From Flux Rope to Dipolarization Front: Global MHD‐AEPIC Simulations of Mercury's Magnetosphere and Magnetotail Dynamics

Mercury's small magnetosphere and strong solar wind driving result in the rapid formation of planetward and tailward‐moving flux ropes (FRs) and dipolarization fronts (DFs) in its magnetotail. These are characterized by large variations in the dipole‐aligned magnetic field component over s timescales and are often associated with fast planetward or tailward plasma flows. At present, the dynamic evolution of DFs and FRs and the relationship between them remains poorly understood at Mercury. To contextualize single‐point observations of these events by the MESSENGER spacecraft, we present coupled fluid‐kinetic simulations of Mercury's magnetosphere using the Magnetohydrodynamics with Adaptively Embedded Particle‐in‐Cell (MHD‐AEPIC) code. Driven by steady southward interplanetary magnetic field and nominal solar wind conditions about aphelion, the simulation produces recurrent DFs in the tail with peak occurrence rates of DFs per minute and jumps in magnetic field ranging from nT, mirroring those observed by MESSENGER. By tracking the 3D, time‐resolved propagation of the simulated DFs, we find that of events originate directly through reconnection at the nearest x‐line to the planet, while the remainder originate from FRs that undergo secondary reconnection or “re‐reconnection” closer to the planet to create DF‐like signatures. Our results suggest that this secondary reconnection process leads to localized heating of electrons along the reconnecting flux tube up to temperatures of 4 keV, which may help account for the suprathermal electrons observed in Mercury's low‐altitude current sheet.