MESSENGER Observations of Cold and Dense Plasma Sheet in Mercury's Magnetosphere

The cold and dense plasma sheet (CDPS) often forms through the direct entry of solar wind particles and is therefore an important indicator of solar wind entry into a planetary magnetosphere. This study investigates the CDPS in Mercury's magnetotail using MESSENGER magnetic field and plasma measurements. A total of 128 CDPS events are identified and they exhibit a dawn-dusk asymmetry with more events observed on the dawnside. The CDPS is approximately twice as thick as the average of the plasma sheet. Furthermore, over 70% of CDPS events occur during northward interplanetary magnetic field (IMF) intervals and are associated with sunward and relatively weak IMF magnitudes. Our results demonstrate that the solar wind plasma could form a significant component of Mercury's magnetotail. The observed dawn-dusk asymmetry and IMF dependence suggest that the formation of CDPS could be associated with the high-latitude cusp reconnection and/or magnetic reconnection at the local dawnside magnetopause.