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

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Title: From Flux Rope to Dipolarization Front: Global MHD‐AEPIC Simulations of Mercury's Magnetosphere and Magnetotail Dynamics.
Authors: Cushen, Alexander T.1 (AUTHOR) atcushen@umich.edu, Jia, Xianzhe1 (AUTHOR), Slavin, James1 (AUTHOR), Chen, Yuxi1 (AUTHOR), Sun, Weijie2 (AUTHOR), Toth, Gabor1 (AUTHOR)
Source: Journal of Geophysical Research. Space Physics. May2026, Vol. 131 Issue 5, p1-22. 22p.
Subject Terms: Magnetic reconnection, Helical structure, Solar wind, Magnetospheric physics, Plasma heating, Space plasmas, Magnetosphere
Abstract: 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 Bz $\left({B}_{z}\right)$ over ∼1 ${\sim} 1$ 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 ∼4 ${\sim} 4$ DFs per minute and jumps in magnetic field ranging from 9−59 $9-59$ nT, mirroring those observed by MESSENGER. By tracking the 3D, time‐resolved propagation of the simulated DFs, we find that 55% $55\%$ 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. Plain Language Summary: The planet Mercury has an intrinsic magnetic field, which partially shields it from the solar wind plasma ejected from the Sun. However, some plasma is able to penetrate close to the planet, which also enables additional magnetic flux from the solar wind to temporarily accumulate within the planet's shadow. This anomalous magnetic flux often arrives in short bursts, termed dipolarization fronts, as they act to intensify the field in alignment with the planet's dipolar magnetic field. We perform computer simulations of this process, and find that the dipolarization fronts observed close to the planet may originate from one of two different mechanisms. One mechanism, which is more broadly recognized, allows the fronts to form more directly from the solar wind magnetic field. The other process, which is detailed in this study, has an intermediate step, where the magnetic field first forms a helical magnetic structure called a flux rope. Although both processes result in broadly similar impacts, we identify some systematic differences between them in how they heat up the plasma in the vicinity of Mercury's nightside surface. Key Points: Fluid‐kinetic simulations of Mercury's magnetosphere for southward IMF result in frequent dipolarization front formation in the plasma sheetA large fraction of dipolarization fronts originate in planetward‐moving flux ropes formed between multiple x‐lines in the near tailThese flux ropes reconnect with the planet's dipole field to produce dipolarization front signatures and heated reconnection outflows [ABSTRACT FROM AUTHOR]
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Abstract: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 Bz $\left({B}_{z}\right)$ over ∼1 ${\sim} 1$ 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 ∼4 ${\sim} 4$ DFs per minute and jumps in magnetic field ranging from 9−59 $9-59$ nT, mirroring those observed by MESSENGER. By tracking the 3D, time‐resolved propagation of the simulated DFs, we find that 55% $55\%$ 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. Plain Language Summary: The planet Mercury has an intrinsic magnetic field, which partially shields it from the solar wind plasma ejected from the Sun. However, some plasma is able to penetrate close to the planet, which also enables additional magnetic flux from the solar wind to temporarily accumulate within the planet's shadow. This anomalous magnetic flux often arrives in short bursts, termed dipolarization fronts, as they act to intensify the field in alignment with the planet's dipolar magnetic field. We perform computer simulations of this process, and find that the dipolarization fronts observed close to the planet may originate from one of two different mechanisms. One mechanism, which is more broadly recognized, allows the fronts to form more directly from the solar wind magnetic field. The other process, which is detailed in this study, has an intermediate step, where the magnetic field first forms a helical magnetic structure called a flux rope. Although both processes result in broadly similar impacts, we identify some systematic differences between them in how they heat up the plasma in the vicinity of Mercury's nightside surface. Key Points: Fluid‐kinetic simulations of Mercury's magnetosphere for southward IMF result in frequent dipolarization front formation in the plasma sheetA large fraction of dipolarization fronts originate in planetward‐moving flux ropes formed between multiple x‐lines in the near tailThese flux ropes reconnect with the planet's dipole field to produce dipolarization front signatures and heated reconnection outflows [ABSTRACT FROM AUTHOR]
ISSN:21699380
DOI:10.1029/2026JA035356