MESSENGER Observations of Planetary Ion Characteristics in the Vicinity of Kelvin-Helmholtz Vortices at Mercury

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft regularly observed the magnetospheric flanks of Mercury during its orbital phase. Data from the Magnetometer (MAG) and the Fast Imaging Plasma Spectrometer (FIPS) allow us to investigate the statistical properties of planetary ions (Na⁺) in the presence of Kelvin-Helmholtz (KH) waves at the duskside magnetopause. We collect the data from orbits with clear signatures of KH waves under northward interplanetary magnetic field, as well as from adjacent orbits that do not have KH signatures, and we compare the energy characteristics between the KH and non-KH events. Although low planetary counts in FIPS data make the comparison of these characteristics difficult, we find that in the presence of KH waves: (1) large counts of planetary ions are observed and (2) differences in Na⁺ energy spectra are only seen inside the magnetosphere, where they show a deceleration signature for ions with energies above 2.0 keV/e. These results suggest that planetary ions are not escaping from the magnetosphere and that electric field structures related to KH waves can decelerate planetary ions originating from the magnetotail region. The understanding of the energy distribution of planetary ions in the magnetospheric flanks of Mercury is important for a better understanding of plasma convection in the magnetosphere.