Excitation of atmospheric helium

Abstract The possible excitation mechanisms for triplet metastable helium in the upper atmosphere are considered in some detail. It is concluded that excitation by photoelectron impact is the most likely production mechanism. The results obtained are in satisfactory agreement with twilight observation at λ 10,830. Although electron recombination with helium ions is found to give negligible contribution to λ 10,830 emission, the expected variation in the abundance of helium ions through the solar cycle is examined. It is suggested that the atmosphere at solar minimum may contain appreciably less helium ions than it does at solar maximum. Attention is directed to the possible effects of the long mean free paths of 20 eV electrons produced above about 200km. The present calculations, which do not include non-local effects, refer to low magnetic latitudes where electrons are constrained vertically by the horizontal field lines. Observations at different latitudes would be desirable and might provide valuable information on the importance of non-local effects. The diurnal variation of 10,830 A emission is considered in some detail. It is concluded that little dayglow enhancement over twilight is to be expected. Indeed at solar minimum the noontime intensity may well be appreciably less than the twilight value.

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