Venus nightside ionospheric irregularities and their relationship to VLF bursts

Abstract

The plasma density and magnetic field are highly spatially and temporally variable in the Venus nightside ionosphere. This variability may affect the propagation and the generation of plasma waves. We determine how these irregularities are related to plasma wave activity. Previously, we have found two types of signals in the lower ionosphere, 100‐Hz narrow‐band signals and high‐frequency wideband bursts. We investigate how these two different wave phenomena depend on the structure of the ionosphere. We calculate the ratios of the instantaneous values of electron density, magnetic field, and electron temperature relative to their average (1 min) values, and also their gradient scale lengths. The 5.4‐kHz burst activity is related to density depressions and obvious electron temperature enhancements, but varies little with magnetic field strength. The 100‐Hz narrow‐band signals are often associated with significantly enhanced magnetic field and depressions of electron density. Both higher field strength and lower density result in a lower β (ratio of thermal pressure to magnetic pressure) plasma, which favors the propagation of whistler waves. We find that neither type of signal is associated with gradients of the magnetic field, but the 5.4‐kHz wideband signals have higher occurrence rates at higher electron density and temperature gradients, while the 100‐Hz narrow‐band burst rate increases only slightly for steeper gradients. A local gradient‐driven instability cannot explain most of the 100‐Hz narrow‐band signals, but some 5.4‐kHz signals may be associated with pressure gradients.