PLASMA-WAVE EVIDENCE FOR LIGHTNING ON VENUS

Plasma wave data from the Pioneer Venus Orbiter provide the largest bo dy of data cited as evidence for lightning on Venus. These data are al so the most controversial, mainly because of the ambiguity in mode ide ntification due to limited spectral information. We review some of the more recent studies of the plasma wave data at Venus, and we demonstr ate that the characteristics of the 100 Hz waves are consistent with w histler-mode waves propagating vertically from below the ionosphere. W e further show that in situ instabilities are too weak to generate whi stler-mode waves, mainly because the thermal pressure is comparable wi th the magnetic field pressure in the ionosphere of Venus. The lower h ybrid drift instability has also been suggested as an alternative sour ce for the 100 Hz waves. However, the wave properties are more consist ent with whistler-mode propagation; the lower hybrid drift instability requires very short gradient scale lengths to overcome damping due to collisions. We also note that an apparent association between Langmui r probe anomalies and 100 Hz waves is much lower than previously repor ted, once we apply a consistent intensity threshold for identifying wa ve bursts. The lightning hypothesis remains the most probable explanat ion of the plasma waves detected at low altitudes in the nightside ion osphere of Venus.