COLLISIONAL JOULE DISSIPATION IN THE IONOSPHERE OF VENUS - THE IMPORTANCE OF ELECTRON HEAT-CONDUCTION

The ionosphere of an unmagnetized planet, such as Venus, is characteri zed by relatively high Pedersen conductivity in comparison to the terr estrial ionosphere because of the weak magnetic field. Collisional Jou le dissipation of plasma waves might therefore be an important source of heat within the Venus ionosphere. However, any assessment of the im portance of collisional Joule dissipation must take into account the c ooling provided by electron heat conduction due to temperature gradien ts. Once heat conduction is included we find that collisional Joule di ssipation is significant only in the bottomside ionosphere; waves obse rved at or near the dayside ionopause, or at higher altitudes (> 150 k m) within the nightside ionosphere do not cause significant heating th rough collisional Joule dissipation. However, lightning-generated whis tler mode waves propagate through the highly collisional bottomside io nosphere, and we have performed detailed wave propagation calculations where we self-consistently calculate the heating due to Joule dissipa tion and the cooling due to heat conduction. The heat conduction alway s exceeds the collisional cooling from elastic collisions. Because the high collision frequency at low-altitude results in a low thermal con ductivity, a steep temperature gradient is required to provide the hea t flux. However, this gradient thermally decouples the bottomside iono sphere from higher altitudes. Collisional Joule dissipation of lightni ng generated whistlers is not likely to have any consequences for the global ionospheric energy budget. Cooling by inelastic collisions, spe cifically the vibrational excitation of CO2, further reduces the botto mside temperature. It is the inelastic cooling rate that determines th e atmospheric heating rate, any excess heat again being carried away t hrough 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. Thi s high a temperature may cause electronic excitation of the neutrals, which could result in optical or ultraviolet emissions from the ionosp here due to lightning. This possibility requires further investigation but requires the incorporation of additional inelastic cooling proces ses, such as electronic excitation of the neutral atmosphere.