earth's magnetotail with the first one to reach explosive growth dominating the dynamics of the tail.
The microphysics of collisionless shocks
NASA Astrophysics Data System (ADS)
Wilson, Lynn Bruce, III
2010-11-01
Shock waves in interplanetary (IP) space are of considerable interest due to their potential to damage ground based electronic systems and their ability to energize charged particles. The energization of charged particles at IP shocks has the obvious extrapolation to supernova shock waves, which are thought to be a candidate for generating the most energetic particles in the universe. The observations and theory behind collisionless shock wave evolution suggest that IP shocks should, for the most part, be stable structures which require energy dissipation. In a regular fluid, like our atmosphere, energy dissipation is accomplished through binary particle collisions transferring the loss of bulk flow kinetic energy to heat. Plasmas are mostly collisionless fluids, thus requiring other means by which to dissipate energy. The studies herein were performed using wave and particle data primarily from the Wind spacecraft to investigate the microphysics of IP shock energy dissipation mechanisms. Due to their lower Mach numbers, more simplified geometry, and quasi-perpendicular nature, IP shock waves are an excellent laboratory to study wave-particle related dissipation mechanisms. Utilization of multiple data sets from multiple high time resolution instruments on board the Wind spacecraft, we have performed studies on the transition region microphysics of IP shocks. The work began with a statistical study of high frequency (≥ 1 kHz) waveform capture data during 67 IP shocks with Mach numbers ranging from ∼1-6 found ion-acoustic wave amplitudes correlated with the fast mode Mach number and shock strength. The ion-acoustic waves (IAWs) were estimated to produce anomalous resistivities roughly seven orders of magnitude above classical estimates. Another study was an examination of low frequency waves (0.25 Hz < f < 10 Hz) at five quasi-perpendicular IP shocks found the wave modes to be consistent with oblique precusor whistler waves at four of the events. The strongest
Notes on Magnetohydrodynamics of Magnetic Reconnection in Turbulent Media
NASA Astrophysics Data System (ADS)
Browning, Philippa; Lazarian, Alex
2013-10-01
Astrophysical fluids have very large Reynolds numbers and therefore turbulence is their natural state. Magnetic reconnection is an important process in many astrophysical plasmas, which allows restructuring of magnetic fields and conversion of stored magnetic energy into heat and kinetic energy. Turbulence is known to dramatically change different transport processes and therefore it is not unexpected that turbulence can alter the dynamics of magnetic field lines within the reconnection process. We shall review the interaction between turbulence and reconnection at different scales, showing how a state of turbulent reconnection is natural in astrophysical plasmas, with implications for a range of phenomena across astrophysics. We consider the process of magnetic reconnection that is fast in magnetohydrodynamic (MHD) limit and discuss how turbulence—both externally driven and generated in the reconnecting system—can make reconnection independent on the microphysical properties of plasmas. We will also show how relaxation theory can be used to calculate the energy dissipated in turbulent reconnecting fields. As well as heating the plasma, the energy dissipated by turbulent reconnection may cause acceleration of non-thermal particles, which is briefly discussed here.
Notes on Magnetohydrodynamics of Magnetic Reconnection in Turbulent Media
NASA Astrophysics Data System (ADS)
Browning, Philippa; Lazarian, Alex
Astrophysical fluids have very large Reynolds numbers and therefore turbulence is their natural state. Magnetic reconnection is an important process in many astrophysical plasmas, which allows restructuring of magnetic fields and conversion of stored magnetic energy into heat and kinetic energy. Turbulence is known to dramatically change different transport processes and therefore it is not unexpected that turbulence can alter the dynamics of magnetic field lines within the reconnection process. We shall review the interaction between turbulence and reconnection at different scales, showing how a state of turbulent reconnection is natural in astrophysical plasmas, with implications for a range of phenomena across astrophysics. We consider the process of magnetic reconnection that is fast in magnetohydrodynamic (MHD) limit and discuss how turbulence—both externally driven and generated in the reconnecting system—can make reconnection independent on the microphysical properties of plasmas. We will also show how relaxation theory can be used to calculate the energy dissipated in turbulent reconnecting fields. As well as heating the plasma, the energy dissipated by turbulent reconnection may cause acceleration of non-thermal particles, which is briefly discussed here.
High power heating of magnetic reconnection in merging tokamak experimentsa)
NASA Astrophysics Data System (ADS)
Ono, Y.; Tanabe, H.; Yamada, T.; Gi, K.; Watanabe, T.; , T., Ii; Gryaznevich, M.; Scannell, R.; Conway, N.; Crowley, B.; Michael, C.
