Mercury’s Low-Latitude Boundary Layer: Identification of Distinct Proton Populations Using MESSENGER Spacecraft Data

We have performed a comprehensive analysis of Mercury’s Low-Latitude Boundary Layer (LLBL) using data from the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft during its orbital phase. Using combined magnetic field and ion measurements, we identified 202 LLBL events and categorized them into three distinct types based on proton energy distributions: single-population, double-population, and energy-shifted LLBLs. While the first two types have been observed at Earth, the third is reported here for the first time in a planetary magnetosphere. Each type exhibits unique ion characteristics and distinct local time distribution: (a) single-population LLBLs show magnetosheath-like energy spectra; (b) double-population LLBLs contain two coexisting components with energy similar to that of the magnetosheath and magnetosphere; and (c) energy-shifted LLBLs exhibit a smooth energy increase from the magnetosheath level to the magnetospheric level. These types are clearly separated in local time, with single-population LLBLs near the subsolar region, double-population LLBLs extending to dusk, and energy-shifted LLBLs on the dawnside, reflecting distinct plasma transport and energization processes. Energy-shifted LLBLs exhibit significant perpendicular energization in the dawn region, where magnetosheath ions entering the magnetosphere drift dawnward and are further energized by cross-field potential drops. Mercury’s smaller geometric radius and magnetic field curvature, which lead to stronger ion drift, likely make this drift-driven energization more pronounced than at Earth. These results offer new insight into plasma transport across Mercury’s magnetopause and lay the foundation for future BepiColombo investigations, which will provide higher temporal and energy resolution multi-point observations of Mercury’s magnetospheric dynamics.