MESSENGER observations of the plasma environment near Mercury

The MESSENGER Fast Imaging Plasma Spectrometer (FIPS) measured the bulk plasma characteristics of Mercury's magnetosphere and solar wind environment during the spacecraft's first two flybys of the planet on 14 January 2008 (M1) and 6 October 2008 (M2), producing the first measurements of thermal ions in Mercury's magnetosphere. In this work, we identify major features of the Mercury magnetosphere in the FIPS proton data and describe the data analysis process used for recovery of proton density (n(p)) and temperature (T-p) with a forward modeling technique, required because of limitations in measurement geometry. We focus on three regions where the magnetospheric flow speed is likely to be low and meets our criteria for the recovery process: the M1 plasma sheet and the M1 and M2 dayside and nightside boundary-layer regions. Interplanetary magnetic field (IMF) conditions were substantially different between the two flybys, with intense reconnection signatures observed by the Magnetometer during M2 versus a relatively quiet magnetosphere during M1. The recovered ion density and temperature values for the M1 quiet-time plasma sheet yielded n(p)similar to 1-10 cm(-3), T-p similar to 2 x 10(6) K, and plasma beta similar to 2. The nightside boundary-layer proton densities during M1 and M2 were similar, at n(p)similar to 4-5 cm(-3), but the temperature during M1 (T-p similar to 4-8 x 10(6) K) was 50% less than during M2 (T-p similar to 8 x 106 K), presumably due to reconnection in the tail. The dayside boundary layer observed during M1 had a density of 16 cm-3 and temperature of 2 x 106 K, whereas during M2 this region was less dense and hotter (n(p)similar to 8 cm(-3) and T-p similar to 10 x 10(6) K), again, most likely due to magnetopause reconnection. Overall, the southward interplanetary magnetic field during M2 clearly produced higher T-p in the dayside and nightside magnetosphere, as well as higher plasma beta in the nightside boundary, similar to 20 during M2 compared with similar to 2 during M1. The proton plasma pressure accounts for only a fraction (24% for M1 and 64% for M2) of the drop in magnetic pressure upon entry into the dayside boundary layer. This result suggests that heavy ions of planetary origin, not considered in this analysis, may provide the "missing" pressure. If these planetary ions were hot due to "pickup" in the magnetosheath, the required density for pressure balance would be an ion density of similar to 1 cm(-3) for an ion temperature of similar to 10(8) K. (C) 2011 Elsevier Ltd. All rights reserved.