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 mi/me=1836. In this high-mi/me-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 ∂pexy/∂x and ∂peyz/∂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.

The Adiabatic 1D Kinetic Equilibrium of the Electron Diffusion Region During Anti‐Parallel Magnetic Reconnection

“…Consistent with the results for 1D current layer driven by electron pressure anisotropy (Egedal, 2023), previous work has suggested that the EDR current layers of anti-parallel reconnection are similarly linked to pressure anisotropy (Egedal et al, 2013(Egedal et al, , 2023Le et al, 2010). Illustrated in Figure 2a, the electron anisotropy with T e‖ ≫ T e⊥ develops self-consistently within the inflow regions.…”

“…For values of β e∞ considered here and typical of the Earth's magnetosphere it is found that Êrec ≪ 1, such that Êrec only marginally influences the rapid motion of the individual electrons through the EDR (Egedal et al, 2023) (for the particular run considered here we have Êrec ≃ 0.025, see Figure 10c in Ref. Egedal et al (2023)). As a result, the structure of the inflow region and the EDR is well accounted for by the adiabatic electron equilibrium governed by Equation 2, where the low values of Êrec only yields small perturbations away from the 1D equilibrium geometry.…”

“…In Ref. Egedal et al (2023), l u is shown to regulate the magnitude of the off-diagonal pressure tensor elements near the center of the EDR; for example, P exy ≃ (x/l u )(P exx P eyy ).…”

Fast Tearing Mode Driven by Demagnetized Electrons

“…An ion-scale TCS model in the weak ion pressure anisotropy approximation was proposed by Arnold and Sitnov (2023). Egedal et al (2023) presented a model of electron-scale TCS driven by strong electron pressure anisotropy in the reconnection region.…”

“…Egedal et al. (2023) presented a model of electron‐scale TCS driven by strong electron pressure anisotropy in the reconnection region.…”