A B-x-interconnected magnetosphere model for Mercury

The access of solar wind plasma to the surface of Mercury depends on the ma gnetic fields in the vicinity of the planet. We present the structure of th e Hermean magnetosphere obtained by the Toffoletto-Hill (J. Geophys. Res. 9 8 (1993)) model of a magnetically interconnected ("open") magnetosphere mod ified for the size of Mercury and the strength of its magnetic field. We ca lculate open regions for the access of incident particles to the surface as a function of the interplanetary magnetic field (IMF) direction and magnit ude. 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 p art of the sodium emissions. We conclude that increased ion-sputtering due to solar wind-magnetosphere interactions may explain the temporal and spati al variations of the sodium exosphere seen at Mercury. We predict that emis sions should be stronger in the southern hemisphere for a positive B-x comp onent and in the northern hemisphere for a negative B-x. The B-z component regulates the size and position of the open field line region. More negativ e IMF B-z corresponds to more equatorial open flux regions. (C) 2001 Publis hed by Elsevier Science Ltd.