The source of impulsive 100 Hz electric field signals detected in the nightside ionosphere of Venus

The Pioneer Venus Orbiter Electric Field Detector (PVO OEFD) detected numerous impulsive electric field events in its 100 Hz channel when in nightside ionospheric troughs. What the source of the signals is has been under debate for over 10 years. Some researchers claim that lightning is generating whistler waves which are then being detected, while other researchers have supported the view that the signals are caused by a local generation of a plasma instability due to the conditions inside the troughs. We believe that the evidence collected by the PVO clearly disproves a lightning source and, in fact, points to an electrostatic wave as being the producer of the transients. We studied several wave modes in an effort to determine the source of the signals. We looked at the two-stream instability, the ion-acoustic instability, the gentle-bump instability, the lower-hybrid-drift instability and the Alfven wave mode.; We found that not one of the wave modes studied could account for all of the signals. It would appear that there is not a large enough current within the troughs to support the two-stream instability. While the ion-acoustic instability may well be present within some troughs, the conditions needed to produce 100 Hz waves may not be universal trough conditions. Thus, this wave mode is not a likely generator of all the signals. The gentle-bump instability cannot produce 100 Hz waves and any Alfven waves will be damped.; The lower-hybrid-drift instability seems to be the most likely candidate of the wave modes studied for producing the transient signals. The frequencies of the lower-hybrid-waves, however, appear to be too low to be detected by the OEFD. We believe that either the frequencies that we calculated are too small due to factors not taken into account, or there is a cascade of energy from the lower-hybrid waves to waves with frequencies that can be detected by the OEFD, or there is a combination of the two.