Polarization of Impulsive Signals at Venus - Conclusions

Polarization of the Impulsive Signals Observed in the Nightside Ionosphere of Venus

R. J. Strangeway

Institute of Geophysics and Planetary Physics,
University of California at Los Angeles

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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4. Summary and Conclusions

       Given the restrictions of the Pioneer Venus OEFD, wave mode identification is often ambiguous. In particular, since the OEFD only detects one component of the wave electric field, and only at four discrete frequencies, it is difficult to identify whistler mode waves. One method for identifying the wave mode is to determine the wave polarization, taking advantage of the spacecraft spin. This has been shown to be quite successful when studying electrostatic plasma waves upstream of the bow shock [Crawford et al., 1990]. We have pointed out that for typical nightside ionosphere plasma densities and magnetic field strengths, the whistler mode wave electric field should be mainly perpendicular to the magnetic field. Hence we might expect to be able to determine the wave mode of the impulsive bursts in the nightside ionosphere from the polarization.

       Specific examples [Scarf and Russell, 1988] have been published to show that the waves detected in the nightside ionosphere are whistler mode. However, the signals are usually rather impulsive in nature, and often only last for some fraction of the spin period. It is reasonable to ask if the examples published by Scarf and Russell are typical or just fortuitous. In order to address this question we have presented a statistical study of the wave polarization using data from the third nightside periapsis season.

       In performing the statistical analysis of the wave polarization we assume that the direction of maximum variance of the wave power gives the orientation of the wave electric field. However, the maximum variance method does not discriminate between real and artificial signals, and the method may be aliased because of the impulsive properties of the signals. Indeed, we find that much of the 100-Hz data are contaminated by interference signals which are probably associated with the spacecraft interaction with the ionosphere. Even when we restrict our analysis to burst intervals in the RvD&S data set, we find that the polarization statistics are biased by interference. It should be noted that the interference only affects the polarization study, previous studies which determined percent occurrence rates are unaffected by the interference.

       The spin phase at which the interference signal is maximum appears to be roughly ordered by the radius vector from the spacecraft to the center of the planet. Since the magnetic field is mainly horizontal in the nightside ionosphere, we obtain an apparent perpendicular polarization that is due to the interference signals. To counteract this, we have derived a "cleaned" data set in which those intervals which appeared to be contaminated by interference are deleted from the data base by visual inspection. This data set was found to display no preferred polarization direction for 100-Hz waves.

       Recently, Strangeway [1991], Sonwalkar et al. [1991], and C.-M. Ho et al., 1991 have pointed out that not all 100-Hz waves are necessarily whistler mode waves. If the source of the whistler mode waves is subionospheric, then the large increase in refractive index encountered by the waves on entering the ionosphere will cause the wave vector to be aligned along the density gradient, which we assume is vertical. Since whistler mode waves can only propagate within the resonance cone, horizontal ambient magnetic fields will preclude whistler mode waves. Consequently, we have further subdivided the data into intervals for which whistler mode propagation is or is not allowed. The cleaned data set showed little bias due to interference, and the wave fields were found to be polarized perpendicular to the ambient magnetic field for propagation inside the resonance cone.

       The cleaned data set also showed that 100-Hz waves detected outside the resonance cone tend to be polarized parallel to the ambient field. A similar result was found for the higher frequencies, which did not suffer from interference. These higherfrequency signals are mainly observed in the postdusk local time sector [Russell et al., 1989], and as discussed by Russell [1991], may be analogous to the signals detected above terrestrial thunderstorms [Kelley et al., 1985]. In the terrestrial case the waves are also polarized along the magnetic field.

       The polarization statistics are consistent with the results of Sonwalkar et al. [1991] and C.-M. Ho et al., 1991 that the impulsive signals detected in the nightside ionosphere of Venus fall into two classes. Prior to these studies the classification was usually based on the wave frequency alone, with the 100-Hz bursts assumed to be whistler mode waves, and the higher frequencies being due to some anomalous wave propagation mechanism. It now appears that the 100-Hz waves also fall into these two, classes. We find that roughly half of the intervals containing 100-Hz bursts are associated with vertical fields and are consistent with whistler mode waves propagating from below the ionosphere. However, since the magnetic field is mainly horizontal in the nightside ionosphere, the normalized rate of occurrence is higher for whistler mode waves. A similar result was found in the burst rate study of C.-M. Ho et al., 1991.

       In conclusion, the results presented here support the hypothesis that atmospheric lightning is responsible for the waves observed in the nightside ionosphere of Venus. The local time dependence of the higher-frequency burst rate [Russell et al., 1989; Ho et al., 1991] is the most telling argument for a lightning source for the nonwhistler mode waves, but the additional information that these waves are parallel polarized suggests that the signals may be similar to those reported by Kelley et al. [1985]. It should be emphasized, however, that a local electrostatic instability might generate parallel polarized waves, and the polarization data alone are not sufficient to discriminate between sources at the higher frequencies. With regard to the 100-Hz waves the whistler mode identification argues against a local source. First, the 100-Hz waves were found to be polarized perpendicular to the ambient magnetic field when whistler mode waves could propagate vertically from below. It is not clear why any local instability would depend on the ambient magnetic field orientation. Second, Maeda and Grebowsky [1989] suggested that whistler mode waves could be generated in situ, but Strangeway [1990] pointed out that the high refractive index of whistler mode waves means that the thermal electron Landau damping is important and any instability must have a large growth rate to overcome this damping. Since it appears unlikely that whistler mode waves are generated locally in the nightside ionosphere of Venus, atmosphere lightning is a probable source.

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