Disturbance-Dependent Expansion of Magnetic Field Variability in Mercury's Northern Magnetospheric Cusp

Mercury’s northern magnetospheric cusp is a primary pathway for solar wind plasma to the surface, exosphere, and magnetosphere. Using the full MESSENGER orbital data set, we investigate how cusp magnetic variability responds to global magnetospheric activity. We quantify high-frequency magnetic field variability with the scalar metric σ_b(B) derived from filtered MAG measurements and relate it to the magnetic disturbance index (DI) derived from MESSENGER magnetometer data Anderson et al. (2013).

We find that DI exerts a strong, control on σ_b(B) the cusp. The high-DI intervals show an enhanced and spatially expanded magnetic variability within the northern cusp region. This enhancement is consistent with the increases in planetary heavy-ion signatures, particularly Na⁺ and He²⁺, attributed to intensified cusp driven ion sputtering from incoming H⁺ at the surface. The resulting ions are distributed within the magnetosphere by e.g. E × B drift, providing a feedback between cusp precipitation, planetary ion supply, and enhanced magnetospheric disturbance Raines et al. (2022).