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The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft while in orbit about Mercury observed highly localized, ~3‐s‐long reductions in the dayside magnetospheric magnetic field, with amplitudes up to 90% of the ambient intensity. These magnetic field depressions are termed cusp filaments because they were observed from just poleward of the magnetospheric cusp to midlatitudes, i.e., ~55° to 85°N. We analyzed 345 high‐ and low‐altitude cusp filaments identified from MESSENGER magnetic field data to determine their physical properties. Minimum variance analysis indicates that most filaments resemble cylindrical flux tubes within which the magnetic field intensity decreases toward its central axis. If the filaments move over the spacecraft at an estimated magnetospheric convection speed of ~35 km/s, then they have a typical diameter of ~105 km or ~7 gyroradii for 1 keV H+ ions in a 300 nT magnetic field. During these events, MESSENGER's Fast Imaging Plasma Spectrometer observed H+ ions with magnetosheath‐like energies. MESSENGER observations during the spacecraft's final low‐altitude campaign revealed that these cusp filaments likely extend down to Mercury's surface. We calculated an occurrence‐rate‐normalized integrated particle precipitation rate onto the surface from all filaments of (2.70 ± 0.09) × 1025 s−1. This precipitation rate is comparable to published estimates of the total precipitation rate in the larger‐scale cusp. Overall, the MESSENGER observations analyzed here suggest that cusp filaments are the magnetospheric extensions of the flux transfer events that form at the magnetopause as a result of localized magnetic reconnection.

Coronal mass ejections (CMEs) and other transient solar phenomena play important roles in magnetospheric and exospheric dynamics. Although a planet may interact only occasionally with the interplanetary consequences of these events, such transient phenomena can result in departures from the background solar wind that often involve more than an order of magnitude greater ram pressure and interplanetary electric field applied to the planetary magnetosphere. For Mercury, an order of magnitude greater ram pressure combined with high Alfvén speeds and reconnection rates can push the magnetopause essentially to the planet's surface, exposing the surface directly to the solar wind flow. In order to understand how the solar wind interacts with Mercury's magnetosphere and exosphere, previous studies have used the Wang‐Sheeley‐Arge (WSA)‐ENLIL solar wind modeling tool to calculate basic and composite solar wind parameters at Mercury's orbital location. This model forecasts only the background solar wind, however, and does not include major transient events. The Cone extension permits the inclusion of CMEs and related solar wind perturbations and thus enables characterization of the effects of strong solar wind disturbances on the Mercury system. The Cone extension is predicated on the assumption of constant angular and radial velocities of CMEs to integrate these phenomena into the WSA‐ENLIL coupled model. Comparisons of the model results with observations by the MESSENGER spacecraft indicate that the WSA‐ENLIL+Cone model more accurately forecasts total solar wind conditions at Mercury and has greater predictive power for magnetospheric and exospheric processes than the WSA‐ENLIL model alone.

Observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in Mercury's magnetotail demonstrate for the first time that Na+ ions exert a dynamic influence on Mercury's magnetospheric system. Na+ ions are shown to contribute up to ~30% of the ion thermal pressure required to achieve pressure balance in the premidnight plasma sheet. High concentrations of planetary ions should lead to Na+ dominance of the plasma mass density in these regions. On orbits with northward‐oriented interplanetary magnetic field and high (i.e., >1 cm−3) Na+ concentrations, MESSENGER has often recorded magnetic field fluctuations near the Na+ gyrofrequency associated with the Kelvin‐Helmholtz (K‐H) instability. These nightside K‐H vortices are characteristically different from those observed on Mercury's dayside that have a nearly constant wave frequency of ~0.025 Hz. Collectively, these observations suggest that large spatial gradients in the hot planetary ion population at Mercury may result in a transition from a fluid description to a kinetic description of vortex formation across the dusk terminator, providing the first set of truly multiscale observations of the K‐H instability at any of the diverse magnetospheric environments explored in the solar system.

We present the first observations of Mercury's plasma mantle, a primary region for solar wind entry into the planetary magnetosphere, located in the high‐latitude magnetotail. MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) observations from two orbits on 10 November 2012 have been analyzed. The main plasma mantle features are (1) a steady decrease in proton density as MESSENGER moved deeper into the magnetotail; (2) frequent flux transfer events throughout the magnetosheath and into the magnetotail, suggesting that these events are the primary source for solar wind plasma injection; (3) a diamagnetic depression, due to the presence of plasma, as pressure balance is maintained; and (4) a clear proton velocity dispersion, resulting from lower‐energy protons being transported deep into the magnetosphere as higher‐energy protons escape downtail. From these velocity dispersions we infer cross‐magnetosphere electric potentials of 23 kV and 29 kV, consistent with estimates determined from measurements of magnetopause reconnection rate and tail loading and unloading events.