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

Next: 2. The Morphology of the VLF/ELF Bursts
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1.       Introduction

      Whether or not lightning occurs on Venus has been an issue of considerable debate over many years. Much of that debate has centered on the observation of plasma wave bursts in the ELF/VLF range, as measured by the Pioneer Venus Orbiter Electric field Detector (OEFD) in the nightside ionosphere of Venus. Russell [1991] recently reviewed the evidence for lightning on Venus, and he describes the results of many of the earlier studies of the OEFD data. In particular, he notes that the plasma wave data are more consistent with a lightning source within the Venus cloud layer, rather than active volcanism as originally suggested by Scarf and Russell [1983]. Thus the interpretation of the plasma wave data has more to say about the atmosphere of Venus, rather than issues concerning active volcanism.

      Lightning within the cloud deck of Venus has important implications for the dynamics and chemistry of the Venus atmosphere. In their search for an optical signature for lightning Borucki et al. [1991] pointed out that, based on our knowledge of terrestrial lightning, the meteorological conditions in the atmosphere of Venus may not be appropriate for the generation of lightning. Sulfuric acid, which is the main constituent in the clouds at Venus, has nearly the same dielectric constant as water, but it is not clear that sufficiently large particles are formed to allow charge separation. Thus, if lightning does occur on Venus, we may need to re-evaluate the mechanisms responsible for charge separation within clouds. With regard to the importance of atmospheric lightning, Borucki et al. [1991] noted that lightning of sufficiently high rate can cause the formation of pre-biological molecules. Russell [1991] also noted that a high lightning rate may be an issue for the safety of space probes, in addition to possible modifications of the atmospheric chemistry. The plasma wave data obtained by the Pioneer Venus Orbiter are the most extensive, and these data are hence suitable in determining lightning rates at Venus. It is therefore important to determine the likelihood that the wave bursts do correspond to lightning.

      In support of the lightning interpretation of the VLF/ELF bursts, Russell [1991] pointed out that there is other evidence for lightning on Venus. Optical measurements from the Venera 9 spacecraft [Krasnopol'sky, 1983], and the observation of impulsive electromagnetic signals by the Venera landers [Ksanfomality et al., 1983] both provide evidence for lightning within the Venus atmosphere. More recently, radio observations during the flyby of Venus by the Galileo spacecraft have also been interpreted as evidence for lightning [Gurnett et al., 1991]. The Galileo observations are probably the least controversial, since the data were acquired several planetary radii from Venus, in the solar wind. It is highly unlikely that plasma instabilities can generate impulsive signals around 1 MHz in the solar wind.

      On the other hand, there have also been negative results in the search for lightning at Venus, most notably in searches for optical signatures. Data from the VEGA balloons did not show any evidence for lightning. Borucki et al. [1991], using data from the star sensor on board the Pioneer Venus spacecraft, did not find any optical flashes whose rate exceeded the false alarm rate. However, both these searches tended to concentrate over the dawn local time sector, while it appears that lightning is mainly a dusk related phenomenon.

      Irrespective of the other evidence for lightning, there is still the issue of whether or not the plasma waves observed in the nightside ionosphere are due to lightning. In testing the lightning hypothesis we are investigating the plasma wave properties of a weakly magnetized and weakly collisional plasma which is quite different from the terrestrial ionosphere. In particular, as we will emphasize throughout this review, the energy density of the thermal plasma can be comparable to the magnetic field energy density. This has important implications for both wave propagation and also possible plasma instabilities.

      Our approach in this review is to compare and contrast the expected plasma wave signatures from the lightning hypothesis with the various plasma instabilities proposed as alternatives. We hope to demonstrate the strengths and weaknesses of the different hypotheses. It is not out intention to prove, nor do we expect to find, that all plasma waves observed in the nightside ionosphere of Venus are due to atmospheric lightning. Rather, our purpose is to assemble sufficient evidence to determine the most probable source for the majority of the plasma waves observed at Venus. Given the nature of the plasma wave data, this assessment must be based on statistics, rather than case studies. The question then becomes, given the various statistical properties of the waves, which hypothesis best explains the bulk (but not necessarily all) of the data?

      The structure of this review is as follows. In the next section we discuss the morphology of the wave bursts. We demonstrate why the ELF (100 Hz) waves are probably whistler-mode waves, and further that they entered the ionosphere from below, consistent with a lightning source. In the third section we discuss the likelihood that plasma instabilities can generate whistler-mode waves at low altitudes. Because of the relatively high electron thermal pressure, we show that whistler-mode waves tend to be damped, rather than driven unstable. In the fourth and fifth section we discuss the most recent alternative explanation for the 100 Hz waves, that they are associated with density fluctuations corresponding with short wavelength lower hybrid resonance waves driven unstable through a gradient drift instability. However, this instability requires very steep density gradients to produce a large enough drift to overcome the damping due to collisions. In the final section we will summarize the discussion presented in this review.

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