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Hot oxygen and the ion energy budget

Several authors have deduced the product of the neutral atomic oxygen number density [O] and the O + -O collision cross section Q O+-O by combining measurements of O + with a theory of O + -O interaction. These have been interpreted variously in terms of [O] by choosing a value for Q O+-O or in terms of Q O+-O by choosing a value for [O]. A distinct difference exists between the values of Q O+-O deduced from energy balance and momentum balance analyses. Explanations for this discrepancy may lie in an error in or omission of a term in either the energy balance or momentum balance relations. We examine here the possibility that a reservoir of hot (∼4000°K) neutral atomic oxygen at the top of the thermosphere may form a missing heat source for the energy balance analysis. We find that this hot O can account for the discrepancy between the energy balance and momentum balance determinations of Q O+-O if hot O accounts for 1-2% of the O at 400 km altitude near solar minimum and smaller amounts at solar maximum. This pattern may be related to the drop in the altitude of the exobase to around 400 km near solar minimum. In such cases, hot O may vie with the electrons as the main heat source for the ions.