Measurements of Metastable Helium in the Thermosphere by Resonance Lidar

Abstract

Fluorescence lidars probing the absorption lines of metallic atoms have been used to measure temperature and wind in the mesosphere/lower thermosphere region for the last four decades. Extending the fluorescence lidar technique into the middle and upper thermosphere requires targeting metastable helium because the number density of metallic species decreases rapidly with altitude. The helium lidar was first proposed in 1997 by Gerrard et al., but until recently the realization has failed due to technical challenges. At the German Aerospace Center (DLR) we built a pathfinder experiment and obtained profiles of metastable helium density by lidar up to an altitude of 700 km in the first year of operation, and the latest results suggest that the metastable helium layer extends even higher. We present an overview of the density measurements we have obtained so far and report on our attempts to retrieve temperature and wind from measurements of the Doppler broadening and Doppler shift of the helium absorption line. We also discuss the scientific potential of future lidar systems for space weather monitoring based on the anticipated improvements to our lidar system and scaling of transmitter power.