Generation of metastable helium and the 1083 nm emission in the upper thermosphere

Abstract

Models of metastable helium, He(23S), production in the upper thermosphere and lower exosphere show that creation by recombination of He+ may be nonnegligible relative to the photoelectron impact on He(11S) source. Owing to large ground state He abundance in the winter and to photoelectrons from an illuminated conjugate thermosphere, the strongest 1083 nm intensities (arising from He(23S) solar resonance) occur during the winter at Arecibo. During late spring and early fall, however, recombination of the nighttime He+ layer begins to compete as a source of metastable He, and the contribution to the early twilight 1083 nm airglow brightness from recombinant He(23S) reaches more than 10%, regardless of solar activity levels, and becomes increasingly dominant for solar zenith angles greater than 105°. A hot metastable component from recombination potentially renders ambiguous interpretation of the 1083 nm spectral profile in terms of exospheric temperature. The presence of such a population may explain reported observations of 1083 nm line widths that increase with shadow height, implying twilight temperatures much hotter than those expected of a thermalized neutral population.