NOTES ON THE DIVERSITY OF THE PROPERTIES OF RADIO-BURSTS OBSERVED ON THE NIGHTSIDE OF VENUS

We report on further studies of radio wave bursts detected by the Orbi ting Electric Field Detector (OEFD) on PVO in the nightside ionosphere of Venus. We have tested a total of 25 cases of wave burst activity f or evidence of whistler-mode propagation to the spacecraft from impuls ive subionospheric sources. As in a previous study of 11 of these case s (Sonwalkar et al., 1991), we find at least two distinct classes of e vents, one, mostly involving bursts at 100 Hz only, that passes certai n tests for whistler-mode propagation, and another, mostly involving b ursts in two or more of the four PVO narrowband channels (at 100 Hz, 7 30 Hz, 5.4 kHz, and 30 kHz), that fails to pass the tests. The subiono spheric lightning hypothesis continues to be tenable as a candidate ex planation for many of the 100 Hz-only events, but its plausibility cou ld be better evaluated if mechanisms could be found to explain the exi stence of a significant number of 100 Hz-only cases that do not pass a ll the applicable whistler-mode tests, as well as the existence at a w ide range of altitudes of multichannel cases that are clearly not prop agating whistler-mode waves. The wideband bursts are often observed at altitudes above 1000 km and frequently occur in regions of locally re duced electron density. Those observed at high altitude (and possibly at low altitude as well) are believed to be generated near the spacecr aft, possibly by an as yet unknown mechanism responsible for similar b urst observations made near Earth and other planets.