The role of photoemission in the coupling of the Mercury surface and magnetosphere

Photoelectrons are emitted from the surface of Mercury and take part in cha rge exchanges with the magnetosphere. We investigate the role of photoemiss ion in closing field-aligned currents and in balancing the flow of magnetos pheric electrons which precipitate to the surface. The conductance of the p hotoelectron sheath and sub-surface rock material are estimated and compare d with that of the exosphere. It is shown that the loss cone angle is alway s larger than 30-40 degrees on the dayside, and that significant electron p recipitation takes place. It is concluded that the closure of held-aligned current is unlikely because the conductance of the surface environment is t oo low, but that photoemission may alter the isotropy of the magnetospheric electron energy distribution and induce plasma instabilities. (C) 1999 Els evier Science Ltd. All rights reserved.