Ion trajectories in Mercury's magnetosphere

Abstract

This thesis presents new tracings of ions in Mercury's magnetosphere that model the variability of the solar wind, sodium and potassium surface fluxes. The magnetic field is given from a modified version of the analytic Toffoletto-Hill (TH93) open magnetosphere model, which also gives the electric potential along open field lines. Its applicability is extended into the closed field line region by the Ding (1995) potential solver, which computes the realistic electric potential that is self-consistent with the magnetic field. Three cases of the solar wind and the interplanetary magnetic field (IMF) are tested, two at aphelion and one at perihelion. Photoions are launched from the scale height for each species, while solar wind ions are backtraced from the surface and to the magnetopause. Photoion results reveal that the escape rate to the solar wind responds to external conditions only loosely: between aphelion and perihelion, the escape ratio was seen to range from 30 to just 40 percent. Therefore, impacts dominate. The prediction that recycling reduces by a factor of 1.5 at perihelion could help explain why the sodium atmosphere is denser at aphelion. The flux capable of sputtering varies between 10--15 of the total dayside flux for strongly southward IMF Bz = -10 nT. In addition, tracings of Hermean ions show that differential escape losses do not exist for potassium photoions such that may explain the variable Na/K ratio in the Hermean atmosphere. Solar wind ion tracings confirm that precipitation to Mercury's surface may happen along closed field lines not only at perihelion, but even at aphelion for realistic cases of southward IMF. The computed total sputtering flux increased by a factor of 1.7 from aphelion to perihelion. Most of the flux capable of sputtering is deposited on a region that is extended in longitude but limited in latitude. We conclude that ion sputtering caused by the solar wind can explain the high-latitude variability seen in imaging data of Mercury's sodium atmosphere. This work is relevant to the NASA/Messenger and ESA/BepiColombo missions, both for mission planning and analysis of results