Fabry Perot observations of helium 10830 Å emission at Millstone Hill

The He(23S) triplet at 10830 Å is observed using a Fabry‐Perot interferometer optimized for the near infrared. These observations, made at the zenith at the Millstone Hill Optical Facility during evening twilight, are the first to resolve the individual members of the helium triplet from the surrounding OH(5‐2) lines. The introduction of an aeronomically unique germanium detector Noto et al., [1993] was crucial to this achievement. The instrument is configured with a spectral resolution of 1/2 the helium line width at 1000°K, assuring that the line widths accurately reflect the velocity distribution of metastable helium. Although the intensities observed are within a factor of two of helium 10830 Å intensities observed by [Tinsley, 1968, Tinsley & Christensen et. al, 1971], the observed temperatures are different from the expected behavior of a neutral species in the upper‐thermosphere during twilight.