First Identification of Foreshock Plasma Populations at Mercury

Observations of foreshock plasma populations at the planet Mercury are presented for the first time, using measurements by the Fast Imaging Plasma Spectrometer (FIPS) aboard MESSENGER. Mercury, the only other rocky planet in the Solar System with an intrinsic dipole field, exists in a unique and exciting parameter space due to the planet's relatively weak magnetic dipole and its proximity to the Sun. Previous investigations have questioned whether foreshock populations can exist in this environment because the small spatial scale of its bow shock leaves less opportunity for familiar Earth foreshock acceleration processes to occur. Observations in this work, however, show 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; of the 31 best observed events in this study, only 4 Diffuse plasma populations were observed when this angle was estimated to be above 35 degrees, and only 2 Field Aligned Beam (FAB) populations were observed when this angle was estimated to be below 35 degrees. These observations also suggest energization mechanisms for both populations. In the case of FABs, application of the method of Paschmann et al. [1980] to this data shows, much like in that work as applied at Earth, that a simple reflection model can account for the multiple-keV ion beams in Mercury's foreshock. This fact suggests that the Shock Drift Acceleration process, which accelerates ions into beams in Earth's quasi-perpendicular foreshock, likely operates effectively at Mercury as well. 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.