March 26, 2010, 12:00 pm - 1:00 pm
March 26, 2010, 12:00 pm - 1:00 pm
MESSENGER Observations of the Plasma Environment Near Mercury
Jim Raines (University of Michigan)
The MESSENGER spacecraft measured the bulk plasma characteristics of Mercury’s magnetosphere and solar wind environment during two flybys of the planet on 14 January 2008 and 6 October 2008. Data was collected with the Fast Imaging Plasma Spectrometer (FIPS), included on the MESSENGER payload to measure planetary ion composition in the Mercury space environment and relate it to the composition of the planet’s crust. FIPS provides plasma and compositional parameters for particles from 46 eV to 13 keV energy per charge at 8-s time resolution and has a near-hemispheric instantaneous field of view (FOV), which is oriented toward the planet to maximize collection of planetary ions. This orientation, coupled with obstruction in the forward direction by the spacecraft heat shield, has the unfortunate side effect of excluding the typical plasma flow direction in the solar wind or magnetosphere. Despite these limitations, we have devised procedures to recover density and temperature from FIPS measurements, through use of a software instrument model which includes the time-dependent instrument field of view, as well as spacecraft position and velocity coordinates. We then, in conjunction with magnetic field data, identified macroscopic features of Mercury's plasma environment observed during the three flybys, including bow-shock and magnetopause crossings as well as the magnetospheric plasma sheet and lobe. We located two regions in each flyby where magnetospheric flow is likely reduced, perhaps even stagnant, which we term dayside and nightside boundary layer regions, and recovered parameters from these portions of the data. In these regions, recovery is greatly simplified by the assumption of small plasma velocity toward the planet, thus providing a good first application of our recovery procedure. The recovered ion density and temperature values show considerable variability across the regions. However, the proton plasma pressure does not fully account for the drop in magnetic pressure upon entry into the boundary layer regions, implying that planetary ion pressure is the primary cause of the reduced magnetic field in these regions.