Generation of Metastable Helium and the 1083nm Emission in the Upper Thermosphere

Models of metastable helium, He(2^3S), production in the upper thermosphere and lower exosphere over Arecibo show that creation by recombination of He^+ can be non-negligible relative to the photoelectron impact on He(1^1S) source. Due to large ground-state He abundance in the winter, and to photoelectrons from an illuminated conjugate thermosphere, the strongest 10830Å intensities (arising from He(2^3S) solar resonance) occur during the winter. The contribution to the 10830Å airglow brightness from He^+ recombination reaches more than 10% in the morning twilight when He^+ peak concentrations are more than ∼30% of the topside composition, and He^+ recombination becomes increasingly dominant for solar zenith angles greater than 100°. Measurements of the topside ionosphere at Arecibo have shown that He^+ layer concentrations in the winter and near the equinoxes are often as high as 50% and significant He^+ concentrations can persist throughout the night. A hot metastable component from recombination renders ambiguous interpretation of the 10830Å spectral profile in terms of exospheric temperature. The presence of such a population may explain reported observations of 10830Å line widths that increase with shadow height, implying twilight temperatures much hotter than those expected of a thermalized neutral population. Modeling of 10830Å line profiles comprised of both thermal and nonthermal He(2^3S) components is investigated to assess the role of He^+ recombination in the generation of metastable He and the implications for the derivation of neutral temperature in the upper thermosphere.