Magnetic flux pileup and plasma depletion in Mercury's subsolar magnetosheath

Abstract

Measurements from the Fast Imaging Plasma Spectrometer (FIPS) and Magnetometer (MAG) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft during 40 orbits about Mercury are used to characterize the plasma depletion layer just exterior to the planet's dayside magnetopause. A plasma depletion layer forms at Mercury as a result of piled‐up magnetic flux that is draped around the magnetosphere. The low average upstream Alfvénic Mach number ( M A ~3–5) in the solar wind at Mercury often results in large‐scale plasma depletion in the magnetosheath between the subsolar magnetopause and the bow shock. Flux pileup is observed to occur downstream under both quasi‐perpendicular and quasi‐parallel shock geometries for all orientations of the interplanetary magnetic field (IMF). Furthermore, little to no plasma depletion is seen during some periods with stable northward IMF. The consistently low value of plasma β , the ratio of plasma pressure to magnetic pressure, at the magnetopause associated with the low average upstream M A is believed to be the cause for the high average reconnection rate at Mercury, reported to be nearly 3 times that observed at Earth. Finally, a characteristic depletion length outward from the subsolar magnetopause of ~300 km is found for Mercury. This value scales among planetary bodies as the average standoff distance of the magnetopause. Key Points Sub‐solar plasma depletion occurs at Mercury for nearly all upstream conditions Large‐scale depletion at Mercury results in increased dayside reconnection The characteristic depletion length scale at Mercury is ~280 km