Long-term VEXMAG observations of upstream proton cyclotron waves at Venus

DELVA, MAGDA; VOLWERK, MARTIN; ZHANG, TIELONG; MAZELLE, CHRISTIAN; BERTUCCI, CESAR; VOEROES, ZOLTAN

Congreso; 37th COSPAR Scientific Assembly; 2008

Magnetometer data from two Venus-years of the Venus Express mission are investigated for the occurrence of proton cyclotron waves. As a new feature at Venus, 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 detailed study of representative cases as well as an overview of the specific wave properties over long term observations from start of the nominal orbit (May 2006) is presented, i.e. specific aspects of the spectra, analysis in the magnetic field principal axes system, direction of propagation etc. The waves occur till large distances from the planet ( 9 Rv) and for a large range of angles (Vsw, B). The question of wave generation is discussed and different generation mechanisms are suggested, with emphasis on instabilities driven by field-aligned planetary ion beams. The occurrence of PCWs up to large distances from the planet raises the question of the existence of an extended reservoir of planetary neutral hydrogen.