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A theory of photoelectron transport that includes both elastic and inelastic collisional processes has been developed to describe the behavior of photoelectrons in the upper atmosphere. Explicit solutions to the transport equations are obtained for a sunlit exponential atmosphere in which local production occurs, and for a predawn exponential atmosphere in which an influx of photoelectrons arrives from a magnetically conjugate region. The results indicate that elastic scattering significantly reduces the rate of photoelectron escape from a sunlit atmosphere while maintaining the up/down isotropy to higher altitudes than would be possible if elastic scattering did not occur. With regard to a predawn atmosphere, a substantial fraction (∼50%) of incoming photoelectrons are backscattered by means of elastic collisions. To illustrate a more realistic situation, the behavior of 28-ev photoelectrons in 750° and 1500°K model atmospheres is also discussed.
Banks et al. (1970) studied this question.