Observations by MESSENGER show that Mercury's magnetosphere is immersed in a comet-like cloud of planetary ions. The most abundant, Na+, is broadly distributed but exhibits flux maxima in the magnetosheath, where the local plasma flow speed is high, and near the spacecraft's closest approach, where atmospheric density should peak. The magnetic field showed reconnection signatures in the form of flux transfer events, azimuthal rotations consistent with Kelvin-Helmholtz waves along the magnetopause, and extensive ultralow-frequency wave activity. Two outbound current sheet boundaries were observed, across which the magnetic field decreased in a manner suggestive of a double magnetopause. The separation of these current layers, comparable to the gyro-radius of a Na+ pickup ion entering the magnetosphere after being accelerated in the magnetosheath, may indicate a planetary ion boundary layer.

[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.

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Abstract. Surface-bounded exospheres have been detected at the Moon, Mercury, and Europa and almost certainly exist about other objects. Historically, the first of these systems to be observed was the lunar exosphere, where He and Ar were detected by the Apollo spacecraft. The Hermean exosphere is archetypical of these systems in that it is part of a coupled system including the surface and magnetosphere interacting dynamically with the solar wind and fields. Studies of the Hermean exosphere heretofore have neglected or only superficially considered these interactions. We will review the current state of knowledge of the exospheres of Mercury and the Moon and discuss areas in which our knowledge is most incomplete. We will focus on the exosphere as part of a coupled system including the surface at its base and the particle, field, and interplanetary environment as both a source and sink for neutrals. Apollo era instruments made unambiguous detections of 36mr, 4ømr, Early Work and OverviewIn a review prior to the Mariner 10 encounter, Banks et al. [1970] reported no observational evidence for the presence of an atmosphere about Mercury. They predicted an exosphere (N < 2 x 10 TM cm -2) or thin atmosphere resulting from diffusion or effusion from the interior, solar wind implantation with subsequent thermal evaporation, or sputtering. By scaling terrestrial noble gas production and diffusion rates and assuming a lunar-like solar wind interaction, they suggested that the most abundant constituents should be the noble gases derived from the solar wind and radiogenic sources, the most abundant being 4He, followed by 4øAr and 2øNe.

Abstract. Mercury's sodium atmosphere is known to be highly variable both temporally and spatially. During a week-long period from November 13 to 20, 1997, the total sodium content of the Hermean atmosphere increased by a factor of 3, and the distribution varied daily. We demonstrate a mechanism whereby these rapid variations could be due to solar wind-magnetosphere interactions. We assume that photon-stimulated desorption and meteoritic vaporization are the active source processes on the first (quietest) day of our observations. Increased ion sputtering results whenever the magnetosphere opens in response to a southward interplanetary magnetic field (IMF) or unusually large solar wind dynamic pressure. The solar wind dynamic pressure at Mercury as inferred by heliospheric radial tomography increased by a factor of 20 during this week, while the solar EUV flux measured by the Solar EUV Monitor (SEM) instrument on board the Solar and Heliospheric Observatory (SOHO) increased by 20%. While impact vaporization provides roughly 25% of the source, it is uniformly distributed and varies very little during the week. The variations seen in our data are not related to Caloris basin, which remained in the field of view during the entire week of observations. We conclude that increased ion sputtering resulting from ions entering the cusp regions is the probable mechanism leading to large rapid increases in the sodium content of the exosphere. While both the magnitude and distribution of the observed sodium can be reproduced by our model, in situ measurements of the solar wind density and velocity, the magnitude and direction of the interplanetary magnetic field, and Mercury's magnetic moments are required to confirm the results.