Collisional Joule dissipation in the ionosphere of Venus: The importance of electron heat conduction

J. Geophys. Res., 101, 2279-2295, 1996
(received March 20, 1995; revised August 18, 1995; accepted August 21, 1995.)
Copyright 1996 by the American Geophysical Union.
Paper number 95JA02587.

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1.       Introduction

      Recently, Cole and Hoegy [1995] suggested that collisional Joule dissipation of VLF waves is a significant source of heating within the ionosphere of Venus. Depending on the amount of collisional Joule dissipation, this could have important ramifications for the Venus ionosphere. For sufficiently high Joule dissipation the heating due to VLF waves would have to be included in the energy budget of the ionosphere. Even more significantly, if the heating rate is large enough to cause unrealistically high electron temperatures, then some of the earlier interpretations of the plasma waves observed at Venus may have to be revised, especially the identification of VLF bursts in the nightside ionosphere as being due to lightning-generated plasma waves.

      There are two basic VLF wave phenomena observed within or near the Venus ionosphere for which collisional Joule dissipation may be important. The first of these is observed at the dayside ionopause, mainly in the 100-Hz channel of the Pioneer Venus Orbiter electric field detector (OEFD). The OEFD measures plasma waves at four frequencies, 100 Hz, 730 Hz, 5.4 kHz, and 30 kHz [Scarf et al., 1980c]. The 100-Hz waves observed at the dayside ionopause were initially identified as whistler mode waves generated at the bow shock, propagating through the magnetosheath to the ionopause [e.g., Scarf et al., 1980b], and it was argued that the waves were transporting energy, providing heat to the dayside ionopause though Landau damping.

      More recently, Szego et al. [1991] suggested that the waves were lower hybrid resonance waves generated by the relative drift between O ions of planetary origin and the shocked solar wind. As the waves are generated locally, Landau damping will act to inhibit wave growth, rather than provide a means for energy transport. Strangeway and Crawford [1993] pointed out that Szego et al. [1991] neglected Landau damping, and the damping of lower hybrid waves argues for an alternative wave mode and instability. Huba [1993] suggested that the waves were instead an ion acoustic mode. Thus the mode identification of the waves observed at the dayside ionopause has not been resolved. Nevertheless, we should determine if collisional Joule dissipation is important for these waves, as that may provide an alternative means for transferring wave energy to the plasma, in addition to the dissipation associated with resonant wave-particle interactions.

      In the opening paragraph we already alluded to the second wave phenomenon which may be subject to collisional Joule dissipation: the observation of VLF bursts at low altitudes in the nightside ionosphere. These waves have been attributed to lightning in the Venus atmosphere [e.g., Scarf et al., 1980a; Scarf and Russell, 1983; Russell, 1991]. If these waves are indeed due to lightning, then they must propagate through the most dense, and most highly collisional, region of the ionosphere. It is consequently very important that we determine how large the Joule dissipation for these waves may be. In particular, is the heating rate so large that the ionosphere cannot adequately absorb the energy? In which case, we might expect there to be detectable changes in the ionosphere due to lightning. We note that there have been no reports of lightning-related signatures within the ionosphere of Venus, apart from the VLF bursts. Small-scale irregularities were reported by Grebowsky et al. [1991], who interpreted the irregularities as being due to a local instability. This instability was identified as a lower hybrid drift instability [Huba, 1992; Huba and Grebowsky, 1993]. Although initially suggested as an alternative to the lightning interpretation, the lower hybrid drift instability does not explain the majority of the VLF bursts [Strangeway, 1995a, b]. If we find that collisional Joule dissipation is significant for the nightside bursts, then perhaps we should revisit the issue of correlation between wave bursts and density fluctuations, as this may explain the 20% correlation between waves and density fluctuations found by Strangeway [1995b].

      Within the nightside ionosphere two types of wave burst have been attributed to lightning. The first of these occurs at 100 Hz [Scarf et al., 1980a], the second is broad-banded in nature [Singh and Russell, 1986], being detected in the higher-frequency channels of the OEFD. Later work [Russell et al., 1988, 1989a], which did not contain telemetry errors apparently included in the analysis of Singh and Russell [Taylor and Cloutier, 1988; Russell and Singh, 1989] showed that the wideband bursts peaked at low altitudes and that the burst rate was maximum near the dusk terminator. It was consequently argued that lightning at Venus was generated mainly in the dusk local time sector [Russell, 1991]. We will not address the Joule dissipation of these wideband signals in this paper, mainly because the source of these high-frequency signals is not understood. They are observed in the propagation stop band between the electron gyrofrequency and the plasma frequency and so cannot be radiation from below the ionosphere [Strangeway, 1991a].

      On the other hand, there is evidence that the 100-Hz bursts are propagating though the ionosphere from below. The wave burst rates are largest at low altitudes [Russell et al., 1988; Ho et al., 1991]. The waves are polarized perpendicularly to the ambient magnetic field, provided the data are restricted to signals that are within the whistler mode resonance cone under the assumption of vertical propagation [Sonwalkar et al., 1991, Strangeway, 1991b]. Furthermore, the burst rate decreases much less rapidly for increasing altitude if the data are restricted to within the resonance cone [Ho et al, 1992]. The resonance cone test, which only applies to waves that are propagating from below the ionosphere, is a strong indicator of a subionospheric source such as atmospheric lightning for the 100-Hz waves. Waves that propagate through the ionosphere are likely to be subject to collisional Joule dissipation.

      It is the purpose of this paper to determine whether or not collisional Joule dissipation is important for plasma waves observed within the Venus ionosphere. In the next section we discuss the ionospheric heat budget, giving expressions for the different terms within the heat budget and the associated collision frequencies. In section 3 we compare orders of magnitude for the heating and cooling terms. We find that electron heat conduction can easily accommodate the heating due to collisional Joule dissipation, except at very low altitudes within the ionosphere. Thus waves observed near the dayside ionopause of Venus or at higher altitudes (> 150 km) in the nightside ionosphere do not supply any significant heating through collisional Joule dissipation. This finding does not preclude additional dissipation due to resonant wave-particle interactions, but a detailed estimation of this form of dissipation is beyond the scope of the present paper. In section 4 we perform detailed wave propagation calculations, following the methodology of Huba and Rowland [1993] but with the added step of iteratively modifying the electron temperature altitude profile so as to balance the heat budget. In section 4 we artificially set the inelastic cooling rate through vibrational excitation of CO equal to zero. This facilitates the demonstration of how the electron temperature profile changes to accommodate the heating by Joule dissipation through heat conduction to higher altitudes. In section 5 we allow vibrational cooling to occur, and we find that this further reduces the electron heating. In addition, we point out that it is the inelastic collision cooling rate that determines how much heat enters the neutral atmosphere, not the Joule dissipation rate. Any excess heat is carried away through heat conduction. Last, we give some concluding remarks in section 6.

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Text and figures by R.J. Strangeway
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