Remote Sensing of Global Lower Thermospheric Winds: Sensing Techniques and Sensor Design

In this chapter, we present the concept and the design of a high-sensitivity heterodyne spectrometer operating at terahertz (THz) frequency for global lower thermospheric neutral wind, temperature, and atomic oxygen density measurements from a low Earth orbit. The instrument, THz Limb Sounder (TLS), is aimed to provide, for the first time, global neutral wind/temperature/density profile measurements globally during day and night, with focus at altitudes of 100-180 km where most of the ion-neutral energy/momentum couplings take place. The instrument is an ambient-temperature, Schottky-diode-powered heterodyne subharmonic radiometer-spectrometer that extends the limb sounding technique to profile lower thermospheric density/temperature/wind by measuring the Doppler line shape of atomic oxygen (OI) fine structure emission (J = 0 -> J = 1) at 2.06 THz. This atomic oxygen line emission is one of the two fine structure emissions originating in the thermosphere and is very bright and distributed nearly uniformly globally (at all latitudes including high-latitude aurora particle precipitation regions) and temporally (at all local times during both day and night), thus ideal for thermospheric remote sensing. The TLS instrument design, measurement methodology, receiver performances, and the expected measurement capability are presented and discussed here.