Abstract
The transfer of energy from an ionizing photon to the atoms and molecules of the neutral gas in the F-region of the atmosphere is investigated. It is found that photoionization heating should be divided into two parts: (1) photoelectron heating associated with the slowing down of the fast photoelectrons formed by photoionization; and (2) reaction heating associated with the chemical reactions undergone by the ions formed in the photoionization process. The photoelectron heating will take place near the time and place of photoionization while the reaction heating will occur at the time and place of the ionic reactions. Photoelectron and reaction heating rates per unit column are computed for the daytime, and reaction heating rates per unit column are computed for the nighttime. It is concluded that: (1) chemical reactions at night lead to a small but significant amount of F-region heating; and (2) reaction heating during the day is nearly proportional to the cosine of the solar zenith angle except near sunrise and sunset.
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