Jiu‐Tong Zhao

The cold and dense plasma sheet (CDPS) often forms through the direct entry of solar wind particles and is therefore an important indicator of solar wind entry into a planetary magnetosphere. This study investigates the CDPS in Mercury's magnetotail using MESSENGER magnetic field and plasma measurements. A total of 128 CDPS events are identified and they exhibit a dawn-dusk asymmetry with more events observed on the dawnside. The CDPS is approximately twice as thick as the average of the plasma sheet. Furthermore, over 70% of CDPS events occur during northward interplanetary magnetic field (IMF) intervals and are associated with sunward and relatively weak IMF magnitudes. Our results demonstrate that the solar wind plasma could form a significant component of Mercury's magnetotail. The observed dawn-dusk asymmetry and IMF dependence suggest that the formation of CDPS could be associated with the high-latitude cusp reconnection and/or magnetic reconnection at the local dawnside magnetopause.

Mercury is exposed to various space weathering processes, which alter the surface and cause emission of atoms and molecules into the exosphere. The exosphere thus allows remote insights into the surface composition and erosion processes, with the emission of refractory elements such as Ca and Mg typically being dominated by micrometeoroid‐impact vaporization. Here we present simulations of the enhanced sputtering by ion impacts during interplanetary coronal mass ejections (ICMEs), predicting an exosphere regime change for strong space weather events. Due to higher ion fluxes and erosion of Mercury's dayside magnetosphere, the sputter source increases by up to a factor of 380. As a result, sputtering becomes the dominant release process for Ca and Mg, temporarily causing a reconfiguration from a dawn‐centered, MIV‐dominated exosphere to a dayside‐centered, sputter‐dominated exosphere. Our study thus highlights and quantifies the variability in Mercury's space environment and the system's strong coupling to space weather effects.

Electron radiation belts, reservoirs of stably trapped energetic electrons hazardous to space technology, are a prevalent feature of most magnetized planetary environments. However, their existence in the compact magnetosphere of Mercury has been a subject of debate for decades. The presence of energetic electrons was first identified by Mariner 10’s observations in the 1970s but was subsequently questioned by MESSENGER’s limited energetic particle measurements 40 years later. Here we integrate MESSENGER’s more sensitive indirect energetic electron measurements with new analysis techniques, particle simulations and theory to discover a structured radiation belt with a variable morphology subject to solar wind activity. This radiation belt can persist for several Earth days and, thus, seems to be quasi-permanent under weak solar wind forcing. However, it becomes unstable under strong driving as electron drift orbits bifurcate about the magnetic equator within the dayside magnetosphere and particles are rapidly lost. Our results establish that even compact magnetospheres can host radiation belts and, furthermore, that Mercury is a natural laboratory for radiation-belt physics usually inaccessible at Earth: one of an extreme, rapidly driven state.

Mercury experiences the most intense and variable solar wind (SW) conditions in the solar system due to its close, eccentric orbit about the Sun. In addition to variation driven by coronal source and solar cycle, the SW arriving at Mercury varies periodically as the planet's heliocentric distance changes by over 50% per orbit. We use 30 years of SW measurements near Earth and scale them to Mercury's orbital distance to determine the importance of this “seasonal” change in the SW. We find the seasonal change to be substantial, systematic, and significant, and that it dominates over other sources of variability (e.g., fast vs. slow SW) and even exceeds the relative change at Earth from ICMEs compared to the ambient SW. As a result, Mercury's magnetosphere is expected to exhibit substantial seasonal effects.

