  We report on the temporal variability of the occurrence of waves at the local
proton cyclotron frequency upstream from the Martian bow shock from Mars Global
Surveyor observations during the first aerobraking and science phasing orbit
periods. Observations at high southern latitudes during minimum-to-mean solar
activity show that the wave occurrence rate is significantly higher around
perihelion southern summer solstice and lower around the same hemisphere's
spring and autumn equinoxes. A similar trend is observed in the hydrogen (H)
exospheric density profiles over the Martian South Pole obtained from a model
including UV thermospheric heating effects. In spite of the complexity in the
ion pick-up and plasma wave generation and evolution processes, these results
support the idea that variations in the occurrence of waves could be used to
study the temporal evolution of the distant Martian H corona and its coupling
with the thermosphere at altitudes currently inaccessible to direct
measurements.
