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.
The ionosphere of an unmagnetized planet, such as Venus, is characterized by relatively high Pedersen conductivity in comparison to the terrestrial ionosphere because of the weak magnetic field. Collisional Joule dissipation of plasma waves might therefore be an important source of heat within the Venus ionosphere. However, any assessment of the importance of collisional Joule dissipation must take into account the cooling provided by electron heat conduction due to temperature gradients. Once heat conduction is included we find that collisional Joule dissipation is significant only in the bottomside ionosphere; waves observed at or near the dayside ionopause, or at higher altitudes (> 150 km) within the nightside ionosphere do not cause significant heating through collisional Joule dissipation. However, lightning-generated whistler mode waves propagate through the highly collisional bottomside ionosphere, and we have performed detailed wave propagation calculations where we self-consistently calculate the heating due to Joule dissipation and the cooling due to heat conduction. The heat conduction always exceeds the collisional cooling from elastic collisions. Because the high collision frequency at low-altitude results in a low thermal conductivity, a steep temperature gradient is required to provide the heat flux. However, this gradient thermally decouples the bottomside ionosphere from higher altitudes. Collisional Joule dissipation of lightning generated whistlers is not likely to have any consequences for the global ionospheric energy budget. Cooling by inelastic collisions, specifically the vibrational excitation of CO, further reduces the bottomside temperature. It is the inelastic cooling rate that determines the atmospheric heating rate, any excess heat again being carried away through heat conduction. We find that for typical wave field amplitudes of 10 mV/m the bottomside is heated to a few eV, while intense fields (100 mV/m) result in bottomside temperatures of a few tens of eV. This high a temperature may cause electronic excitation of the neutrals, which could result in optical or ultraviolet emissions from the ionosphere due to lightning. This possibility requires further investigation but requires the incorporation of additional inelastic cooling processes, such as electronic excitation of the neutral atmosphere.
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