The access of incident solar wind plasma to the surface of Mercury is dependent on the magnetic fields in the vicinity of the planet. We present the structure of the Hermean magnetosphere obtained by the Toffoletto-Hill (1993) model of a magnetically interconnected ("open") magnetosphere modified for the strength of the magnetic field of Mercury and scaled to account for the difference in planetary radii between Mercury and the Earth. We calculate open regions for the access of incident particles to the surface as a function of the interplanetary magnetic field (IMF) direction and of the solar wind pressure. Predictions are also made for cases of limited field penetration. These results are compared with existing sodium data. Although quantitatively crude, the model gives a good qualitative approach on how to interpret the sodium emission. We predict emission is stronger in the southern hemisphere for a positive Bx component, and in the north for a negative Bx. The Bz component regulates the size and position of the open field line region, with more negative Bz's corresponding to more equatorial open flux regions.