The force balance of electrons during kinetic anti-parallel magnetic reconnection

Abstract

Fully kinetic simulations are applied to the study of 2D anti-parallel reconnection, elucidating the dynamics by which the electron fluid maintains force balance within both the ion diffusion region (IDR) and the electron diffusion region (EDR). Inside the IDR, magnetic field-aligned electron pressure anisotropy (pe ∥ ≫ pe⊥) develops upstream of the EDR. Compared to previous investigations, the use of modern computer facilities allows for simulations at the natural proton to electron mass ratio m i / m e = 1836. In this high-m i / m e-limit, the electron dynamics change qualitatively, as the electron inflow to the EDR is enhanced and mainly driven by the anisotropic pressure. Using a coordinate system with the x-direction aligned with the reconnecting magnetic field and the y-direction aligned with the central current layer, it is well known that for the much studied 2D laminar anti-parallel and symmetric scenario the reconnection electric field at the X-line must be balanced by the ∂ p exy/ ∂ x and ∂ p eyz/ ∂ z off-diagonal electron pressure stress components. We find that the electron anisotropy upstream of the EDR imposes large values of ∂ pexy/ ∂ x within the EDR, and along the direction of the reconnection X-line, this stress cancels with the stress of a previously determined theoretical form for ∂ peyz/ ∂ z. The electron frozen-in law is instead broken by pressure tensor gradients related to the direct heating of the electrons by the reconnection electric field. The reconnection rate is free to adjust to the value imposed externally by the plasma dynamics at larger scales.