2015-05-01
Significant ion/electron heating of magnetic reconnection up to 1.2 keV was documented in two spherical tokamak plasma merging experiment on MAST with the significantly large Reynolds number R˜105. Measured 1D/2D contours of ion and electron temperatures reveal clearly energy-conversion mechanisms of magnetic reconnection: huge outflow heating of ions in the downstream and localized heating of electrons at the X-point. Ions are accelerated up to the order of poloidal Alfven speed in the reconnection outflow region and are thermalized by fast shock-like density pileups formed in the downstreams, in agreement with recent solar satellite observations and PIC simulation results. The magnetic reconnection efficiently converts the reconnecting (poloidal) magnetic energy mostly into ion thermal energy through the outflow, causing the reconnection heating energy proportional to square of the reconnecting (poloidal) magnetic field Brec2 ˜ Bp2. The guide toroidal field Bt does not affect the bulk heating of ions and electrons, probably because the reconnection/outflow speeds are determined mostly by the external driven inflow by the help of another fast reconnection mechanism: intermittent sheet ejection. The localized electron heating at the X-point increases sharply with the guide toroidal field Bt, probably because the toroidal field increases electron confinement and acceleration length along the X-line. 2D measurements of magnetic field and temperatures in the TS-3 tokamak merging experiment also reveal the detailed reconnection heating mechanisms mentioned above. The high-power heating of tokamak merging is useful not only for laboratory study of reconnection but also for economical startup and heating of tokamak plasmas. The MAST/TS-3 tokamak merging with Bp > 0.4 T will enables us to heat the plasma to the alpha heating regime: Ti > 5 keV without using any additional heating facility.
Asymmetric Reconnection in the Terrestrial Reconnection EXperiment
NASA Astrophysics Data System (ADS)
Olson, Joseph; Egedal, Jan; Forest, Cary; Wallace, John; TREX Team; MPDX Team
2014-10-01
The Terrestrial Reconnection EXperiment (TREX) is a new and versatile addition to the Wisconsin Plasma Astrophysics Laboratory (WiPAL) at the University of Wisconsin-Madison. TREX is optimized for the study of kinetic reconnection in various regimes and to provide the first laboratory evidence in support of a new model describing the dynamics of trapped electrons and correlating pressure anisotropy. The initial configuration implemented in TREX is specially designed to study asymmetric reconnection scenarios. These are particularly relevant to the dayside magnetopause in which the plasma beta of the solar wind and of the magnetosphere can differ by factors of 100-1000. The configuration utilizes the Helmholtz coils to produce a static, uniform magnetic field up to 275 G through the 3 m spherical vacuum vessel. Plasma is produced on a 10 s rep rate while two internal coils are pulsed, creating an opposing magnetic field to induce reconnection with asymmetric high and low beta inflows. A Langmuir and Bdot probe array is swept in between pulses to build up the magnetic profiles in the reconnection region. Preliminary data from these initial runs will be presented.
Spontaneous three-dimensional magnetic reconnection in merging toroidal plasma experiment
Ii, Toru; Ono, Yasushi
2013-01-15
We investigated a new phenomenon of three-dimensional (3D) magnetic reconnection in TS-4 torus plasma merging experiments by directly measuring the 3D structures of the current sheet. Removal of all toroidal asymmetry of the device reveals that a strong external drive of reconnection inflow increases the toroidal asymmetry of the current sheet only during the reconnection. This spontaneous 3D deformation of the current sheet increases the reconnection outflow as well as the reconnection electric field, probably because local compression of the current sheet to a thickness less than the ion gyroradius triggers its strong dissipation of the current sheet, responsible for the onset of 3D reconnection. These mechanisms indicate that the 3D reconnection is a newly observed spontaneous process of fast reconnection.
Spontaneous three-dimensional magnetic reconnection in merging toroidal plasma experiment
NASA Astrophysics Data System (ADS)
Ii, Toru; Ono, Yasushi
2013-01-01
We investigated a new phenomenon of three-dimensional (3D) magnetic reconnection in TS-4 torus plasma merging experiments by directly measuring the 3D structures of the current sheet. Removal of all toroidal asymmetry of the device reveals that a strong external drive of reconnection inflow increases the toroidal asymmetry of the current sheet only during the reconnection. This spontaneous 3D deformation of the current sheet increases the reconnection outflow as well as the reconnection electric field, probably because local compression of the current sheet to a thickness less than the ion gyroradius triggers its strong dissipation of the current sheet, responsible for the onset of 3D reconnection. These mechanisms indicate that the 3D reconnection is a newly observed spontaneous process of fast reconnection.
