TESTING RADIO-BURSTS OBSERVED ON THE NIGHTSIDE OF VENUS FOR EVIDENCE OF WHISTLER MODE PROPAGATION FROM LIGHTNING

Radio burst events recorded on the nightside of Venus by the orbiting electric field detector (OEFD) on Pioneer Venus Orbiter (PVO) have been interpreted as originating in subionospheric lightning. This lightning source interpretation has been subject to repeated challenges. During many of the burst observations, activity occurred in the lowest, or 100 Hz, filter band channel only, while in a smaller number of cases, activity occurred at two or more of the four filter band frequencies 100 Hz, 730 Hz, 5.4 kHz, and 30 kHz. Previous work with the data has been primarily statistical in nature. In some studies, only events with activity limited to the 100-Hz channel were considered; 100 Hz had been found to be lower than typical values (approximately 100-1000 Hz) of the ambient electron gyrofrequency, and such cases appeared to be candidates for whistler mode propagation from lightning sources to the satellite. In general it was recognized that if the higher-frequency signals were of subionospheric origin, their observation from PVO would require an ionospheric penetration mechanism other than the conventional one associated with excitation of the cold plasma whistler mode at the lower ionospheric boundary. In the present work, methods have been developed for testing the hypothesis that particular burst events were the result of whistler mode propagation of signals from subionospheric lightning sources. The tests allow prediction of the resonance cone angle, wave normal direction, refractive index, wave dispersion, and wave polarization and are believed to represent an improved way of categorizing OEFD burst data for purposes of investigating source/propagation mechanisms. The tests, which are capable of refinement, were applied to observations from 11 periods along seven orbits. Most of these cases had been illustrated in the literature in support of conflicting interpretation of the observations. The key wave normal test was applied to each of 11 cases, and the dispersion and polarization tests were also applied to be limited extent that the properties of the particular data sets would permit. The results obtained from the limited data sample indicate that there are at least two main categories of burst events, one for which the assumed vertical wave normal angle was within the allowed cone of angles for whistler mode propagation and one for which this was not the case. Lightning is thus considered to be a candidate source for at least some of the OEFD bursts. Its further assessment as a source must await studies of additional events and, in particular, examination of cases to which the more stringent dispersion and polarization tests can be applied. Four of the five burst events that were found to be inconsistent with the hypothesis of whistler mode propagation from lightning involved receptions at multiple OEFD filter band frequencies, while one involved 100 Hz only. A search for the cause of such events should include possible mechanisms of ionospheric wave penetration of frequencies both above and below the gyrofrequency, as well as plasma instability mechanisms local to the spacecraft.