*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.

**Next:**
Appendix

**Previous:**
5. Cooling Through Inelastic Collisions

**Top:**
Title and Abstract

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.

**Next:**
Appendix

**Previous:**
5. Cooling Through Inelastic Collisions

**Top:**
Title and Abstract

Text and figures by R.J. Strangeway

Converted to HTML by Chris Casler

Last modified: Feb. 10,1996