First observations of Mercury's plasma mantle by MESSENGER

Abstract

We present the first observations of Mercury's plasma mantle, a primary region for solar wind entry into the planetary magnetosphere, located in the high‐latitude magnetotail. MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) observations from two orbits on 10 November 2012 have been analyzed. The main plasma mantle features are (1) a steady decrease in proton density as MESSENGER moved deeper into the magnetotail; (2) frequent flux transfer events throughout the magnetosheath and into the magnetotail, suggesting that these events are the primary source for solar wind plasma injection; (3) a diamagnetic depression, due to the presence of plasma, as pressure balance is maintained; and (4) a clear proton velocity dispersion, resulting from lower‐energy protons being transported deep into the magnetosphere as higher‐energy protons escape downtail. From these velocity dispersions we infer cross‐magnetosphere electric potentials of 23 kV and 29 kV, consistent with estimates determined from measurements of magnetopause reconnection rate and tail loading and unloading events