Analysis of Low Frequency Whistler Wave Occurrences in the Night-side Venus Ionosphere

This body of work deals with a detailed analysis of plasma, magnetic, and electric field data from Pioneer Venus Orbiter (PVO) to determine if the data are consistent with the possibility of lightning on Venus. This has been a strong topic of debate in the planetary physics community. In a recent Nature article [Ingersoll, 2007], Ingersoll provides a synopsis of the case against lightning on Venus. He states that there should not be lightning on Venus, whose clouds, at roughly 55-60km above the surface, are like terrestrial smog clouds, which do not produce lightning. Ingersoll then goes on to recount that no visible evidence (flashes) has been detected on the night or day-side of Venus. An article published by Gurnett [ Gurnett, et al., 2001] details the non-detection of high frequency radio waves characteristic of terrestrial lightning (0.125 to 16MHz) during Cassini's two fly-bys of Venus, contrasted with the definite detection of RF waves during Cassini's later earth fly-by. However, the detection of low frequency whistler waves by Venus Express has revived claims that the source of these whistler waves is lightning in the lower atmosphere of Venus [ Russell, et al., 2007]. Numerous other papers have been published on different aspects of the debate, such as a paper addressing telemetry interference being incorrectly interpreted as evidence for lightning [Taylor, et al., 1988], another paper suggesting the detected events are a local phenomenon [Taylor, et al., 1983], and a paper documenting some of the optical searches for Venusian lightning [Taylor, et al. , 1994]. This work is a comprehensive reconsideration of 14 years of PVO plasma data on a season by season basis, as the spacecraft goes to low altitudes on the night-side of Venus. In this effort, intelligent software filters have been developed to find, sort, and analyze the frequency of occurrence of low frequency whistler waves. The results of this investigation show that the source cannot be in the lower atmosphere of Venus, since at the lowest altitudes (140-156km), the signals disappear. Therefore, an ionospheric source for these whistler waves must be considered.