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

[1] 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.