Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p44a..06z&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P44A-06
Physics
[2706] Magnetospheric Physics / Cusp, [2756] Magnetospheric Physics / Planetary Magnetospheres, [2784] Magnetospheric Physics / Solar Wind/Magnetosphere Interactions, [5405] Planetary Sciences: Solid Surface Planets / Atmospheres
Scientific paper
We report global observations of the ionized exosphere of Mercury measured by the Fast Imaging Plasma Spectrometer (FIPS) on MESSENGER during the spacecraft's first months in orbit since March 18, 2011. We observe considerable variability, presumably due to changes in the solar wind and heliospheric magnetic field near Mercury, but some large-scale properties were persistent throughout the observation period. Global maps of heavy ion fluxes at Mercury are clearly peaked in the northern cusp region, pointing to the importance of the polar regions as sources of Mercury's ionized exosphere. On the nightside, plasma is regularly observed near equatorial regions. The observed fluxes of He ions are more evenly distributed, indicating a spatially more uniform source of neutral He in Mercury's environment. The observed ion distributions suggest either rapid energization in the inner magnetosphere, to energies greater than 10 keV within only 0.2 Mercury radii, or that ions energized in the magnetosheath or other regions are transported to lower latitudes. We derive densities and temperatures of key constituents using a model-based technique to account for observation geometry and effects of limited field of view. Although, on average, protons dominate by number density, the pressure of Na ions can exceed that of protons in the cusp and nightside equatorial regions. This initial survey of Mercury's plasma ion environment indicates that the mass loading and thermal pressure effects of the heavier planetary ions, especially the oxygen and sodium groups, will be very important, and perhaps even dominant, in determining magnetospheric structure and dynamics.
Anderson Benjamin J.
Baker Daniel N.
Gershman Daniel J.
Gilbert Jason A.
Gloeckler George
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