Do Mercury's Dipolarization Fronts Originate From Flux Ropes? MHD-AEPIC Simulations and Observations of Mercury's Magnetosphere

Mercury’s small magnetosphere and strong solar wind driving results in short-lived, highly dynamic substorms where large amounts of magnetic flux is processed in the magnetotail through nightside reconnection. This processing is realized through the rapid formation of planetward and tailward-moving flux ropes and dipolarization fronts, which lead to plasma heating and flux transport in the low-altitude plasma sheet. The MESSENGER spacecraft observed dipolarization fronts and flux ropes during its orbital campaign from 2011-2015, but open questions about their dynamics, relationship to each other, and role in broader magnetospheric processes persist. To contextualize these observations, we present coupled fluid-kinetic simulations of Mercury's magnetosphere using the MHD-AEPIC code, implemented through the Space Weather Modeling Framework. This model utilizes a Hall-MHD solver for the global magnetosphere coupled to an embedded particle-in-cell code, which simulates the magnetotail dynamics. By tracking the 3D, time-resolved propagation of dipolarization fronts, we find that only ~60 of events originate directly through single x-line reconnection, while the remainder originate from flux ropes that undergo secondary reconnection closer to the planet to create DF-like signatures. We present case studies of both event types, finding that the secondary reconnection process leads to localized heating of the electron fluid along the reconnecting flux tube to temperatures of >4 keV. We compare these characteristics to dipolarization fronts detected by MESSENGER, finding that this model may help account for some of the observed magnetic signatures, associated electron injections, and dawn-dusk distribution asymmetries. Future observations by BepiColombo will be important for further characterizing the frequency and impact of this process within Mercury's magnetotail.