SMALL-SCALE PLASMA IRREGULARITIES IN THE NIGHTSIDE VENUS IONOSPHERE

The individual volt-ampere curves from the Pioneer Venus orbiter electron temperature probe showed evidence for small-scale density irregularities, or short-period plasma waves, in regions of the nightside ionosphere where the orbiter electric field detector observed waves in its 100-Hz channel. A survey of the nightside volt-ampere curves has revealed several hundred examples of such irregularities (approximately 2% of all the current-voltage (I-V) sweeps from below 2000 km and 7% of those below 200 km). The I-V structures correspond to plasma density structures with spatial scale sizes in the range of about 100-2000 m, or alternatively they could be viewed as waves having frequencies extending toward 100 Hz. They are often seen as isolated events, with spatial extent along the orbit frequently less than 80 km. The density irregularities or waves occur in or near prominent gradients in the ambient plasma concentrations both at low altitudes where molecular ions are dominant and at higher altitudes in regions of reduced plasma density where O+ is the major ion. Electric field 100-Hz bursts occur simultaneously, with the majority of the structured I-V curves providing demonstrative evidence that at least some of the E field signals are produced within the ionosphere. Potential sources for the irregularities are extraionospheric particle precipitation and spatial structures and turbulence resulting from the day to night ion transport within the ionosphere.