Inner and outer electron diffusion region of antiparallel collisionless reconnection: Density dependence

Abstract

We study inflow density dependence of substructures within electron diffusion region (EDR) of collisionless symmetric magnetic reconnection. We perform a set of 2.5D particle-in-cell simulations which start from a Harris current layer with a uniform background density n b. A scan of n b ranging from 0:02 n 0 to 2 n 0 of the peak current layer density (n 0) is studied keeping other plasma parameters the same. Various quantities measuring reconnection rate, EDR spatial scales, and characteristic velocities are introduced. We analyze EDR properties during quasisteady stage when the EDR length measures saturate. Consistent with past kinetic simulations, electrons are heated parallel to the B field in the inflow region. The presence of the strong parallel anisotropy acts twofold: (1) electron pressure anisotropy drift gets important at the EDR upstream edge in addition to the E Â B drift speed and (2) the pressure anisotropy term Àr Á P ðeÞ =ðneÞ modifies the force balance there. We find that the width of the EDR demagnetization region and EDR current are proportional to the electron inertial length $d e and $d e n 0:22 b , respectively. Magnetic reconnection is fast with a rate of $0:1 but depends weakly on density as $n À1=8 b. Such reconnection rate proxies as EDR geometrical aspect or the inflow-to-outflow electron velocity ratio are shown to have different density trends, making electric field the only reliable measure of the reconnection rate.

Bridging hybrid- and full-kinetic models with Landau-fluid electrons

“…Eq. ( 11)) and thus a lower stationary reconnection rate is consistent with previous works (Divin et al 2019).…”

“…The same trend for the quasi-stationary reconnection rates (i.e., higher with isotropic and isothermal electrons, lower in the full-kinetic case) was also reported in Le et al (2016). Although the stationary reconnection rates are smaller than the "usual" value of about 0.1 (Cassak et al 2017), it is worth noticing that this rate can indeed also depend on the background density (see Wu et al 2011;Divin et al 2019) or, more precisely, on the ratio between the background (i.e., asymptotic) and the peak (i.e., in the middle of the Harris sheet) values of the density. In fact, most simulations adopt a background-to-peak density ratio of n b /n H,0 = 0.1 − 0.2 4 , in order to better compare with early simulations setup (usually GEM-like configurations, see Birn et al 2001) and with some magnetospheric data.…”

Bridging hybrid- and full-kinetic models with Landau-fluid electrons: I. 2D magnetic reconnection

“…The same trend for the quasi-stationary reconnection rates (i.e., higher with isotropic and isothermal electrons, lower in the full-kinetic case) was also reported in Le et al (2016). Although the stationary reconnection rates are smaller than the "usual" value of about 0.1 (Cassak et al 2017), it is worth noticing that this rate can indeed also depend on the background density (see Wu et al 2011;Divin et al 2019) or, more precisely, on the ratio between the background (i.e., asymptotic) and the peak (i.e., in the middle of the Harris sheet) values of the density. In fact, most simulations adopt a background-to-peak density ratio of n b /n H,0 = 0.1 − 0.2 4 , in order to better compare with early simulations setup (usually GEM-like configurations, see Birn et al 2001) and with some magnetoshperic data.…”

Calculating the Electron Diffusion Region Aspect Ratio With Magnetic Field Gradients

“…Equation 2 gives a theoretical aspect ratio value for a given set of background plasma conditions which can be compared with experimental values of aspect ratio calculated with Equation 1. The scaling relations used in Equation 2 were found for reconnection with an anti-parallel magnetic field configuration and a limited range of upstream background densities, and may require modification for guide field reconnection (Divin et al, 2019;Le et al, 2013). The dimensions in Equation 1 and Equation 2 are also defined differently: Equation 1 describes the dimensions of the region where electrons have non-gyrotropic orbits, whereas L z and L x in Equation 2 are the dimensions of the region with electron non-gyrotropy and a positive out-of-plane electric field component.…”