SMALL-SCALE DENSITY IRREGULARITIES IN THE NIGHTSIDE VENUS IONOSPHERE - COMPARISON OF THEORY AND OBSERVATIONS

The lower-hybrid-drift instability has been proposed as a mechanism to generate small-scale density and 100 Hz electric field fluctuations in the nightside Venus ionosphere [Huba, 1992]. The primary purpose of this paper is to compare this theory with observations of density irregularities. Marginal stability boundaries (gamma = 0) for the lower-hybrid-drift instability and the occurrence of small-scale density fluctuations are presented as a function of magnetic field B and density n. For density gradient scale lengths in the range 2 - 10 km, 80 - 85% of the density fluctuations lie in the unstable B/n parameter regime (gamma > 0). A secondary purpose of the paper is to present stability boundaries for the onset of instability at a wavelength for which the Pioneer Venus Orbiter (PVO) could measure a Doppler-shifted frequency of 100 Hz. The conditions on B and n for instability in this situation are more stringent than those for marginal stability, especially at low densities (n less-than-or-equal-to 5 x 10(3) cm-3). In general, the instability is most likely to be generated in regions of low beta: high magnetic field strength and low density, as found in ionospheric holes or troughs.