Plasma Wave Evidence for Lightning on Venus

J. Atmos. Terr. Phys., 57, 537-556, 1995
(Received in final form 19 May 1994; accepted 27 June 1994)
Copyright © 1995, Elsevier Science Ltd

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6.       Conclusions

      There is now a large body of evidence that supports the hypothesis that atmospheric lightning is a source of plasma waves in the nightside ionosphere of Venus. Figure 12 gives a sketch of how signals generated by lightning are detected by the Pioneer Venus Orbiter. The sketch shows a portion of the nightside ionosphere containing an ionospheric hole. The magnetic field within a hole is thought to be generated by IMF field lines that are transported to the nightside ionosphere. Therefore the field lines within a hole must ultimately be connected to the IMF, and holes are thought to come in pairs [Brace et al., 1982]. The field lines that exit the bottom of the figure are assumed to be connected to another hole, where they pass back out of the ionosphere into the solar wind. Given the high magnetic field strength within an ionospheric hole, it is likely that some of the field passes through the ionosphere and enters the atmosphere below. At low frequencies electromagnetic waves generated by lightning may propagate some distance in the surface ionosphere waveguide. A hole will provide a region where whistler-mode radiation can escape because of the enhanced magnetic field and reduced density. Thus we expect the 100-Hz bursts to be detected at large distances from the source. At higher frequencies it is possible that the spacecraft detects "near-field" effects, perhaps due to direct coupling to the ionosphere, and we expect the high frequency bursts to occur close to the lightning source.

Fig. 12.     Diagram illustrating the hypothesis that lightning is a source for plasma waves in the nightside ionosphere of Venus.

      On escaping from the atmosphere whistler-mode waves will be refracted vertically, assuming a horizontally stratified medium, and we can determine if the waves are within the whistler-mode resonance cone solely from the orientation of the magnetic field with respect to the vertical. The resonance cone test, which applies only for a wave source below the ionosphere of Venus, clearly shows that many of the 100 Hz wave bursts are whistler-mode waves propagating from below the ionosphere. Vertical refraction explains why the burst rate of 100 Hz waves is a maximum for vertical magnetic fields. The burst rate for 100 Hz waves inside the resonance decreases much more slowly with increasing altitude, in contrast to the non-whistler-mode high frequency bursts and the 100 Hz bursts outside the resonance cone. The waves within the resonance cone are polarized perpendicular to the ambient field.

      One alternative explanation for the 100 Hz waves is that they are whistler-mode waves generated in situ by a plasma wave instability. However, because of the weak magnetic field in the nightside ionosphere of Venus, the electron beta () can be large. Thus we expect 100 Hz waves to be detected in "ionospheric holes", where the plasma density is low, and the ambient field is large, with a large vertical component. The damping due to thermal electrons will be lowest in such a region. However, even in holes, there is sufficient damping to quench any instability due to precipitating electrons that may have come from the solar wind.

      More recently, the lower hybrid drift instability has been postulated as an alternative explanation of the 100 Hz waves. As with the whistler-mode, the lower hybrid waves are expected to occur in regions of low . However, the lower hybrid drift instability requires small electron Larmor radii, and hence large fields, since the wavelength of the waves must be 100 m to be Doppler-shifted to 100 Hz through spacecraft motion. Additionally, the required gradient scale length must be very short, 2.5 km, so that the resultant gradient drift velocity is large enough to overcome the damping due to collisions in high density regions. The ion Larmor radius is typically 5 km.

      The lower hybrid instability may better explain the Langmuir probe anomalies investigated by Grebowsky et al. [1991]. Grebowsky et al. reported a high degree of coincidence between 100 Hz wave bursts and Langmuir probe anomalies, but they did not use a consistent identification criterion for the wave bursts. We find a much lower degree of coincidence ( 20%). This level of coincidence appears to be because both the wave bursts and Langmuir probe anomalies are mainly detected in regions of low , rather than being due to a common source.

      The major question remaining for the 100 Hz waves concerns how much of the energy generated by lightning gains access to the ionosphere. Huba and Rowland [1993] have determined the attenuation due to collisions as the waves enter the atmosphere. They found that for peak densities of 10 cm, approximately 0.1% of the incident wave energy could be transmitted through the ionospheric density peak, provided the vertical magnetic field was 30 nT. The attenuation scale depends quite strongly on the ambient magnetic field strength and plasma density. Strangeway et al. [1993] compared the observations at very low altitudes ( 130 km) obtained during the Pioneer Venus Orbiter entry phase with predictions for the attenuation scale. The observed attenuation scale lengths were 1 km, consistent with lightning generated whistler-mode waves propagating through a moderately dense plasma with a vertical field < 10 nT. However, possible spacecraft interactions with the neutral atmosphere cannot be completely discounted as a source for the waves at very low altitudes.

      The wide-band bursts detected at low altitudes may also be due to lightning, and may be evidence for direct coupling of lightning into the ionosphere of Venus. However, this interpretation is somewhat speculative, and additional study of coupling mechanisms and alternative sources is warranted. Some insight into the nature of the broadband waves may be obtained through comparison with the recent GEOTAIL results [Matsumoto et al., 1994]. The GEOTAIL data indicate that broadband signals observed in the geomagnetic tail consist of short wave packets, which have a broad frequency signature when sampled as a function of frequency. Doppler-shift may cause additional broadening of the signal. By analogy with these signals, the broadband bursts detected at Venus may correspond to short duration wave packets, as expected from an impulsive source such as lightning.

      In conclusion, the preponderance of the evidence points towards atmospheric lightning as the dominant source of plasma waves at low altitudes within the nightside Venus ionosphere. Some may reject this explanation, citing an as yet unknown plasma instability as an alternative. However, in the absence of any viable alternative, and given the consistency of the observations with the lightning hypothesis, the plasma wave data acquired by the Pioneer Venus Orbiter provide strong evidence for the existence of lightning in the atmosphere of Venus.

Acknowledgments-I wish to thank C. T. Russell and C. M. Ho for many useful discussions on lightning and the plasma wave observations at Venus. I also wish to thank J. M. Grebowsky and J. D. Huba for many fruitful exchanges. The late F. L. Scarf was the original Principal Investigator for the Orbiter Electric Field Detector. His efforts were the motivation behind much of the work presented in this paper. This work was supported by NASA grant NAG2-485.

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