What Can We Learn about Magnetotail Reconnection from 2D PIC Harris-Sheet Simulations?
NASA Astrophysics Data System (ADS)
Goldman, M. V.; Newman, D. L.; Lapenta, G.
2016-03-01
The Magnetosphere Multiscale Mission (MMS) will provide the first opportunity to probe electron-scale physics during magnetic reconnection in Earth's magnetopause and magnetotail. This article will address only tail reconnection—as a non-steady-state process in which the first reconnected field lines advance away from the x-point in flux pile-up fronts directed Earthward and anti-Earthward. An up-to-date microscopic physical picture of electron and ion-scale collisionless tail reconnection processes is presented based on 2-D Particle-In-Cell (PIC) simulations initiated from a Harris current sheet and on Cluster and Themis measurements of tail reconnection. The successes and limitations of simulations when compared to measured reconnection are addressed in detail. The main focus is on particle and field diffusion region signatures in the tail reconnection geometry. The interpretation of these signatures is vital to enable spacecraft to identify physically significant reconnection events, to trigger meaningful data transfer from MMS to Earth and to construct a useful overall physical picture of tail reconnection. New simulation results and theoretical interpretations are presented for energy transport of particles and fields, for the size and shape of electron and ion diffusion regions, for processes occurring near the fronts and for the j × B (Hall) electric field.
NASA Astrophysics Data System (ADS)
Cassak, P.; Doss, C.; Palmroth, M.; Hoilijoki, S.; Pfau-Kempf, Y.; Ganse, U.; Dorelli, J.
2015-12-01
Flux ropes caused by magnetic reconnection commonly form at the dayside magnetopauses of Earth and other planets, such as Mercury and Jupiter. They are convected tailward due to their interaction with the solar wind and as the result of reconnection. The leading model for their tailward propagation speed at Earth's magnetopause has been described using boundary layer physics (Cowley and Owen, Planet. Space Sci., 37, 1461, 1989). We revisit this topic, noting that during times when the reconnection at both X-lines bracketing the flux ropes remain active, there should be consistency with the scaling laws of asymmetric magnetic reconnection with a flow shear. The convection speed of an isolated reconnecting X-line as a function of arbitrary upstream plasma parameters, including the reconnecting magnetic fields, densities, and upstream flow in the plane of the fields, was recently calculated analytically and tested with two-fluid simulations (Doss et al., J. Geophys. Res., submitted). Here, we present fully electromagnetic kinetic particle-in-cell simulations of local asymmetric reconnection with a flow shear that confirm the prediction in collisionless plasmas relevant to planetary magnetospheres. It is notable that the X-line convects even for sub-Alfvenic flow shear and can reconnect even for flow speeds exceeding twice the magnetosheath Alfven speed, which counters previous models. The application of these results for flux rope motion in global magnetospheric simulations of Earth is discussed, as are applications to the magnetospheres of other planets.
Weak collisionless shocks in laser-plasmas
NASA Astrophysics Data System (ADS)
Cairns, R. A.; Bingham, R.; Trines, R. G. M.; Norreys, P.
2015-04-01
We obtain a theory describing laminar shock-like structures in a collisionless plasma and examine the parameter limits, in terms of the ion sound Mach number and the electron/ion temperature ratio, within which these structures exist. The essential feature is the inclusion of finite ion temperature with the result that some ions are reflected from a potential ramp. This destroys the symmetry between upstream and downstream regions that would otherwise give the well-known ion solitary wave solution. We have shown earlier (Cairns et al 2014 Phys. Plasmas 21 022112) that such structures may be relevant to problems such as the existence of strong, localized electric fields observed in laser compressed pellets and laser acceleration of ions. Here we present results on the way in which these structures may produce species separation in fusion targets and suggest that it may be possible to use shock ion acceleration for fast ignition.
Electron scale structures and magnetic reconnection signatures in the turbulent magnetosheath
NASA Astrophysics Data System (ADS)
Yordanova, E.; Vörös, Z.; Varsani, A.; Graham, D. B.; Norgren, C.; Khotyaintsev, Yu. V.; Vaivads, A.; Eriksson, E.; Nakamura, R.; Lindqvist, P.-A.; Marklund, G.; Ergun, R. E.; Magnes, W.; Baumjohann, W.; Fischer, D.; Plaschke, F.; Narita, Y.; Russell, C. T.; Strangeway, R. J.; Le Contel, O.; Pollock, C.; Torbert, R. B.; Giles, B. J.; Burch, J. L.; Avanov, L. A.; Dorelli, J. C.; Gershman, D. J.; Paterson, W. R.; Lavraud, B.; Saito, Y.
