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(2 3 S) in the thermosphere has long been a target for remote-sensing instruments seeking to fill that gap; however, passive measurements of He(2 3 S) fluorescence are limited by low brightness, and interpretation of these observations is complicated by the > 500 km depth of the He(2 3 S) layer. Here, we demonstrate a lidar instrument that is able to stimulate and detect He(2 3 S) fluorescence, and we present measured profiles of He(2 3 S) density. These measurements provide crucial validation to space weather models, support predictions of peak number density ( ~ 1 cm -3 ) 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(2 3 S) line, thereby retrieving profiles of neutral wind speed and temperature, opening a window for studying space weather phenomena.