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.