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

An earlier model of the electron distribution function accounts for the pressure anisotropy that develops in the inflow regions during anti‐parallel magnetic reconnection. However, the model is not applicable to the electron diffusion region (EDR), where the electron magnetic moments break as an adiabatic invariant. The earlier model is here generalized through the use of the current‐sheet adiabatic invariant of a 1D current layer, to become applicable also to the EDR. The new generalized model reproduces the main features of the EDR and its vicinity observed in a fully kinetic simulation, formally proving the 1D equilibrium relationship between the upstream electron pressure anisotropy and the EDR electron jets. Plain Language Summary: Ever since 1953 when Dungey introduced the concept of magnetic reconnection in the Earth's magnetotail, the process has been investigated as the potential cause of the Aurora phenomenon. During reconnection the magnetic field lines break and reform with changed connectivity as they pass through a small electron diffusion region (EDR). In the magnetotail the EDRs responsible for the Aurora often forms about 20 Earths radii away from Earth, spectacularly breaking the standard rules of plasma physics in ways not fully understood. In the present letter we show how the structure of the EDR can largely be accounted for through a relatively simple kinetic model. The model relies on an adiabatic invariant of the electron motion perpendicular to the magnetotail current sheet, and it accounts for the electron pressure anisotropy which builds at the edge of the EDR. In turn, we show how the thermal forces associated with this pressure anisotropy are responsible for driving the strong electron jets observed within in the EDR. The model is validated against a state‐of‐the‐art supercomputer simulation of the reconnection process. Key Points: During anti‐parallel magnetic reconnection the electron diffusion region (EDR) has a largely 1D structureThe cross‐section of the 1D structure is governed by the invariance of the electron current sheet action integral, Jz=∮vzdz ${\mathcal{J}}_{z}=\oint {v}_{z}dz$Electron pressure anisotropy forms upstream of the EDR, couples to the Speiser orbits, and drives the electron jets of the EDR [ABSTRACT FROM AUTHOR]