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Abstract Mercury possesses a miniature yet dynamic magnetosphere driven primarily by magnetic reconnection occurring regularly at the magnetopause and in the magnetotail. Using the newly developed Magnetohydrodynamics with Adaptively Embedded Particle‐in‐Cell (MHD‐AEPIC) model coupled with planetary interior, we have performed a series of global simulations with a range of upstream conditions to study in detail the kinetic signatures, asymmetries, and flux transfer events (FTEs) associated with Mercury's dayside magnetopause reconnection. By treating both ions and electrons kinetically, the embedded PIC model reveals crescent‐shaped phase‐space distributions near reconnection sites, counter‐streaming ion populations in the cusp region, and temperature anisotropies within FTEs. A novel metric and algorithm are developed to automatically identify reconnection X‐lines in our 3D simulations. The spatial distribution of reconnection sites as modeled by the PIC code exhibits notable dawn‐dusk asymmetries, likely due to such kinetic effects as X‐line spreading and Hall effects. Across all simulations, simulated FTEs occur quasi‐periodically every 4–9 s. The properties of simulated FTEs show clear dependencies on the upstream solar wind Alfvénic Mach number (M_A) and the interplanetary magnetic field orientation, consistent with MESSENGER observations and previous Hall‐MHD simulations. FTEs formed in our MHD‐AEPIC model tend to carry a large amount of open flux, contributing ∼3%–36% of the total open flux generated at the dayside. Taken together, our MHD‐AEPIC simulations provide new insights into the kinetic processes associated with Mercury's magnetopause reconnection that should prove useful for interpreting spacecraft observations, such as those from MESSENGER and BepiColombo. 

Abstract The magnetotail lobe region at Mercury is characterized by low plasma density and low magnetic field variability compared to the nightside magnetosheath and central plasma sheet. At Mercury, as well as other planets, lobe magnetic fields play a crucial role in storing and releasing magnetic flux in response to changing upstream solar wind conditions such as interplanetary magnetic field (IMF) orientation and solar wind dynamic pressure (P_dyn). This makes the region significant for studying the magnetospheric interaction with the intense solar wind conditions at Mercury's orbit. Here, we identify and analyze magnetotail lobe observations made by the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft during its 4 years orbital phase. We empirically determined a set of criteria using magnetometer (MAG) and the Fast Imaging Plasma Spectrometer instruments onboard MESSENGER to identify lobe magnetic field intervals. From 3,332 MESSENGER orbits, we identify 1,242 lobe field intervals. We derive an expression for the average lobe magnetic field strength in nanotesla with respect to radial distance downtail:B_lobe(r) = (135 ± 8) * r^{(−2.1±0.3)} + (31 ± 8). The lobe magnetic field exhibits both small‐scale (∼3 min) and orbit‐to‐orbit (∼8–12 hr) variability in magnetic field strength compared to this averaged field strength expression. The orbit‐to‐orbit variability in lobe field strength is not significantly correlated with estimated IMF orientation, but is directly correlated withP_dyn. Thus, our findings provide evidence for the pressure balance between the inward facingP_dynon the nightside magnetopause and the outward facing magnetic pressure supplied by the lobes. 

Abstract An important discovery of MESSENGER is the occurrence of dayside disappearing magnetosphere (DDM) events that occur when the solar wind dynamic pressure is extremely high and the interplanetary magnetic field (IMF) is both intense and southward. In this study, we investigate the DDM events at Mercury under extreme solar wind conditions using a three‐dimensional (3‐D) global hybrid simulation model. Our results show that when the solar wind dynamic pressure is 107 nPa and the magnitude of the purely southward IMF is 50 nT, most of the dayside magnetosphere disappears within 10 s after the interaction between the solar wind and the planetary magnetic field starts. During the DDM event, the ion flux is significantly enhanced at most of the planetary dayside surface and reaches its maximum value of about 10¹⁰ cm⁻² s⁻¹at the low‐latitude surface, which is much larger than that under normal solar wind conditions. During the DDM events, the dayside bow shock mostly disappears for about 9 s and then reappears. Moreover, the time evolution of magnetopause standoff distance under different solar wind conditions is also studied. When the solar wind dynamic pressure exceeds 25 nPa and the IMF is purely southward, a part of the dayside magnetosphere disappears. Under the same IMF, the higher the solar wind dynamic pressure, the faster the magnetopause standoff distance reaches the planetary surface. When the solar wind conditions are normal (with a dynamic pressure of 8 nPa) or the IMF is purely northward, the dayside magnetosphere does not disappear. The results provide a clear physical image of DDM events from a 3‐D perspective.