SPATIAL-DISTRIBUTION OF PLASMA-WAVE ACTIVITY IN THE NIGHTSIDE IONOSPHERE OF VENUS

In this study we use 14 years of Pioneer Venus Orbiter Electric Field Detector (OEFD) data to define the characteristics of VLF burst activi ty in the nightside ionosphere of Venus. Our statistical results show that there are essentially four types of VLF signals. The first type o f signal is only observed in the 100 Hz channel and not in any of the higher frequency channels (730 Hz, 5.4 kHz or 30 kHz). Occurrence of t hese waves is controlled by the magnetic field with a weaker dependenc e on electron density. The occurrence rate decreases with increasing a ltitude to a height of 600 km. For higher altitudes beyond 600 km the occurrence rate remains roughly constant. The statistics of these sign als are what one would expect for whistler mode waves from a subionosp heric source. The second type of signal is broadband wave activity app earing below 300 km in the low altitude ionosphere. These signals ofte n occur in all four channels of the OEFD. These signals are also thoug ht to come from a subionospheric source. The third type of signal is s trong mid-frequency broadband burst signals appearing near the edge of the planetary optical shadow. They are probably ion acoustic waves ge nerated by a current driven instability associated with plasma clouds in the wake. The fourth type of signal is a narrow band wave. It occur s in either of the two high frequency channels in the high altitude ta il region, and is attributed to locally generated Langmuir waves. In a ddition, we also observe spacecraft interference noise in both the 100 and 730 Hz channels. These signals mainly occur near the edge of the planetary optical shadow and have an inbound and outbound asymmetry in activity.