Proton Precipitation in Mercury's Northern Magnetospheric Cusp

• Published in: Journal of geophysical research. Space physics 2022-11, Vol.127 (11), p.n/a
• Main Authors: Raines, Jim M., Dewey, Ryan M., Staudacher, Natalie M., Tracy, Patrick J., Bert, Christopher M., Sarantos, Menelaos, Gershman, Daniel J., Jasinski, Jamie M., Bowers, Charles F., Fisher, Erik, Slavin, James A.
• Format: Article
• Language: English
• Subjects: Aerospace environments; Cusps; Exosphere; FIPS; Fluctuations; Geochemistry; Interplanetary magnetic field; Ion precipitation; Latitude; Magnetic fields; magnetosphere; Magnetospheric cusp; Mercury; Mercury (planet); Mercury surface; MESSENGER Mission; Neutral atoms; Planetary magnetospheres; plasma; precipitation; Proton flux; Proton precipitation; Protons; Regolith; Solar wind; Solar wind ions; sputtering
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2022JA030397

Ion precipitation onto Mercury's surface through its magnetospheric cusps acts as a source of planetary atoms to both Mercury's exosphere and magnetosphere. Through the process of ion sputtering, solar wind ions (∼95% protons) impact the surface regolith and liberate material, mostly as neutral atoms. We have identified 2760 northern magnetospheric cusp crossings throughout the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission, based on enhancements in proton flux observed by the Fast Imaging Plasma Spectrometer (FIPS). We find cusp crossings spanning 50–85° in magnetic latitude with a geometric center typically at 60–70°. The cusp center is stable about its average but its latitudinal extent varies orbit‐to‐orbit. The mean latitude weakly depends on the magnitude of the interplanetary magnetic field (IMF), dropping by about 1.3° magnetic latitude for each increase of 10 nT in IMF strength. We have used these identified cusp boundaries to estimate the flux of protons which will precipitate onto Mercury's surface. We find an average proton precipitation flux of 1.0 × 107 cm−2 s−1, ranging 3.3 × 104–6.2 × 108 cm−2 s−1, and that this flux can vary substantially between subsequent 10‐s measurements. We also tabulated the peak precipitation fluxes for each cusp crossing. They range 9.8 × 104–1.4 × 109 cm−2 s−1, with a mean of 3.7 × 107 cm−2 s−1. We find strong dependencies on the local time of the cusp crossing as well as on Mercury's orbit around the Sun, which warrant further investigation. Key Points Mercury's northern cusp was found at 50°–85° magnetic latitude in 2760 MErcury Surface, Space ENvironment, GEochemistry and Ranging orbits, falling by 1.3° per 10 nT increase in interplanetary magnetic field Average proton precipitation flux was 1.0 × 107 cm−2 s−1, ranging 3.3 × 104–6.2 × 108 cm−2 s−1 in the 707 orbits with B vector in view Proton precipitation flux can vary by two orders of magnitude in subsequent 10‐s measurements with peak fluxes up to 1.4 × 109 cm−2 s−1