Ulf Waves at the Proton Gyrofrequency Upstream of the Martian bow Shock, Based on Mars Global Surveyor's Measurements

Small amplitude MHD waves at the proton gyrofrequency were detected and analyzed using the magnetic field measurements obtained by the Mars Global Surveyor, in the region upstream of the Martian bow shock, between days 87 and 255 of 1998. The origin of the observed ULF waves was found to be protons back-streaming from the bow shock and pick-up protons from the planetary hydrogen exosphere, interacting with the incoming solar wind. A small contribution of oxygen ions has also been detected. Furthermore, the spatial distribution of the waves was investigated, using the spacecraft's position during the time the data were obtained.