OBSERVATION OF INTENSE WAVE BURSTS AT VERY-LOW ALTITUDES WITHIN THE VENUS NIGHTSIDE IONOSPHERE

Intense ELF (100 Hz) bursts were detected by the Pioneer Venus Orbiter plasma wave instrument during the final operations of the spacecraft prior to atmospheric entry. These bursts were detected at approximatel y 130 km altitude around 0400 local time. The wave activity lasted for several tens of seconds. Furthermore the bursts were not symmetric ab out periapsis, unlike instrument noise caused by neutral impacts on th e spacecraft. The bursts had a vertical attenuation scale height of th e order 1 km, consistent with that expected for whistler-mode waves pr opagating through a collisional ionosphere. Since the decay of the sig nals appears to be due to attenuation, the source must persist for sev eral tens of seconds. The wave bursts could therefore be the signature of electromagnetic radiation entering the bottomside ionosphere from several distant sources, as would be expected if lightning were a rela tively persistent phenomenon with the Venus atmosphere.