Field Line Resonance in the Hermean Magnetosphere: Structure and Implications for Plasma Distribution

The first statistical survey of field line resonance (FLR) events is presented using magnetometer data from the entire MErcury Surface, Space ENvironment, GEochemistry and Ranging mission. Ultralow-frequency waves are an important tool for the magnetoseismology of the Hermean magnetosphere; this study provides a completely new window onto the resonance structures and plasma density distribution in the Hermean magnetosphere. Here we assess resonance events from two categories—toroidal resonances characteristic of the classical picture of FLRs in the terrestrial magnetosphere driven by the Kelvin-Helmholtz instability and a more comprehensive approach including all observed transverse resonances with more relaxed polarization criteria. Two hundred twenty-three toroidal FLRs with characteristics consistent with Kelvin-Helmholtz-driven FLRs are found in the dayside Hermean magnetosphere. The fundamental frequencies of these waves are used to provide estimates of plasma mass density in the range of ∼ 1–650 amu/cm3. A further 343 transverse resonances are found which provide very similar density estimates to the Earth-like FLR population. Fundamental and harmonic frequencies from all 566 events are used to fit a power law to plasma mass density along the field lines. The equatorial plasma mass density is predicted to vary approximately with R−7.5. The offset of the Hermean dipole into the northern hemisphere causes significant asymmetries in the standing wave structure. Due to the extreme warping (away from a dipolar configuration) of Mercury's magnetosphere by the solar wind, the fundamental toroidal mode is predicted to oscillate with a notably lower frequency than the fundamental poloidal mode, contrary to relative toroidal and poloidal frequencies modeled for Earth's magnetosphere.