Spatial dimensions of the electron diffusion region in anti-parallel magnetic reconnection

Abstract

Abstract. Spatial dimensions of the detailed structures of the electron diffusion region in anti-parallel magnetic reconnection were analyzed based on two-dimensional fully kinetic particle-in-cell simulations. The electron diffusion region in this study is defined as the region where the positive reconnection electric field is sustained by the electron inertial and non-gyrotropic pressure components. Past kinetic studies demonstrated that the dimensions of the whole electron diffusion region and the inner non-gyrotropic region are scaled by the electron inertial length d e and the width of the electron meandering motion, respectively. In this study, we successfully obtained more precise scalings of the dimensions of these two regions than the previous studies by performing simulations with sufficiently small grid spacing (1/16-1/8 d e ) and a sufficient number of particles (800 particles cell −1 on average) under different conditions changing the ion-to-electron mass ratio, the background density and the electron β e (temperature). The obtained scalings are adequately supported by some theories considering spatial variations of field and plasma parameters within the diffusion region. In the reconnection inflow direction, the dimensions of both regions are proportional to d e based on the background density. Both dimensions also depend on β e based on the background values, but the dependence in the inner region (∼ 0.375th power) is larger than the whole region (0.125th power) reflecting the orbits of meandering and accelerated electrons within the inner region. In the outflow direction, almost only the non-gyrotropic component sustains the positive reconnection electric field. The dimension of this single-scale diffusion region is proportional to the ionelectron hybrid inertial length (d i d e ) 1/2 based on the background density and weakly depends on the background β e with the 0.25th power. These firm scalings allow us to predict observable dimensions in real space which are indeed in reasonable agreement with past in situ spacecraft observations in the Earth's magnetotail and have important implications for future observations with higher resolutions such as the NASA Magnetospheric Multiscale (MMS) mission.

Electron scale structures and magnetic reconnection signatures in the turbulent magnetosheath

“…The spacecraft does not clearly enter the electron diffusion region; however, the nondiagonal elements of electron pressure tensor increase significantly as MMS 1 probably crossed the separatrix region. This is consistent with simulations on the spatial dimensions of the electron diffusion region [e.g., Nakamura et al , ; Swisdak , ].…”

“…Field and plasma parameters in the rotated coordinate system: (a-c) magnetic field components for all spacecraft; (d-f ) electron speed components; (g-i) ion speed components; (j and k) parallel (black) and perpendicular (red) to the background magnetic field electron and ion temperature for all spacecraft, respectively; and (l) ion density.generalized Ohm's law and their relative strength indicate if the spacecraft are crossing the ion or electron diffusion regions[Nakamura et al, 2016]. The different terms in the Ohm's law are plotted inFigures 3a-3e.The (V i × B) z is small…”

Two-stage bulk electron heating in the diffusion region of anti-parallel symmetric reconnection

“…e∞ [32], our runs did not confirm this scaling. The precise length of the diffusion region may be sensitive to boundary conditions and the time selected for measuring its length.…”

Structure of the Current Sheet in the 11 July 2017 Electron Diffusion Region Event

“…It should be noted, however, that for spacecraft not at the center of the current sheet, there should also be a contribution from the electron bulk flow inertia term. The spatial dimensions of the inner EDR in antiparallel reconnection have been examined using PIC simulations, from which scaling laws based on the mass ratio were used to predict the results for a realistic mass ratio (Nakamura et al, 2016;Shay et al, 2007). Taking into account that the edge of the diffusion region is the point where the electric field corresponding to the Lorentz force becomes smaller than the reconnection electric field E M (where E M ′ becomes negative), Shay et al (2007) found that the inner EDR scale size becomes smaller as the mass ratio becomes larger and predicted that for a mass ratio of 1,836 the inner EDR scale size, Δ L ∼ 0.6 d i .…”