THE DAYSIDE MAGNETOSPHERE OF MERCURY

The magnetosphere of Mercury is often referred as “pocket-magnetosphere”, due to its relative size with respect to the planetary radius. Data from Mariner 10 spacecraft shown that the magnetosphere of this planet can be roughly approximated by scaling the Earth’s one (about 1:6.9), nevertheless, several differences are expected due to the lack of an ionosphere, and the likely absence of a stable ring current. Furthermore, at Mercury’s orbit, the IMF BX component is expected to play a crucial role in the solar wind – magnetosphere coupling, causing the magnetosphere of the planet to be actually “open”, even for IMF BZ > 0. Models indicate that a prominent feature is represented by the wide open area(s) on the Mercury’s dayside magnetopause, due to the broad cusp regions, through which the magnetosheath plasma can enter and then circulate into the magnetosphere. Thanks to the acceleration gained by crossing the rotational discontinuity at the magnetopause, the ions injected along the reconnected field lines can reach energies up to tens of keV, depending on the value of the Alfvenic speed on both sides of the magnetopause. Finite gyro-radius effect and gradient drift cause the spreading of the initial distribution over a broad region. The absence of a dense atmosphere allows a large faction of the impinging plasma to reach the planet’s surface, where it can contributes via ion-sputtering to the release processes of heavy atoms form the soil. Results obtained by modeling the Mercury’s magnetosphere by means of an ad hoc modified Toffoletto-Hill (TH39) model, is presented. The analysis is focused on the dayside magnetospheric configurations, and on the plasma entry and circulation patterns.