The structure of Mercury's dayside magnetosphere is investigated during three extreme solar wind dynamic pressure events. Two were the result of coronal mass ejections (CMEs), and one was from a high-speed stream (HSS). The inferred pressures for these events are~45 to 65 nPa. The CME events produced thick, low-β (where β is the ratio of plasma thermal to magnetic pressure) plasma depletion layers and high reconnection rates of 0.1-0.2, despite small magnetic shear angles across the magnetopause of only 27 to 60°. For one of the CME events, brief,~1-2 s long diamagnetic decreases, which we term cusp plasma filaments, were observed within and adjacent to the cusp. These filaments may map magnetically to flux transfer events at the magnetopause. The HSS event produced a high-β magnetosheath with no plasma depletion layer and large magnetic shear angles of 148 to 166°, but low reconnection rates of 0.03 to 0.1. These results confirm that magnetic reconnection at Mercury is very intense, and its rate is primarily controlled by plasma β in the adjacent magnetosheath. The distance to the subsolar magnetopause is reduced during these events from its mean of 1.45 Mercury radii (R M ) from the planetary magnetic dipole to between 1.03 and 1.12 R M . The shielding provided by induction currents in Mercury's interior, which temporarily increase Mercury's magnetic moment, was negated by reconnection-driven magnetic flux erosion.

We report the properties of a novel type of sub-proton scale magnetic hole found in two dimensional particle-in-cell simulations of decaying turbulence with a guide field. The simulations were performed with a realistic value for ion to electron mass ratio. These structures, electron vortex magnetic holes (EVMHs), have circular cross-section. The magnetic field depression is associated with a diamagnetic azimuthal current provided by a population of trapped electrons in petal-like orbits. The trapped electron population provides a mean azimuthal velocity and since trapping preferentially selects high pitch angles, a perpendicular temperature anisotropy. The structures arise out of initial perturbations in the course of the turbulent evolution of the plasma, and are stable over at least 100 electron gyroperiods. We have verified the model for the EVMH by carrying out test particle and PIC simulations of isolated structures in a uniform plasma. It is found that (quasi-)stable structures can be formed provided that there is some initial perpendicular temperature anisotropy at the structure location. The properties of these structures (scale size, trapped population, etc.) are able to explain the observed properties of magnetic holes in the terrestrial plasma sheet. EVMHs may also contribute to turbulence properties, such as intermittency, at short scale lengths in other astrophysical plasmas. V C 2015 AIP Publishing LLC.

[1] Analysis of MESSENGER magnetic field observations taken in the southern lobe of Mercury's magnetotail and the adjacent magnetosheath on 11 April 2011 indicates that a total of 163 flux transfer events (FTEs) occurred within a 25 min interval. Each FTE had a duration of $2-3 s and was separated in time from the next by $8-10 s. A range of values have been reported at Earth, with mean values near $1-2 min and $8 min, respectively. We term these intervals of quasiperiodic flux transfer events "FTE showers." The northward and sunward orientation of the interplanetary magnetic field during this shower strongly suggests that the FTEs observed during this event formed just tailward of Mercury's southern magnetic cusp. The point of origin for the shower was confirmed with the Cooling model of FTE motion. Modeling of the individual FTE-type flux ropes in the magnetosheath indicates that these flux ropes had elliptical cross sections, a mean semimajor axis of 0.15 R M (where R M is Mercury's radius, or 2440 km), and a mean axial magnetic flux of 1.25 MWb. The lobe magnetic field was relatively constant until the onset of the FTE shower, but thereafter the field magnitude decreased steadily until the spacecraft crossed the magnetopause. This decrease in magnetic field intensity is frequently observed during FTE showers. Such a decrease may be due to the diamagnetism of the new magnetosheath plasma being injected into the tail by the FTEs.

[1] Several series of large dipolarization events are documented from magnetic field observations in Mercury's magnetotail made by the MESSENGER spacecraft. The dipolarizations are identified by a rapid ($1 s) increase in the northward component of the magnetic field, followed by a slower return ($10 s) to pre-onset values. The changes in field strength during an event frequently reach 40 nT or higher, equivalent to an increase in the total magnetic field magnitude by a factor of $4 or more. The presence of spatially constrained dipolarizations at Mercury provides a key to understanding the magnetic substorm process in a new parameter regime: the dipolarization timescale, which is shorter than at Earth, is suspected to lead to efficient non-adiabatic heating of the plasma sheet proton population, and the high recurrence rate of the structures is similar to that frequently observed for flux ropes and traveling compression regions in Mercury's magnetotail. The relatively short lifetime of the events is attributed to the lack of steady field-aligned current systems at Mercury.

