MESSENGER Investigations of the Plasma Environment at Mercury.

Abstract

We report on the first observations of plasma ions at Mercury, from the Fast Imaging Plasma Spectrometer (FIPS). These observations were made by the MESSENGER spacecraft during two flybys of Mercury and while in orbit from March 2011 through May 2012. We found: 1) Bulk plasma ion measurements confirm that Mercury’s magnetosphere is morphologically similar to Earth: There is hot, dense plasma in the magnetosheath. Plasma ions are concentrated at the cusps and in the central plasma sheet, where solar wind protons and alpha particles usually dominate and with densities and temperatures consistent with expectations from Earth. 2) Planetary ions are distributed throughout Mercury’s magnetosphere, confirming the exosphere as a source of magnetospheric ions. These ions show some density enhancements in common with their parent neutral populations, particularly in the cusp and near the dawn equator. He+ are distributed more uniformly than other planetary ions, which may result from substantially different source regions or processes. Planetary ions have much larger e-folding heights than neutrals across the dayside, indicating significant influence from magnetospheric electric and magnetic fields. 3) These studies have shed light on the dynamics of planetary ions in Mercury’s magnetosphere, the first step in understanding their role in the system. First, the asymmetric enhancement of large-gyroradius Na+ ions in the pre-midnight central plasma sheet may indicate non-adiabatic motion within this region and in transit to it. This behavior could have wide-reaching implications in the surface-exosphere-magnetosphere system. In particular, non-adiabatic motion could alter locations and energies of planetary ions precipitating onto the surface, and the resultant creation of new ions and neutrals from surface constituents. Since Na+-group ions can at times provide up to 25% of total plasma ion pressure in the central plasma sheet, these ions may, at times, share the role of determining magnetospheric dynamics with ions of solar wind origin. Finally, we have shown that planetary Na+-group ions, observed in Mercury’s northern cusp, are swept in to the cusp rather than flowing out, as at Earth. Often correlated with dayside reconnection, this novel behavior serves to energize planetary ions in Mercury’s active magnetosphere.