Investigation of the Low-Latitude Boundary Layer (LLBL) in Mercury's Magnetosphere

The MESSENGER spacecraft, which orbited Mercury from 2011 to early 2015, provided crucial insights into the structure and dynamics of Mercury's magnetosphere, including the identification of the Low Latitude Boundary Layer (LLBL). LLBL forms a mixed region of the magnetospheric and magnetosheath plasma, playing a crucial role in transferring mass and energy from the solar wind into the planetary magnetosphere. A statistical study by Liljablad et al. (2015) examined the properties of the LLBL during MESSENGER's first orbital year. More recently, the BepiColombo spacecraft crossed the LLBL in Mercury's duskside magnetosphere during its third Mercury flyby in 2023. Using the Mercury Plasma Particle Experiment (MPPE) instruments, specifically the ion analyzer (MIA) and mass spectrum analyzer (MSA), clear ion energy dispersion ranging from a few eV/e to 40 keV/e was observed (Harada et al., 2024; Hadid et al., 2024).

This study aims to build on these findings by conducting a comprehensive analysis of the LLBL using all MESSENGER data collected throughout its orbital period. The Magnetic field (MAG) and ion data (FIPS) revealed 351 LLBL cases. Considering the energy variation of the maximum differential flux of protons from the magnetopause toward the magnetosphere, 38 cases exhibited decreasing H⁺ energy dispersion, while 88 showed increasing H⁺ energy dispersion. Notably, the average H⁺ temperature is higher in LLBLs with increasing dispersion compared to those with decreasing or no dispersion. A clear dawn-dusk asymmetry was observed: 85% of H⁺ decreasing cases occurred on the duskside, while 89% of H⁺ increasing cases were on the dawnside.  Interestingly, in many LLBL cases, the energy dispersion of He²⁺ ions differed from that of H⁺, particularly in the majority of increasing cases, though He²⁺ data is limited. Following orbit insertion, the 3D distribution functions measured by the ion sensors (MIA and MSA) aboard the BepiColombo magnetospheric orbiter will enable a more detailed analysis.