[1] We present a survey of Kelvin-Helmholtz (KH) waves at Mercury's magnetopause during MESSENGER's first Mercury year in orbit. The waves were identified on the basis of the well-established sawtooth wave signatures that are associated with nonlinear KH vortices at the magnetopause. MESSENGER frequently observed such KH waves in the dayside region of the magnetosphere where the magnetosheath flow velocity is still subsonic, which implies that instability growth rates at Mercury's magnetopause are much larger than at Earth. We attribute these greater rates to the limited wave energy dissipation in Mercury's highly resistive regolith. The wave amplitude was often on the order of 100 nT or more, and the wave periods were $10-20 s. A clear dawn-dusk asymmetry is present in the data, in that all of the observed wave events occurred in the postnoon and duskside sectors of the magnetopause. This asymmetry is likely related to finite Larmor-radius effects and is in agreement with results from particle-in-cell simulations of the instability. The waves were observed almost exclusively during periods when the north-south component of the magnetosheath magnetic field was northward, a pattern similar to that for most terrestrial KH wave events. Accompanying plasma measurements show that the waves were associated with the transport of magnetosheath plasma into the magnetosphere.

Electron-scale magnetic depressions in the terrestrial plasma sheet are studied using Cluster multispacecraft data. The structures, which have an observed duration of~5-10 s, are approximately 200-300 km wide in the direction of propagation, and they show an average reduction in the background magnetic field of 10-20%. A majority of the events are also associated with an increase in the high-energy high pitch angle electron flux, which indicates that the depressions are presumably generated by electrons with relatively high velocity perpendicular to the background magnetic field. Differences in the recorded electron spectra in the four spacecraft indicates a possible nongyrotropic structure. Multispacecraft measurements show that a subset of events are cylindrical, elongated along the magnetic field, and with a field-parallel scale size of at a minimum 500 km. Other events seem to be better described as electron-scale sheets, about 200-300 km thick. We find that no single formation mechanism can explain this variety of events observed. Instead, several processes may be operating in the plasma sheet, giving rise to similar magnetic field structures in the single-spacecraft data, but with different 3-D structuring. The cylindrical structures have several traits that are in agreement with the electron vortex magnetic holes observed in 2-D particle-in-cell simulations of turbulent relaxation, whereas the sheets, which show nearly identical signatures in the multispacecraft data, are better explained by propagating electron solitary waves.

MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) magnetic field and plasma measurements taken during crossings of Mercury's magnetotail from 2011 to 2014 have been examined for evidence of substorms. A total of 26 events were found during which an Earth‐like growth phase was followed by clear near‐tail expansion phase signatures. During the growth phase, just as at Earth, the thinning of the plasma sheet and the increase of the magnetic field intensity in the lobe are observed, but the fractional increase in field intensity could be ∼3 to 5 times that at Earth. The average timescale of the growth phase is ∼1 min. The dipolarization that marks the initiation of the substorm expansion phase is only a few seconds in duration. During the expansion phase, lasting ∼1 min, the plasma sheet is observed to thicken and engulf the spacecraft. The duration of the substorm observed in this paper is consistent with previous observations of Mercury's Dungey cycle. The reconfiguration of the magnetotail during Mercury's substorm is very similar to that at Earth despite its very compressed timescale.

A large study of Kelvin-Helmholtz (KH) waves at the magnetopause of Mercury covering 907 days of data from the MErcury Surface Space ENvironment GEochemistry Ranging spacecraft have resulted in 146 encounters of not only nonlinear KH waves but also linear surface waves, including the first observations of KH waves at the dawnside magnetopause. Most of the waves are in the nonlinear phase (90%) occur at the duskside magnetopause (93%), under northward magnetosheath magnetic field conditions (89%) and during greater magnetosheath B z (23 nT) values than in general. The average period and amplitude is 30 ± 14 s and 14 ± 10 nT, respectively. Unlike duskside events, dawnside waves do not appear at the magnetopause flank (< 6 magnetic local time). This is in agreement with previous observations and modeling results and possibly explained by finite Larmor radius effects and/or a lack of a large-scale laminar flow at the dawnside magnetopause boundary.