M. Kuznetsova

Geospace Environmental Modeling (GEM) Magnetic Reconnection Challenge

The Geospace Environmental Modeling (GEM) Reconnection Challenge project is presented and the important results, which are presented in a series of companion papers, are summarized. Magnetic reconnection is studied in a simple Harris sheet configuration with a specified set of initial conditions, including a finite amplitude, magnetic island perturbation to trigger the dynamics. The evolution of the system is explored with a broad variety of codes, ranging from fully electromagnetic particle in cell (PIC) codes to conventional resistive magnetohydrodynamic (MHD) codes, and the results are compared. The goal is to identify the essential physics which is required to model collisionless magnetic reconnection. All models that include the Hall effect in the generalized Ohms law produce essentially indistinguishable rates of reconnection, corresponding to nearly Alfvenic inflow velocities. Thus the rate of reconnection is insensitive to the specific mechanism which breaks the frozen-in condition, whether resistivity, electron inertia, or electron thermal motion. The reconnection rate in the conventional resistive MHD model, in contrast, is dramatically smaller unless a large localized or current dependent resistivity is used. The Hall term brings the dynamics of whistler waves into the system. The quadratic dispersion property of whistlers (higher phase speed at smaller spatial scales) is the key to understanding these results. The implications of these results for trying to model the global dynamics of the magnetosphere are discussed.

The Diffusion Region in Collisionless Magnetic Reconnection

The structure of the dissipation region in collisionless magnetic reconnection is investigated by means of kinetic particle-in-cell simulations and analytical theory. Analyses of simulations of reconnecting current sheets without guide magnetic field, which keep all parameters fixed with the exception of the electron mass, exhibit very similar large scale evolutions and time scales. A detailed comparison of two runs with different electron masses reveals very similar large scale parameters, such as ion flow velocities and magnetic field structures. The electron-scale phenomena in the reconnection region proper, however, appear to be quite different. The scale lengths of these processes are best organized by the trapping length of bouncing electrons in a field reversal region. The dissipation is explained by the electric field generated by nongyrotropic electron pressure tensor effects. In the reconnection region, the relevant electron pressure tensor components exhibit gradients which are independent of t...

Collisionless magnetic reconnection: Electron processes and transport modeling

Particle-in-cell simulations are used to investigate collisionless magnetic reconnection in thin current sheets, based on the configuration chosen for the Geospace Environment Modeling (GEM) magnetic reconnection challenge [Birn et al., this issue]. The emphasis is on the overall evolution, as well as details of the particle dynamics in the diffusion region. Here electron distributions show clear signatures of nongyrotropy, whereas ion distributions are simpler in structure. The investigations are extended to current sheets of different widths. Here we derive a scaling law for the evolution dependence on current sheet width. Finally, we perform a detailed comparison between a kinetic and Hall-magnetohydrodynamic model of the same system. The comparison shows that although electric fields appear to be quite similar, details of the evolution appear to be considerably different, indicative of the role of further anisotropies in the ion pressures.

Collisionless reconnection supported by nongyrotropic pressure effects in hybrid and particle simulations

This paper presents the detailed comparative analysis of full particle and hybrid simulations of collisionless magnetic reconnection. The comprehensive hybrid simulation code employed in this study incorporates essential electron kinetics in terms of the evolution of the full electron pressure tensor in addition to the full ion kinetics and electron bulk flow inertia effects. As was demonstrated in our previous publications, the electron nongyrotropic pressure effects play the dominant role in supporting the reconnection electric field in the immediate vicinity of the neutral X point. The simulation parameters are chosen to match those of the Geospace Environmental Modeling (GEM) “Reconnection Challenge.” It is that these comprehensive hybrid simulations perfectly reproduce the results of full particle simulations in many details. Specifically, the time evolutions of the reconnected magnetic flux and the reconnection electric field, as well as spatial distributions of current density and magnetic field at all stages of the reconnection process, are found to be nearly identical for both simulations. Comparisons of variations of characteristic quantities along the x and z axes centered around the dominating X points also revealed a remarkable agreement. Noticeable differences are found only in electron temperature profiles, i.e., in the diagonal electron pressure tensor components. The deviation in the electron heating pattern in hybrid simulations from that observed in particle simulations, however, does not affect parameters essential for the reconnection process. In particular, the profiles of the off-diagonal components of the electron pressure tensor are found to be very similar for both runs and appear unaffected by heat flux effects. Both simulations also demonstrate that the Ey component of the electric field is nearly constant inside the diffusion region where ions are nonmagnetized. We demonstrate that the simple analytical estimate for the reconnection electric field as a convection electric field at the edge of the diffusion region very well reproduces the reconnection electric field observed in the simulations.

