A Bx-interconnected magnetosphere model for Mercury

The access of solar wind plasma to the surface of Mercury depends on the magnetic fields in the vicinity of the planet. We present the structure of the Hermean magnetosphere obtained by the Toffoletto–Hill (J. Geophys. Res. 98 (1993)) model of a magnetically interconnected (“open”) magnetosphere modified for the size of Mercury and the strength of its magnetic field. We calculate open regions for the access of incident particles to the surface as a function of the interplanetary magnetic field (IMF) direction and magnitude. These results are compared with existing sodium data obtained during a week-long period of observations in November 1997. Although quantitatively crude, the model gives a qualitative approach on how to interpret a good part of the sodium emissions. We conclude that increased ion-sputtering due to solar wind–magnetosphere interactions may explain the temporal and spatial variations of the sodium exosphere seen at Mercury. We predict that emissions should be stronger in the southern hemisphere for a positive Bx component, and in the northern hemisphere for a negative Bx. The Bz component regulates the size and position of the open field line region. More negative IMF Bz corresponds to more equatorial open flux regions.