VLF bursts in the night ionosphere of Venus: Effects of the magnetic field

Abstract

An examination of the occurrence rate of bursts at 0.1, 0.73, 5.4 and 30 kHz in the night ionosphere of Venus and the control of these bursts by the orientation and magnitude of the ionospheric magnetic field shows that orientation has little effect on the occurrence rates but that the field strength has a strong effect. Restricting analysis to intervals where the magnitude field is greater than 15 nT we find that these bursts are mainly a dusk phenomenon at all frequencies. Although strong magnetic fields are more likely to contain these bursts than weak magnetic fields, the local time distribution of strong magnetic fields and of bursts are quite different. Thus the peak in occurrence of a burst at 21:00 L.T. must mainly reflect the source location and not ionospheric propagation conditions. Using these data we make the first contour map of the 100 Hz occurrences. The uncorrected 100 Hz local time latitude map is similar to earlier maps at higher frequencies which have been corrected for the altitude dependence, with the exception that somewhat higher occurrence rates are found in the dawn sector at 100 Hz. This similarity is due to the fact that there is little altitude dependence of the 100 Hz occurrences for strong ( B ⩾15nT) magnetic fields. Maps at higher frequency are also calculated for the three higher frequency channels of the PVO plasma were instrument restricted to intervals of stronger magnetic fields. These maps resemble earlier maps with slightly higher peak occurrence rates, maximizing at about 70% at 21:00 L.T. for 0.73 and 5.4 Hz and 30% at 30 kHz.