THEORY OF SMALL-SCALE DENSITY AND ELECTRIC-FIELD FLUCTUATIONS IN THE NIGHTSIDE VENUS IONOSPHERE

Recently, it has been reported that small-scale (lambda approximately 0.1-2 km) density irregularities occur during 100-Hz electric field bursts in the nightside ionosphere of Venus. The correlation of field and plasma fluctuations suggests that a local plasma instability may be responsible for the turbulence. In this paper we provide a detailed analysis of the lower-hybrid-drift instability as a mechanism to generate the observed irregularities. We develop a fully electromagnetic theory that is relevant to the finite beta-plasma in Venus' ionosphere and include collisional effects (e.g., electron-ion, electron-neutral, and ion-neutral collisions). The key features of the analysis that favor this instability are (1) it is a flute mode and propagates orthogonal to the ambient magnetic field, (2) it is a relatively short wavelength mode and the Doppler-shifted frequency can be greater than or similar to 100 Hz, (3) it can produce both electric field and density fluctuations, as well as magnetic field fluctuations in a finite beta-plasma, and (4) it is most unstable in low-beta-plasmas (i.e., beta less than or similar to 1) so that it is likely to occur in the low-density, high-magnetic-field ionospheric holes. These features are consistent with observational results.