Polarization of Impulsive Signals at Venus - Introduction
J. Geophys. Res., 96, 22, 741-22, 752, 1991
(Received: June 11, 1991; accepted: October 1, 1991)
Copyright 1991 by the American Geophysical Union.
Paper Number 91JE02506.
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Impulsive plasma wave signals observed in the nightside ionosphere of Venus by the Pioneer Venus Orbiter electric field detector (OEFD) are often cited as evidence for lightning in the atmosphere of Venus, as discussed in the review by Russell . In his review, Russell points out that other evidence for lightning exists, but the question of lightning at Venus has not been clearly resolved. Searches for the optical signature of lightning using the Pioneer Venus star sensor [Borucki et al., 1991] place an upper limit of the lightning rate that is less than the terrestrial rate (100 flashes/s). In contrast, the Galileo plasma wave instrument has detected radio bursts in the several hundred kilohertz range that are consistent with lightning [Gurnett et al., 1991]. Lastly, recent studies of the OEFD data [Ho et al., 1991] have determined VLF burst rates that are comparable with the terrestrial lightning rate.
The hypothesis that the impulsive signals observed with the OEFD were due to lightning was first put forward by Taylor et al.  and expanded on further by Scarf et al. . In the latter paper, Scarf and colleagues pointed out that in general only those signals observed in the lowest-frequency channel of the wave instrument (100 Hz) could be whistler mode waves. As a consequence, signals occurring solely in the 100-Hz channel were counted as possible lightning events in subsequent studies [e.g., Scarf and Russell, 1983].
From their analysis, Scarf and Russell  concluded that the impulsive events were observed primarily over the highlands of Venus and postulated that active volcanism was responsible for the lightning assumed to produce the observed signals. However, as noted by Taylor et al. , the study of Scarf and Russell  suffered from orbital bias. Furthermore, Russell et al. [1988, 1989] have shown that the data are more clearly ordered by local time. It is important to note that the event definition used by Russell et al. is different than the definition used by Scarf; in the former any impulsive signal detected in any of the four frequency channels of the OEFD is considered as a possible lightning event. It is the high-frequency events which show strong local time clustering, with an occurrence rate maximum in the 2000 - 2200 local time range.
The OEFD data attributed to lightning hence fall into two classes. One consists of a relatively broadband signal observed mainly in the postdusk local time sector. The second consists of waves observed only in the 100-Hz channel at all local times in the nightside ionosphere, and these waves are thought to propagate in the whistler mode. If the Pioneer Venus Orbiter were equipped with either a search coil sensor or with an electric field detector capable of producing well resolved wave spectra, we could determine the mode of propagation with a reasonable degree of certainty directly from the wave measurements. Unfortunately, the OEFD only measures a single electric field component and furthermore has only four frequency filters at 100 Hz, 730 Hz, 5.4 kHz, and 30 kHz. Typically, only the 100-Hz channel is sensitive to whistler mode waves; the ambient magnetic field must be greater than 26 nT for fce > 730 Hz, where fce is the electron gyrofrequency. We must consequently determine the wave mode through less direct methods.
One possibility is to determine whether or not the signals are observed within the whistler mode resonance cone [Strangeway, 1991; Sonwalkar et al., 1991; C.-M. Ho et al., Control of VLF burst activity in the nightside ionosphere of Venus by the magnetic field orientation, submitted to J. Geophys. Res., 1991 (hereinafter referred to as C.-M. Ho et al., 1991)]. Since the refractive index is large in the ionosphere and the variation in refractive index is mainly due to changes in plasma density, refraction will cause the wave vector of whistler mode waves to align along the density gradient in the ionosphere as the waves penetrate the ionosphere from below. For a horizontally stratified ionosphere the wave vector will consequently point vertically upward. Hence a simple test uses the orientation of the magnetic field with respect to the vertical to determine if wave bursts correspond to vertically propagating whistler mode waves. This test was used by C.-M. Ho et al., 1991 who found that the burst rate inside the resonance cone was about 2 to 3 times that for bursts outside the resonance cone.
A more sophisticated version of the resonance cone test was employed by Sonwalkar et al. . They used detailed knowledge of the plasma density, magnetic field strength and orientation, and the orientation and spin phase of the OEFD antenna to predict the degree of spin modulation expected for a whistler mode wave propagating vertically in the ionosphere. This test not only uses the simple binary test of whether or not the waves are within the resonance cone but also tests the degree of polarization of the signal. In general, this depends on the direction of propagation and the plasma parameters. Sonwalkar et al. found that 6 of the 11 cases studied were consistent with whistler mode propagation.
In this paper we will only test for perpendicular or parallel polarization rather than the more complex analysis used by Sonwalkar et al. . We are motivated to do this for two reasons. The first is that Scarf and Russell  published two examples of wave polarization showing perpendicular orientation to argue that the 100-Hz waves are whistler mode waves. We wish to determine if the polarization found by Scarf and Russell is statistically significant. Second, our analysis should complement the more rigorous analysis used by Sonwalkar et al. in their case studies. Because our method is relatively simple, we can analyze many orbits of data. The polarization test is an important additional piece of information to be used for determining the source of the waves observed at Venus. The examples presented by Scarf and Russell  were used to counter a suggestion by Taylor and Cloutier  that the 100-Hz signals were ion acoustic waves, since these might be expected to be parallel polarized.
The outline of the paper is as follows. In the next section we discuss the polarization expected for whistler mode waves in the nightside ionosphere of Venus, and we give some examples of wave polarization. In the third section we present the results of our statistical analysis. First, we consider the effects of interference. We then discuss the results for waves that can propagate in the whistler mode, assuming a subionospheric source. Lastly, we present statistical results for nonwhistler mode waves. The last section summarizes the statistical results and presents some concluding remarks. The appendix describes the methods used to determine the statistical significance of our analysis.
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