Z. W. Ma

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.

Hall magnetohydrodynamic reconnection: The Geospace Environment Modeling challenge

Numerical results are presented on the Geospace Environment Modeling (GEM) reconnection challenge (and its variants) from the Hall magnetohydrodynamics (MHD) code developed at the University of Iowa (UI). Resistivity provides the mechanism for breaking field lines in this study. It is shown that the peak reconnection rate in the quasi-saturated regime is controlled dominantly by ions and has a weak dependence on the resistivity. The reconnection rate is close to those obtained from other particle-in-cell, hybrid, and Hall MHD codes. Some differences between the results from the UI Hall MHD code and other codes are discussed.

Fast impulsive reconnection and current sheet intensification due to electron pressure gradients in semi‐collisional plasmas

A numerical simulation of forced reconnection and current sheet growth due to inward boundary flows in semi-collisional plasmas is presented, and contrasted with the results of an incompressible resistive MHD simulation in the high-Lundquist-number regime. Due to the presence of electron pressure (or Hall currents) in the generalized Ohms law, the reconnection dynamics makes an impulsive transition from a slow linear regime to a nonlinear regime characterized by fast reconnection and current sheet intensification at a near-Alfvenic rate. The current sheet spanning Y-points in the early nonlinear regime shrinks and approaches an X-point geometry. The spatial scale of the collisionless parallel electric field is the ion skin depth, and decoupled from the spatial scale of the parallel current which is much narrower and determined by the Lundquist number.

Kinetic Alfvén waves as a source of plasma transport at the dayside magnetopause

As the shocked solar wind with variable plasma density and magnetic field impinges on the dayside magnetopause, it is likely to generate large-scale Alfven waves at the solar wind magnetosphere interface. However, large gradients in the density and magnetic field at the magnetopause boundary effectively couple large-scale Alfven waves with kinetic Alfven waves. In this paper the authors propose that the wave power converted into kinetic Alfven waves may play an important role in plasma transport at the dayside magnetopause and in electron acceleration along field lines. The transport can occur because, unlike the magnetohydrodynamic (MHD) shear Alfven wave, the kinetic Alfven wave has an associated parallel electric field which breaks down the {open_quotes}frozen-in{close_quotes} condition and decouples the plasma from field lines. The authors calculate the average deviation of the plasma from the field line from which they estimate the diffusion coefficient associated with these {open_quotes}bundles{close_quotes} of decoupled plasma to be approximately 10{sup 9}m{sup 2}/s. The parallel electric field also may lead to acceleration of electrons along field lines in the magnetopause boundary and may possibly provide an explanation for observed counterstreaming electron beams characterized by energies of 50-200 eV. 31 refs., 4 figs.

Ballooning instability of a thin current sheet in the high‐Lundquist‐number magnetotail

Two-dimensional simulations of the magnetotail in the high-Lundquist-number regime indicate the slow growth of thin current sheets and an impulsive intensification of the cross-tail current density at near-Earth distances during a short interval just before the onset of the expansion phase, consistent with multi-satellite observations. Such a two-dimensional magnetotail, symmetric along y and containing a thin current sheet, is found to be unstable to a symmetry-breaking, ideal compressible ballooning instability with high wave number along y. The linear instability is demonstrated by numerical solutions of the ideal ballooning eigenmode equation for a sequence of two-dimensional thin current sheet configurations in the impulsive growth phase. Line-tied boundary conditions at the ionosphere are imposed, and shown to play a crucial role in the stability analysis. It is suggested that the ideal ballooning instability, which has strong spatial variation along y, provides a possible mechanism for disrupting the cross-tail current at onset.

Collisionless reconnection: Effects of Hall current and electron pressure gradient

An analytical treatment is given of two-dimensional, quasi-steady collisionless reconnection on the basis of the generalized Ohms law, including the effects of the Hall current and scalar electron pressure gradient. The equilibrium magnetic field configuration is of the form , containing a neutral line at z = 0 and a constant guide field BT. The dispersion relations of the waves in the ideal region as well as the reconnection layer are discussed, including the effects of plasma beta. When BT = 0, the reconnection layer supports obliquely propagating Alfven-whistler waves, and the reconnection dynamics is controlled by the Hall current. When BT/BP0 ≥ 1, the reconnection layer supports kinetic/inertial Alfven waves, and the reconnection dynamics is controlled by the electron pressure gradient. Analytical estimates are obtained for the nonlinear reconnection rate with and without the guide field from the generalized Ohms law. A recent claim by Shay et al. (1999) that the reconnection rate is a “universal constant” is questioned. Although the leading-order reconnection rate is independent of the mechanism that breaks field lines (resistivity or electron inertia) and the system size as the system size becomes large, it does depend on global conditions such as the boundary conditions driving reconnection. The analytical predictions are tested by means of Hall magnetohydrodynamics simulations. While some of the geometric features of the reconnection layer and the weak dependence of the reconnection rate on resistivity are reminiscent of Petscheks classical model, the underlying wave and particle dynamics mediating reconnection in the presence of the Hall current and electron pressure gradient are qualitatively different.

