Upstream Proton Cyclotron Waves at Venus and Consequences for the Venus Exosphere

Magnetometer data from two Venus-years of the Venus Express mission are investigated for the occurrence of proton cyclotron waves. Recently, proton cyclotron waves (PCW) were detected in the upstream region, in and upstream of the foreshock region and over a large volume of space; they are a direct indication of pick up of planetary protons from the exosphere of Venus and loss of hydrogen to interplanetary space. A statistical survey from long term observations from start of the nominal orbit (May 2006) over two Venus- years gives insight into the waves' spatial occurrence and specific properties. The waves are found up to large distances from the planet (~ 9 Rv); the positions of observation in Solar Wind Magnetic coordinates (with Z along the convective electric field) show no clear organization with the electric field direction. No known mechanisms exist to propagate PCWs across the magnetic fieldlines nor ions into directions of negative electric field. Furthermore, generation of the waves closer to the planet and subsequent propagation in the solar wind outward to the observed position can be excluded for several reasons. Therefore, the PCWs have to be generated locally from available neutral hydrogen, even at large distances. Pick-up from different sources of neutral hydrogen is discussed, leading to a non-satisfying result for non- planetary particles. The PCW observations from VEXMAG are interpreted as indication for the existence of an extended neutral hydrogen exosphere at Venus. The nature of this extended exosphere is still an open issue and under investigation.