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Mercury's flux transfer event (FTE) showers are dayside magnetopause crossings accompanied by large numbers (≥10) of magnetic flux ropes (FRs). These shower events are common, occurring during 52% (1,953/3,748) of the analyzed crossings. Shower events are observed with magnetic shear angles (θ) from 0° to 180° across the magnetopause and magnetosheath plasma β from 0.1 to 10 but are most prevalent for high θ and low plasma β. Individual FR duration correlates positively, while spacing correlates negatively, with θ and plasma β. FR flux content and core magnetic field intensity correlate negatively with plasma β, but they do not correlate with θ. During shower intervals, FRs carry 60% to 85% of the magnetic flux required to supply Mercury's Dungey cycle. The FTE showers and the large amount of magnetic flux carried by the FTE‐type FRs appear quite different from observations at Earth and other planetary magnetospheres visited thus far. Plain Language Summary: Any planet with an interior dynamo will interact with the outward streaming stellar wind and likely form a magnetosphere. The magnetopause is a boundary between the shocked solar wind and planetary magnetic field, which can prevent most of the solar wind from directly entering into the magnetosphere. The multiple X‐line reconnection that frequently occurs in the magnetopause creates helical magnetic fields that are termed magnetic flux ropes (FRs) about which open and interplanetary magnetic fields drape. FTE‐type FRs generally have magnetic field lines with one end embedded in the solar wind and the other end connected to the planet through the magnetospheric cusp. The investigation of FTEs in Mercury's magnetosphere is of particular interest because they often occur in large numbers with extremely small temporal spacing, i.e., FTE showers, that are not seen elsewhere. We find that the properties of the FTE‐type flux ropes in these showers depend upon plasma β in the magnetosheath and the magnetic shear angle across the magnetopause. The magnetic flux carried by these flux ropes dominates magnetic flux transfer between Mercury's dayside and nightside magnetosphere. These new results may contribute significantly to our understanding of solar wind‐magnetosphere‐exosphere coupling at Mercury. Key Points: Flux transfer event (FTE) showers (≥10 flux ropes in a magnetopause crossing) are prevalent when shear angle is large and plasma β is smallFTE‐type flux rope duration, spacing, core field, and flux content during shower events are shown to depend upon shear angle and plasma βFTE‐type flux ropes in shower events carry between 60% and 85% of the magnetic flux required to supply Mercury's Dungey cycle [ABSTRACT FROM AUTHOR]

MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) measurements taken during passes over Mercury's dayside hemisphere indicate that on four occasions the spacecraft remained in the magnetosheath even though it reached altitudes below 300 km. During these disappearing dayside magnetosphere (DDM) events, the spacecraft did not encounter the magnetopause until it was at very high magnetic latitudes, ~66 to 80°. These DDM events stand out with respect to their extremely high solar wind dynamic pressures, Psw ~140 to 290 nPa, and intense southward magnetic fields, Bz ~ −100 to −400 nT, measured in the magnetosheath. In addition, the bow shock was observed very close to the surface during these events with a subsolar altitude of ~1,200 km. It is suggested that DDM events, which are closely associated with coronal mass ejections, are due to solar wind compression and/or reconnection‐driven erosion of the dayside magnetosphere. The very low altitude of the bow shock during these events strongly suggests that the solar wind impacts much of Mercury's sunlit hemisphere during these events. More study of these disappearing dayside events is required, but it is likely that solar wind sputtering of neutrals from the surface into the exosphere maximizes during these intervals. Key Points: The dayside magnetosphere of Mercury is observed to disappear at MESSENGER's orbit during some coronal mass ejection impactsThe cause appears to be extreme solar wind compression and/or reconnection‐driven erosion of Mercury's dayside magnetic fieldThe low altitude of the bow shock during these events strongly suggests that Mercury's dayside surface experiences direct solar wind impact [ABSTRACT FROM AUTHOR]

This study presents a statistical investigation of the force balance and structures in the flux ropes in Mercury's magnetotail plasma sheet by using the measurements of MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER). One hundred sixty‐eight flux ropes were identified from the 14 hot seasons of MESSENGER from 11 March 2011 to 30 April 2015, and 143 of them show clear magnetic field enhancements with the core field being ≥20% higher than the background magnetic field. The investigation on the force balance of these 143 flux ropes shows that magnetic pressure gradient force cannot be solely balanced by magnetic tension force, implying that thermal plasma pressure gradient force cannot be neglected in the flux ropes. We employ a non‐force‐free model considering the contribution of thermal pressure to resolve the physical properties of flux ropes in Mercury's magnetotail. Twenty‐eight flux ropes are obtained through the fitting to the non‐force‐free model. The flux ropes are found to be consistent with the flattened structures, in which the mean semimajor is ∼851 km and semiminor is ∼333 km, both are several times the local proton inertial length. The average core field is estimated to be ∼57.5 nT, and flux content is ∼0.019 MWb, much larger than the previous results obtained from force‐free flux rope model. The importance of thermal pressure gradient in the force balance of the flux ropes and the flattened structure indicates that the flux ropes in Mercury's magnetotail plasma sheet are mostly in early stage of the evolution, and still contain enough plasma to affect their magnetic structures. Key Points: Thermal pressure gradient is significant for the flux ropes in Mercury's magnetotailNon‐force‐free modeling reveals the flatten structure and much higher magnetic flux of the flux ropes different from the previous studiesFlux ropes in this study should be in their early stage of evolution and could be strongly affected by thermal pressure [ABSTRACT FROM AUTHOR]

