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.