Magnetic reconnection is a fundamental physical process that can drive rapid conversion of magnetic energy into plasma bulk flows, thermal heating, and particle acceleration in space and astrophysical plasmas. Classical reconnection theory predicts that the Alfvénic reconnection exhausts are bounded by pairs of slow-mode shocks. However, identifying and characterizing these shocks through in situ spacecraft observations remains a challenge. Here, we report Parker Solar Probe observations of a reconnection exhaust embedded in the heliospheric current sheet at a heliocentric distance of 12.2 R ⨀ . The reconnection exhaust is bounded on both boundaries by compound magnetic structures rather than a pair of pure slow shocks. Each boundary consists of a rapidly evolving, steep, inner slow shock, whose Mach numbers and shock-normal angles change significantly within several minutes, and an outer, gradual compound structure that comprises a slow shock and a rotational discontinuity. These slow shocks are quasi-perpendicular and are accompanied by enhanced proton perpendicular heating. Deep within the reconnection exhaust, high perpendicular temperature together with large plasma β trigger mirror instability and generate mirror-mode structures. These observations provide new insights into the structure of reconnection exhaust boundaries and their role in energy conversion in the near-Sun plasma.

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.

Kelvin‐Helmholtz (K‐H) waves are important in transporting mass, momentum, and energy across magnetic boundaries in space plasma. This study presents the observations of K‐H waves on Mercury's pre‐dawnside magnetopause, where more than 10 linear and non‐linear K‐H waveforms were identified. Adjacent to these waves, compressional ultra‐low frequency waves and ion‐Bernstein modes along with a proton distribution exhibiting a positive phase space density gradient were observed. These observations suggest that energy and mass are transported by the K‐H waves. However, during the same MESSENGER's orbit on the pre‐duskside magnetopause, only a few magnetopause oscillations were observed and there is no accompanied compressional waves and ion‐Bernstein modes. The differences might be attributed to that MESSENGER crossed the dusk magnetopause on the dayside, where K‐H waves may not have fully developed. Another possible reason might be due to the direction of the interplanetary magnetic field. When the quasi‐parallel bow shock is located on the dawnside, it generates a smaller tangential magnetic field in the magnetosheath, which allows K‐H waves to be more easily excited. Our findings provide new insights into occurrences and energy transport of K‐H waves in Mercury's magnetosphere.

Ion impacts on airless bodies such as Mercury alter their surfaces and contribute to their exospheres via sputtering. Their exact contribution in comparison to other effects is still uncertain, but observations by the MESSENGER spacecraft largely indicated influences from micrometeoroids. In this paper, we present an updated modeling of sputtering at Mercury to help estimate the role of sputtering at average solar wind conditions. To achieve this, we account for ion precipitation due to the planet's magnetosphere and for the presence of a porous regolith: We combine H ⁺ and He ⁺⁺ fluxes to the surface from the Amitis hybrid model with sputter yields derived from a regolith simulation in SDTrimSP‐3D. We find that H ⁺ and He ⁺⁺ show similar precipitation patterns, but H ⁺ energies are much more reduced and variable than those of He ⁺⁺ . Globally, H ⁺ and He ⁺⁺ contribute about equal amounts of sputtering. Our laboratory‐calibrated sputter yields are significantly lower than estimates used in previous studies, resulting in a global sputtering source of around 10 ²³ atoms s ⁻¹ . Specifically for Ca and Mg exospheres we find source rates from sputtering that are largely unaffected by Mercury's seasonal orientation and too small by up to around two orders of magnitudes to explain MESSENGER observations. This supports a micrometeoroid‐impact‐dominated source of refractory elements. We find, however, that this is an effect of the reduced magnetospheric precipitation at Mercury. At other bodies such as the Moon, a different regime should be prevalent and sputtering should contribute at least similarly to the exospheres of refractory elements.

Observations from the Juno spacecraft near the M‐shells of the Galilean moons have identified alternating enhancements and reductions of particle fluxes at discrete energies. These banded structures were previously attributed to bounce resonance between particles and standing Alfvén waves generated by moon‐magnetospheric interactions. Here, we show that this explanation is inconsistent with key observational features, and propose an alternative interpretation: the bands are remote signatures of particle absorption at the moons. In this scenario, whether a particle encounters the moon before reaching Juno depends on the number of bounce cycles it undergoes within a fixed drift segment determined by the moon‐spacecraft separation. Therefore, the absorption bands are expected to appear at discrete, equally‐spaced velocities. This is largely consistent with the observations, though discrepancies remain, possibly due to spacecraft charging and/or finite data resolution. This finding improves our understanding of moon‐plasma interactions and may help constrain Jovian magnetospheric models.