Alexander Cushen

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.

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.

At the end of its mission, the MESSENGER spacecraft's orbit intersected Mercury's nightside magnetic equator at low altitudes below 420 km, enabling the first in situ observations of this region, where the magnetic field strength is typically sub‐dipolar. We present 5 events from these orbits where MESSENGER encountered Earth‐like dipolarization regions characterized by enhanced field strengths up to 20 nT above the intrinsic planetary field, and an average decrease and increase in plasma proton density and temperature, respectively, for 1–2 min periods, comparable to Hermean substorm timescales. The events span local times of 1.5 hr pre‐ and post‐midnight, and are present from the magnetic equator up to magnetic latitudes of at least north. Supported by estimates of decreased flux tube entropy during these events, we suggest these dipolarization regions are formed by the pileup of dipolarization fronts and formation of a substorm current wedge.

Safe and reliable lunar landings are crucial for future exploration of the Moon. The regolith ejected by a lander’s rocket exhaust plume represents a significant obstacle in achieving this goal. It prevents spacecraft from reliably utilizing their navigation sensors to monitor their trajectory and spot emerging surface hazards as they near the surface. As part of NASA’s 2024 Human Lander Challenge (HuLC), the team at the University of Michigan developed an innovative concept to help mitigate this issue. We developed and implemented a machine learning (ML)-based sensor fusion system, ARC-LIGHT, that integrates sensor data from the cameras, lidars, or radars that landers already carry but disable during the final landing phase. Using these data streams, ARC-LIGHT will remove erroneous signals and recover a useful detection of the surface features to then be used by the spacecraft to correct its descent profile. It also offers a layer of redundancy for other key sensors, like inertial measurement units. The feasibility of this technology was validated through development of a prototype algorithm, which was trained on data from a purpose-built testbed that simulates imaging through a dusty environment. Based on these findings, a development timeline, risk analysis, and budget for ARC-LIGHT to be deployed on a lunar landing was created.