Venus 'lightning' signals reinterpreted as in situ plasma noise

Electric field noise bursts detected from the Pioneer Venus Orbiter across the nightside of Venus have been interpreted in a number of papers as providing evidence of extensive lightning at Venus. Moreover, the asserted clustering of these 100-Hz signals over mountainous topography has encouraged widespread speculation that present-day volcanism is prevalent. Earlier studies have noted, however, that the 100-Hz signals were often associated with ionospheric perturbations, prompting the suggestion that the electric field signals were being produced in situ and were thus unrelated to the lower atmosphere or surface. In this more detailed analysis we find a statistically persistent correlation between the 100-Hz noise and ion troughs marking the solar wind interaction with the nightside ionosphere. Clusters of closely spaced 100-Hz noise bursts attributed to lightning often “turn on” in the troughs, simultaneously with sharp density gradients in which the plasma concentration drops by an order of magnitude or more and the direction of the magnetic field changes significantly, within a few seconds. Analysis of the 100-Hz noise also shows two other important statistical characteristics, namely (1) the noise is most frequently observed near 180 km and above but much less frequently at lower altitudes and (2) the noise is most often detected when the angle between the spacecraft velocity vector and the direction of the B field approaches 90°. We interpret the foregoing characteristics as evidence that the 100-Hz noise is produced from solar wind/interplanetary magnetic field related plasma perturbations occurring near the spacecraft and that the noise is unrelated to the lower atmosphere and surface of Venus. Unfortunately, we do not have access to sufficient high-resolution data to specifically identify the type of ionospheric noise involved.