Analytic Kinetic Electron Model for Antiparallel Symmetric Reconnection

Antiparallel symmetric reconnection has been studied intensively given its direct relevance to reconnection in the Earth’s magnetotail [1]. Recent results show that the structure of the electron diffusion region (EDR) and its vicinity can largely be accounted for through a relatively simple kinetic model [2,3]. The model relies on an adiabatic invariant of the electron motion perpendicular to the reconnection current layer, J_z ∝ ∫ v_z dz, and it accounts for the electron pressure anisotropy, which builds at the edge of the EDR. In turn, the thermal forces associated with this pressure anisotropy are primarily responsible for the strong electron jets observed within the EDR. In this talk we generalize the model to include non-adiabatic effects, related to the rotation of the current layer direction along the length of the EDR into the outflow directions. Furthermore, direct acceleration by the reconnection electric field yields small corrections to the electron trajectories and a modest enhancement of the EDR current intensity. The new generalized model is compared to a fully kinetic simulation, and successfully reproduces all electron fluid moments obtained for vertical cut through the EDR of the numerical run. The origin of off-diagonal stress in the electron pressure tensor is identified. This stress allows the electron fluid to break the frozen-in law, a fundamental requirement for magnetic reconnection to proceed.

[1] R. B. Torbert et al., Electron-scale dynamics of the diffusion region during symmetric magnetic reconnection in space, Science 10.1126 (2018).

[2] J. Egedal, On a plasma sheath with a small normal magneticfield separating regions of oppositely directed magnetic field, Phys. Plasmas 30, 10.1063/5.0170212 (2023).

[3] J. Egedal, The adiabatic 1D kinetic equilibrium of the electron diffusion region during anti-parallel magnetic reconnection, Geophy. Res. Lett. 51, 10.1029/2024GL108895 (2024).