VLF EMISSIONS IN THE VENUS FORESHOCK - COMPARISON WITH TERRESTRIAL OBSERVATIONS

Enhanced ELF/VLF emissions are observed in the solar wind upstream of the Venus bow shock for periods when the interplanetary magnetic field connects the Pioneer Venus Orbiter spacecraft to the shock surface. Such magnetic connection places the spacecraft in the foreshock region and provides an avenue along which counterstreaming electrons and ions of shock origin escape away from the shock and propagate into the upstream region. Two distinct sets of emissions are observed by the Orbiter electric field detector: Enhanced 30-kHz emissions are found in the electron foreshock and are identified as parallel polarized electron plasma oscillations while large amplitude 5.4-kHz and 730-Hz signatures are measured in the ion foreshock and identified as parallel polarized ion acoustic emissions. The 30-kHz electron foreshock emissions have peak intensity at the electron foreshock boundary and decrease in intensity with penetration into the foreshock, but the enhanced 5.4-kHz emissions in the ion foreshock are observed throughout the entire ion foreshock region. Both the electron and ion foreshock emissions show a decrease in intensity with distance away from the shock surface along the field line connecting the spacecraft to the shock surface. The ion foreshock emissions also show a strong asymmetry between the two foreshock regions formed on either side of the foreshock boundary tangent point (the upstream and downstream foreshocks). This asymmetry may be a consequence of differences in the transmission and/or reflection mechanisms which act both to populate and energize ions in the upstream region. Our study shows that the 30-kHz emissions are tied to the quasi-perpendicular portion of the shock while the 5.4-kHz and 730-Hz emissions are associated predominantly with quasi-parallel regions on the shock. Comparisons of the Venus observations with equivalent terrestrial measurements show them to be surprisingly similar given the large difference in solar wind interaction for the two planets.