This study analyzes 25 ion‐scale flux ropes in the Magnetospheric Multiscale (MMS) observations to determine their structures. The high temporal and spatial resolution MMS measurements enable the application of multispacecraft techniques to ion‐scale flux ropes. Flux ropes are identified as quasi‐one‐dimensional (quasi‐1‐D) when they retain the features of reconnecting current sheets; that is, the magnetic field gradient is predominantly northward or southward, and quasi‐2‐D when they exhibit circular cross sections; that is, the magnetic field gradients in the plane transverse to the flux rope axis are comparable. The analysis shows that the quasi‐2‐D events have larger core fields and smaller pressure variations than the quasi‐1‐D events. These two types of flux ropes could be the result of different processes, including magnetic reconnection with different dawn‐dusk magnetic field components, temporal transformation of flattened structure to circular, or interactions with external environments. Plain Language Summary: Magnetic flux ropes are fundamental magnetic structures in space plasma physics and are commonly seen in the universe, such as, astrophysical jets, coronal mass ejections, and planetary magnetospheres. Flux ropes are important in mass and energy transport across plasma and magnetic boundaries, and they are found in a wide range of spatial sizes, from several tens of kilometers, that is, ion‐scale flux ropes, to tens of millions of kilometers, that is, coronal mass ejections, in the solar system. The ion‐scale flux ropes can be formed during magnetic reconnection and are hypothesized to energize electrons and influence the reconnection rate. Previous examinations of the structure of ion‐scale flux ropes were greatly limited by measurement resolution. The unprecedented Magnetospheric Multiscale (MMS) mission high temporal and spatial resolution measurements provide a unique opportunity to investigate flux rope structures. By employing multispacecraft techniques, this study has provided new insights into the magnetic field variations and dimensionality of ion‐scale flux ropes in the Earth's magnetotail. The results are consistent with the evolution of ion‐scale flux ropes from initially flattened current sheet‐like flux ropes near the time of formation into lower energy state with circular cross section predicted by theory and termed as the "Taylor" state. Key Points: Ion‐scale flux ropes are observed to have either flattened or circular cross sections using MDD and GS reconstructionAnalysis of 25 flux ropes show that circular cross‐section flux ropes have stronger core field and smaller thermal pressures than flattened flux ropesThe two types of flux ropes may be the results of reconnection, temporal evolution, or interactions with external environment [ABSTRACT FROM AUTHOR]

Abstract: The variations of plasma sheet proton properties during magnetospheric substorms at Earth and Mercury are comparatively studied. This study utilizes kappa distributions to interpret proton properties at both planets. Proton number densities are found to be around an order of magnitude higher, temperatures several times smaller, and κ values broader at Mercury than at Earth. Protons become denser and cooler during the growth phase, and are depleted and heated after the dipolarizations in both magnetospheres. The changes of κ at Earth are generally small (<20%), indicating that spectrum‐preserving processes, like adiabatic betatron acceleration, play an important role there, while variations of κ at Mercury are large (>60%), indicating the importance of spectrum‐altering processes there, such as acceleration due to nonadiabatic cross‐tail particle motions and wave‐particle interactions. This comparative study reveals important intrinsic properties on the energization of protons in both magnetospheres. [ABSTRACT FROM AUTHOR]

The energization and heating processes for protons in the near-Mercury tail are examined with MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) observations. In a case study, suprathermal proton particle flux (STPF) and proton temperature are observed to be clearly enhanced during near-Mercury substorm dipolarizations, indicating the proton energization and heating processes. STPF and proton temperature distributions in near-Mercury central plasma sheets display dawn-dusk asymmetries, with higher values in the dawnside plasma sheet, i.e., postmidnight, than in the duskside, i.e., premidnight. Further investigations reveal that these asymmetries are more prominent during active periods in Mercury's magnetosphere, as compared to quiet periods. Magnetic field variations in the ZMSM component display a similar feature, with variations being more prominent on the dawnside than the duskside during active periods. We propose that the dawn-dusk asymmetry in the distributions of protons could be due to the fact that more substorm dipolarizations were initiated on the dawnside of Mercury's magnetotail. [ABSTRACT FROM AUTHOR]

An automated method was applied to identify magnetotail flux rope encounters in MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) magnetometer data. The method identified significant deflections of the north-south component of the magnetic field coincident with enhancements in the total field or dawn-dusk component. Two hundred forty-eight flux ropes are identified that possess well-defined minimum variance analysis (MVA) coordinate systems, with clear rotations of the field. Approximately 30% can be well approximated by the cylindrically symmetric, linearly force-free model. Flux ropes are most common moving planetward, in the postmidnight sector. Observations are intermittent, with the majority (61%) of plasma sheet passages yielding no flux ropes; however, the peak rate of flux ropes during a reconnection episode is ∼5 min−1. Overall, the peak postmidnight rate is ∼0.25 min−1. Only 25% of flux ropes are observed in isolation. The radius of flux ropes is comparable to the ion inertial length within Mercury's magnetotail plasma sheet. No clear statistical separation is observed between tailward and planetward moving flux ropes, suggesting the near-Mercury neutral line (NMNL) is highly variable. Flux ropes are more likely to be observed if the preceding lobe field is enhanced over background levels. A very weak correlation is observed between the flux rope core field and the preceding lobe field orientation; a stronger relationship is found with the orientation of the field within the plasma sheet. The core field strength measured is ∼6 times stronger than the local dawn-dusk plasma sheet magnetic field. [ABSTRACT FROM AUTHOR]