Kinetic quasi‐viscous and bulk flow inertia effects in collisionless magnetotail reconnection

Both electron inertia and nongyrotropic effects were analysed using 2 1/2-dimensional hybrid simulations of collisionless magnetic reconnection. The traditional hybrid approach which treats ions as particles and electrons as an isotropic massless fluid was modified. In the new model the electron mass dependence was introduced both in the expression for the electric field and in the evolution equation of the full electron pressure tensor. This comprehensive hybrid model includes full ion kinetics, incorporates Hall effects, describes the leading terms in electron dynamics responsible for breaking the frozen magnetic flux constraint and allows to consider arbitrary ion/electron temperature and mass ratios. We demonstrate that the kinetic quasi-viscous electron inertia associated with nongyrotropic pressure effects dominates over the electron bulk flow inertia in controlling the structure of the dissipation region around the neutral X line. The reconnection electric field based on the nongyrotropic pressure tends to reduce the current density and to relax gradients in the vicinity of the X line (similar to the localized anomalous viscosity). On the other hand, the reconnection electric field based on electron bulk flow inertia tends to require an increased current density, with gradient scales comparable with the electron skin depth. The dependence on the ion/electron temperature ratio and on the current carrier also is discussed. An analytical analysis which supports the results of the numerical simulations is also presented.

New measure of the dissipation region in collisionless magnetic reconnection.

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 electrons 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.

Vlasov theory of the equilibrium structure of tangential discontinuities in space plasmas

Extensive theoretical work has been performed on the equilibrium structure of tangential discontinuities (TDs) in collisionless plasmas. This paper reviews kinetic models based on steady-state solutions of the Vlasov equation. It is shown that most of the existing models are special cases of a generalized multi-species model. In this generalized model all particle populations -from both outer regions and from inside the layer — are described using a unique formalism for the velocity distribution functions. Because of their historical importance, the Harris and Sestero models are reviewed and deduced from the generalized model. The Lee and Kan model is also a special case of the generalized model. The generalized model, however, is also able to describe TDs with velocity shear and large angles of magnetic field rotation. Such a multi-species model with a large number of free parameters and different gradient scales illustrates many observable features of TDs, including their multiscale fine structure. Particular attention is paid to the magnetopause. Observed magnetopause crossings are simulated. The effects of the relative flow velocity and asymmetrical magnetic field profiles on the structure of the magnetopause and on its stability with respect to tearing perturbations are discussed. We also present calculations that demonstrate the potential of the generalized model in explaining the origin of discrete auroral arcs. Numerical simulations of solar wind TDs with heavy ions and a large spectrum of thicknesses are also feasible. This indicates that such a model is of fundamental importance for understanding the detailed structure of solar wind TDs, like those observed by the interplanetary spacecraft ULYSSES. The problems associated with the one-dimensional, time-independent Vlasov approach are discussed and a variational principle is suggested to reduce the arbitrariness resulting from the large number of free parameters.

The role of electron heat flux in guide-field magnetic reconnection

A combination of analytical theory and particle-in-cell simulations are employed in order to investigate the electron dynamics near and at the site of guide field magnetic reconnection. A detailed analysis of the contributions to the reconnection electric field shows that both bulk inertia and pressure-based quasiviscous processes are important for the electrons. Analytic scaling demonstrates that conventional approximations for the electron pressure tensor behavior in the dissipation region fail, and that heat flux contributions need to be accounted for. Based on the evolution equation of the heat flux three tensor, which is derived in this paper, an approximate form of the relevant heat flux contributions to the pressure tensor is developed, which reproduces the numerical modeling result reasonably well. Based on this approximation, it is possible to develop a scaling of the electron current layer in the central dissipation region. It is shown that the pressure tensor contributions become important at the scale length defined by the electron Larmor radius in the guide magnetic field.

ISEE 3 observations of plasmoids with flux rope magnectic topologies

This paper reports new evidence for the existence of plasmoids with force-free flux rope magnetic topologies. Motivated by the fact that force-free magnetic flux ropes have intense axial fields at their centers, the ISEE 3 observations have been searched for plasma sheet intervals in which the magnetic field intensity exceeds that in the lobes by ≥10% for a minute or longer. A total of 39 “high field regions” were found which met this simple criterion. Further examination showed that they nearly always correspond to the core regions of plasmoids; i.e, intervals of bipolar Bz with durations of minutes, fast tailward flows and clear substorm associations. A new, more realistic plasmoid magnetic field model which represents the core region as a non-linear force-free flux rope was developed and validated using these events. Finally, observations suggesting that plasmoids evolve toward quasi-force-free flux rope configurations as they move down the tail are presented.

Geospace Environment Modeling 2008–2009 Challenge: Ground magnetic field perturbations

helps the users of the modeling products to better understand the capabilities of the models and to choose the approach that best suits their specific needs. Further, metrics!based analyses are important for addressing the differences between various modeling approaches and for measuring and guiding the progress in the field. In this paper, the metrics!based results of the ground magnetic field perturbation part of the Geospace Environment Modeling 2008‐2009 Challenge are reported. Predictions made by 14 different models, including an ensemble model, are compared to geomagnetic observatory recordings from 12 different northern hemispheric locations. Five different metrics are used to quantify the model performances for four storm events. It is shown that the ranking of the models is strongly dependent on the type of metric used to evaluate the model performance. None of the models rank near or at the top systematically for all used metrics. Consequently, one cannot pick the absolute“winner”: the choice for the best model depends on the characteristics of the signal one is interested in. Model performances vary also from event to event. This is particularly clear for root!mean!square difference and utility metric!based analyses. Further, analyses indicate that for some of the models, increasing the global magnetohydrodynamic model spatial resolution and the inclusion of the ring current dynamics improve the models’capability to generate more realistic ground magnetic field fluctuations.