First Observations of Mercury's Plasma Mantle As Seen By MESSENGER
Friday, 19 December 2014
We present the first observations of Mercury's plasma mantle, a major source for solar wind entry into the planet's magnetosphere. The plasma mantle is created when reconnected magnetic flux tubes at the dayside magnetopause convect anti-sunward with the magnetosheath flow. As these flux tubes are assimilated into the lobes they carry solar wind plasma into the magnetotail. MESSENGER Fast Imaging Plasma Spectrometer (FIPS) and Magnetometer observations have been analyzed for two orbits when the FIPS field of view was well oriented to measure the field-aligned plasma flowing just inside the high-latitude tail magnetopause. For both events, frequent flux transfer events are observed in the magnetosheath, cusp, and nightside magnetopause, indicating that magnetic flux was being added to the magnetotail during these two intervals. The main plasma mantle features at Mercury are similar to those found at Earth: (1) a decrease in plasma density as MESSENGER moves from the magnetopause deeper into the tail lobes; and (2) a clear dispersion in the proton energy distribution, indicating that low-energy protons are being transported equatorward and much deeper into the magnetotail than the higher energy particles that escape by streaming tailward along the lobe field lines before they can E×B drift to the plasma sheet, where E and B are the electric and magnetic fields, respectively. The three-dimensional FIPS plasma distributions confirm that the solar wind protons entering the magnetosphere are indeed flowing anti-sunward with weighted average energies of 0.27 and 0.28 keV, corresponding to a velocity of ~230 km s-1. Diamagnetic depressions, indicating the presence of plasma, are observed in the magnetic field data, and the total field magnitude increases throughout the plasma mantle as the plasma disperses. The proton energy dispersion implies a cross-magnetosphere electric potential of ~20–30 kV, which supports the estimates made from the measurement of dayside magnetopause structure.