Planetographic clustering of low-altitude impulsive electric signals in the night ionosphere of Venus

Abstract

The possible occurrence of lightning in the Venus atmosphere has long been a subject of speculation^1,2, so the initial reports of impulsive electromagnetic signals deep in the Venus atmosphere³ and in the night ionosphere⁴ generated much interest. The results of optical experiments, however, were mixed. The infrared spectrometer on Venera 9 observed one sequence of lightning-like activity⁵. The star-sensor of the PVO orbiter⁶ saw none and the Vega balloon photometers⁷ obtained negative but ambiguous results. Evidence for the lightning generation of the electromagnetic signals includes their impulsive nature, their occurrence patterns^8,9, their altitude dependences^8,10 and their electromagnetic polarization¹¹. Although only signals below several hundred hertz can directly propagate to the PVO spacecraft, higher frequency signals could scatter in the inhomogeneities of the night ionosphere¹². As these higher frequencies cannot propagate far into the ionosphere and are seldom observed at altitudes of above 400 km (ref. 13) they mark the location of the generating region. In this paper we map the locations of the observations made at 730 Hz during the first three night-time observing seasons of Pioneer Venus, when the spacecraft reached altitudes low enough to detect these leakage signals. These maps show that there are specific 'active regions' of enhanced signal occurrence and that at least some of these active regions appear to remain fixed in planeto-graphic coordinates from year to year.