Measurements of metastable helium in Earth's atmosphere by resonance lidar.

Monitoring and predicting space weather activity is increasingly important given society's growing reliance on space-based infrastructure but is hampered by a lack of observational data. Airglow at 1083 nm from metastable helium He(2S) in the thermosphere has long been a target for remote-sensing instruments seeking to fill that gap; however, passive measurements of He(2S) fluorescence are limited by low brightness, and interpretation of these observations is complicated by the > 500 km depth of the He(2S) layer. Here, we demonstrate a lidar instrument that is able to stimulate and detect He(2S) fluorescence, and we present measured profiles of He(2S) density. These measurements provide crucial validation to space weather models, support predictions of peak number density ( ~ 1 cm) and the dependence of density on altitude, solar zenith angle, and season, and extend by a factor of 4 the maximum probed altitude range by an atmospheric profiling lidar. These measurements open the door for the development of more sophisticated lidars: by applying well-established spectroscopic lidar techniques, one can measure the Doppler shift and broadening of the He(2S) line, thereby retrieving profiles of neutral wind speed and temperature, opening a window for studying space weather phenomena.