Structure and Dynamics of Three-Dimensional Magnetotail Reconnection

Abstract

We have used a global magnetohydrodynamic simulation and embedded particle-in-cell (PIC) simulation to analyze magnetotail reconnection in and near the electron diffusion region (EDR). Results from the magnetohydrodynamic simulation were used to set the initial and boundary conditions for the large-scale implicit PIC simulation. We examined proxies for the EDR (the nongyrotropy of the electron distribution function, slippage, and the nonideal terms in Ohm's law and the work done [J·E′] in the electron frame). The reconnection was well organized by the Bₓ = 0 surface. All of the proxies gave false positive values but together, along with the magnetic field Bz component and calculations of magnetic field lines, we were able to locate the EDR. Three of the proxies (the slippage, the nonideal terms in Ohm's law, and agyrotropy) give consistent results in the EDR. The EDR is structured and time dependent. Multiple EDRs aligned roughly parallel to the X axis extended from X ~ −31RE to X ~ −38RE with structure in the Y direction. There are regions with both J·E′ < 0 and J·E′ > 0. Wave-like behavior with a scale of ~0.5RE or 2–3dᵢ develops in the reconnected plasma sheet. The structure is closely related to strong electron flows in the YZ plane and to the density gradient at the outer edge of the plasma sheet. These results are consistent with expectations for the lower hybrid drift instability coupled with a shear-type instability such as the Kelvin-Helmholtz instability. Similar results were found in a Harris sheet PIC simulation.