Global view of dayside magnetic reconnection with the dusk-dawn IMF orientation: A statistical study for Double Star and Cluster data

Double Star/TC-1 and Cluster data show that both component reconnection and anti-parallel reconnection occur at the magnetopause when the interplanetary magnetic field ( IMF) is predominantly dawnward. The occurrence of these different features under these very similar IMF conditions are further confirmed by a statistical study of 290 fast flows measured in both the low and high latitude magnetopause boundary layers. The directions of these fast flows suggest a possible S-shaped configuration of the reconnection X-line under such a dawnward dominated IMF orientation.

Core magnetic field enhancement in single X line, multiple X line and patchy reconnection

Magnetic flux transfer events often show a significant increase of the magnetic field strength at the center of the events. Similar magnetic field observations have been reported for structures in or near the plasma sheet of the magnetotail at about 20 RE. We have carried out two-dimensional (2D) and three-dimensional (3D) simulations of single X reconnection (SXR), multipleX line reconnection (MXR), and patchy reconnection to determine and compare the amplification of the magnetic field in the center of the developing flux tubes. The various processes are achieved by appropriate choices of 2D or 3D resistivity models. The simulations show that the increase in magnetic field strength depends on both the property of the initial configuration and the particular reconnection geometry. For the chosen initial conditions the MXR process leads to a larger increase of the core magnetic field than the patchy reconnection and SXR caused by larger magnetic tensions in the MXR process. The 3D processes always lead to a larger amplification than the corresponding 2D processes. In the 3D cases, force imbalance in the y direction will accelerate plasma out of the flux tube. This process reduces the thermal pressure and leads to a further compression of the flux rope, which yields an additional increase in the interior magnetic field strength.

Recent developments in collisionless reconnection theory: Applications to laboratory and space plasmas

Recent developments in the theory and simulation of nonlinear collisionless reconnection hold the promise for providing solutions to some outstanding problems in laboratory and astrophysical plasma physics. Examples of such problems are sawtooth oscillations in tokamaks, magnetotail substorms, and impulsive solar and stellar flares. In each of these problems, a key issue is the identification of fast reconnection rates that are insensitive to the mechanism that breaks field lines (resistivity and/or electron inertia). The classical models of Sweet-Parker and Petschek sought to resolve this issue in the realm of resistive magnetohydrodynamics (MHD). However, the plasmas mentioned above are weakly collisional, and hence obey a generalized Ohm’s law in which the Hall current and electron pressure gradient terms play a crucial role. Recent theoretical models and simulations on impulsive (or triggered) as well as quasi-steady reconnection governed by a generalized Ohm’s law are reviewed. In the impulsive reconnection problem, not only is the growth rate fast but the time-derivative of the growth rate changes rapidly. In the steady-state reconnection problem, explicit analytical expressions are obtained for the geometric characteristics (that is, length and width) of the reconnection layer and the reconnection rate. Analytical results are tested by Hall MHD simulations. While some of the geometric features of the reconnection layer and the weak dependence of the reconnection rate on resistivity are reminiscent of Petschek’s classical model, the underlying wave and particle dynamics mediating the reconnection dynamics in the presence of the Hall current and electron pressure gradient are qualitatively different. Quantitative comparisons are made between theory and observations. Open and unresolved issues are identified.

Sudden enhancement and partial disruption of thin current sheets in the magnetotail due to Hall MHD effects

Multi-satellite observations indicate the development of thin current sheets and a rapid intensification of the cross-tail current density at near-Earth distances during a short interval (<1 min) just before onset, after a period of sluggish growth (∼ 0.5–1.5 hr). These observational features are described by a Hall MHD simulation in which we include the effects of the electron pressure gradient and the Hall current. These Hall MHD effects decouple the spatial scales of the parallel electric field and current density. The thin current sheet exhibits an impulsive pre-onset enhancement at near-Earth distances on a near-Alfvenic time scale, weakly dependent on the value of the Lundquist number. The amplitude of the pre-onset current sheet is larger, and its subsequent disruption faster than in a resistive MHD simulation. Agreement and discrepancies between theory and observation are discussed.