Evidence for recombination as a significant source of metastable helium

A Fabry-Perot spectrometer optimized for maximum near infrared (NIR) transmission and featuring a germanium detector with a quantum efficiency of 86% at 10,830 Å is used to measure the brightness and spectral width of the metastable helium emission in the upper thermosphere. Observations are made at the optical facility associated with the Millstone Hill Incoherent Scatter Radar site in Massachusetts and at the optical facility associated with the Arecibo Observatory in Arecibo, Puerto Rico. Using a spectral resolution of 0.117 Å, these are the first observations to isolate the metastable helium triplet 2p³P_0,1,2-2s³S₁ from nearby OH airglow contamination. Measured 10,830 Å brightness decays with shadow in a manner consistent with an emission that is due to resonant scattering of the solar 10,830 Å emission. Although the technique we describe was conceived as a powerful tool to measure upper thermospheric and exospheric temperatures, the data indicate a surprising increase of temperature with shadow height to values much greater than commonly ascribed to the near solar minimum thermosphere. We assert that the broad 10.830Å spectral line widths, retrieved when the shadow heights are above ∼300 km, are due to a nonthermal metastable helium population. This source of hot, metastable helium is very likely He⁺ recombination in the twilight. That conclusion is contrary to an historical understanding that the dominant source of metastable helium is the impact of photoelectrons with energies in excess of 19.8 eV with ground state helium.