Precipitation of solar wind ions in the Venusian atmosphere : An ASPERA-4 investigation

Precipitation of solar wind ions in the Venusian atmosphere during solar minimum is studied using ASPERA-4 (Analyzer of Space Plasma and Energetic Atoms) data onboard the Venus Express pacecraft. A strong dependence of penetration on interplanetary magnetic field direction has been found. With change in the field orientation from parallel to perpendicular, the probability of observing precipitating protons during an orbit changed from 43% to 5%. For July 2007 data the probability is 24%. An alpha particle penetration case has also been observed in the nightside ionosphere, but this is the only case seen throughout our study. The fluxes of downgoing protons together with occurrence frequencies have also been calculated. It is noted that proton penetration occur during 2% of the observation time inside ionopause boundary. The results were also used to compare with results obtained for Mars and large differences in occurrence frequency has been found. The ingoing flux of alpha particles is at least 10 times smaller compared to Mars. With small number of precipitation cases the results suggest that induced magnetic barrier shielding is effective against solar wind on Venus. The results also contradict the idea that solar wind may be a source of helium in the Venusian atmosphere and that the loss of helium is compensated by the capture of solar wind alpha particles.Validerat; 20120709 (anonymous