Research Update: First Identification of Foreshock Plasma Populations at Mercury

Using observations by the Fast Imaging Plasma Spectrometer (FIPS) aboard MESSENGER, we recently established the existence of foreshock plasma populations at the planet Mercury. Previous investigations had questioned whether foreshock populations could exist in this environment because the small spatial scale of its bow shock leaves less opportunity for familiar Earth foreshock acceleration processes to occur. Our work has shown, however, that there are foreshock populations upstream of the Hermean bow shock, that are similar to those seen in the terrestrial foreshock, in at least 14% of MESSENGER's first 3000 orbits of Mercury. Furthermore, these populations are organized by the bow shock geometry in a manner similar to those at Earth: by the angle between the magnetic field and the shock surface at the population's generation location. In this update, we discuss the conditions for Shock Drift Acceleration, which accelerates ions into beams in Earth's quasi-perpendicular foreshock. It is established that SDA likely operates effectively at Mercury to create Field Aligned Beams. For diffuse populations, it is shown through estimates of the diffusion coefficient and IMF-bow shock connection times that a connection-time-limited diffusive shock acceleration is likely responsible.