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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6.       Conclusions

      Through order of magnitude estimates of the relative importance of the different heating and cooling rates we find that collisional Joule dissipation of plasma waves is likely to be important only at low altitudes in the ionosphere of Venus. This conclusion arises from the inclusion of electron heat conduction in the heat budget equation. Except at the lowest altitudes, the heat flux associated with relatively small temperature gradients is sufficient to match the heating from Joule dissipation.

      Near the dayside ionopause, temperature gradient scales > 1000 km can provide sufficient heat conduction to offset the Joule dissipation. Waves are mainly observed above the ionopause, where ambient plasma densities are of the order 100 cm [Crawford et al., 1993], and the scale lengths can be much longer, several planetary radii. At high altitudes in the nightside ( 150 km), temperature gradient scale lengths > 10 km are sufficient for heat conduction to balance Joule dissipation. Even longer scale lengths (> 50 km) are sufficient in the reduced density regions known as ionospheric holes, where the waves are usually detected.

      Determining the relative significance of Joule dissipation in the bottomside ionosphere requires detailed wave propagation calculations, because the heating caused by Joule dissipation is a consequence of the attenuation of the wave fields. We have performed wave propagation calculations using the scheme of Huba and Rowland [1993], modified to iteratively recalculate the temperature profile until the total heating rate is zero.

      During the Pioneer Venus entry phase the OEFD measured 100 Hz waves around 130 km altitude [Strangeway et al., 1993b]. The waves decreased in amplitude with a scale height of the order 1 km, and with a peak amplitude of between 10 and 10 V m Hz, which corresponds to an electric field amplitude of a few tens of millivolts per meter assuming a bandwidth of 100 Hz. Thus the calculations presented here are consistent with the low altitude entry phase observations, and we might expect bottomside electron temperatures to be elevated to a few tens of eV for the most intense waves. As such, Joule heating by the most intense waves could possibly result in optical or ultraviolet emissions, or even enhanced ionization, which may in turn provide additional evidence for lightning on Venus.

      However, while electron heating may be occurring, the high collision frequencies thermally decouple the bottomside ionosphere from higher altitudes, and we do not expect lightning generated heating to have any catastrophic consequences for the global energy budget of the Venus ionosphere and atmosphere. In particular, it is not the Joule dissipation rate, but the inelastic collision cooling rate that determines the amount of heat entering the neutral atmosphere. Electron heat conduction carries away any excess heat that cannot be absorbed by the neutral atmosphere. Since the inelastic cooling rate, which we have modeled by vibrational excitation of CO, is only weakly dependent on temperature above 0.2 eV [Morrison and Greene, 1978], the cooling rate is approximately independent of the amount of Joule dissipation, and we find electron cooling rates, and hence neutral atmosphere heating rates, of the order 10 W/m for typical wave field amplitudes. This rate appears to be well within the bounds of heating rates which can be accommodated by the neutral atmosphere.

Acknowledgments.

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Text and figures by R.J. Strangeway
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Last modified: Feb. 10,1996