2016-06-01
Collisionless space plasma turbulence can generate reconnecting thin current sheets as suggested by recent results of numerical magnetohydrodynamic simulations. The Magnetospheric Multiscale (MMS) mission provides the first serious opportunity to verify whether small ion-electron-scale reconnection, generated by turbulence, resembles the reconnection events frequently observed in the magnetotail or at the magnetopause. Here we investigate field and particle observations obtained by the MMS fleet in the turbulent terrestrial magnetosheath behind quasi-parallel bow shock geometry. We observe multiple small-scale current sheets during the event and present a detailed look of one of the detected structures. The emergence of thin current sheets can lead to electron scale structures. Within these structures, we see signatures of ion demagnetization, electron jets, electron heating, and agyrotropy suggesting that MMS spacecraft observe reconnection at these scales.
Observed Aspects of Reconnection in Solar Eruptions
NASA Technical Reports Server (NTRS)
Moore, Ronald L.
2010-01-01
Signatures of reconnection in major CME (coronal mass ejection)/flare eruptions and in coronal X-ray jets are illustrated and interpreted. The signatures are magnetic field lines and their feet that brighten in flare emission. CME/flare eruptions are magnetic explosions in which: 1. The field that erupts is initially a closed arcade. 2. At eruption onset, most of the free magnetic energy to be released is not stored in field bracketing a current sheet, but in sheared field in the core of the arcade. 3. The sheared core field erupts by a process that from its start or soon after involves fast tether-cutting reconnection at an initially small current sheet low in the sheared core field. If the arcade has oppositely-directed field over it, the eruption process from its start or soon after also involves fast breakout reconnection at an initially small current sheet between the arcade and the overarching field. These aspects are shown by the small area of the bright field lines and foot-point flare ribbons in the onset of the eruption. 4. At either small current sheet, the fast reconnection progressively unleashes the erupting core field to erupt with progressively greater force. In turn, the erupting core field drives the current sheet to become progressively larger and to undergo progressively greater fast reconnection in the explosive phase of the eruption, and the flare arcade and ribbons grow to become comparable to the pre-eruption arcade in lateral extent. In coronal X-ray jets: 1. The magnetic energy released in the jet is built up by the emergence of a magnetic arcade into surrounding unipolar "open" field. 2. A simple jet is produced when a burst of reconnection occurs at the current sheet between the arcade and the open field. This produces a bright reconnection jet and a bright reconnection arcade that are both much smaller in diameter that the driving arcade. 3. A more complex jet is produced when the arcade has a sheared core field and undergoes an
Formation of Plasmoid Chains in Magnetic Reconnection
Samtaney, R.; Loureiro, N. F.; Uzdensky, D. A.; Schekochihin, A. A.; Cowley, S. C.
2009-09-09
A detailed numerical study of magnetic reconnection in resistive MHD for very large, previously inaccessible, Lundquist numbers (104 ≤ S ≤ 108) is reported. Large-aspect-ratio Sweet-Parker current sheets are shown to be unstable to super-Alfvenically fast formation of plasmoid (magnetic-island) chains. The plasmoid number scales as S3/8 and the instability growth rate in the linear stage as S1/4, in agreement with the theory by Loureiro et al. [Phys. Plasmas 14, 100703 (2007)]. In the nonlinear regime, plasmoids continue to grow faster than they are ejected and completely disrupt the reconnection layer. These results suggest that high-Lundquist-number reconnection is inherently time-dependent and hence call for a substantial revision of the standard Sweet-Parker quasistationary picture for S>104.
Nonlinear regimes of forced magnetic reconnection
Vekstein, G.; Kusano, K.
2015-09-15
This letter presents a self-consistent description of nonlinear forced magnetic reconnection in Taylor's model of this process. If external boundary perturbation is strong enough, nonlinearity in the current sheet evolution becomes important before resistive effects come into play. This terminates the current sheet shrinking that takes place at the linear stage and brings about its nonlinear equilibrium with a finite thickness. Then, in theory, this equilibrium is destroyed by a finite plasma resistivity during the skin-time, and further reconnection proceeds in the Rutherford regime. However, realization of such a scenario is unlikely because of the plasmoid instability, which is fast enough to develop before the transition to the Rutherford phase occurs. The suggested analytical theory is entirely different from all previous studies and provides proper interpretation of the presently available numerical simulations of nonlinear forced magnetic reconnection.