Ground Based Height Profiling Technique to Resolve Altitude Variations of Neutral Winds in the Lower Thermosphere

Few techniques are available for measuring neutral winds at E-region altitudes ranging from 100 to 150 km. In-situ probes are typically unable to remain aloft at these heights for more than a few hours, and remote sensing is largely ineffective. The consequent scarcity of observations is a problem, because winds in this region are of scientific interest (due to the transition from atmospheric behavior to space-like behavior) and of operational interest (because of their impact on satellite orbits or radio signals). Further, strong altitude gradients of wind and temperature are known to occur at these heights, so any useful measurement techniques must account for these gradients and, ideally, resolve them. One approach is to record Doppler spectra of the oxygen 558 nm emission using a Fabry-Perot interferometer (FPI). However, within the aurora, the characteristic energy of electron precipitation can change by many keV on time scales of minutes or less. As a result, aurorally excited 558 nm emission heights may vary both across the sky and over time. This behavior coupled with altitude gradients in the wind field would violate the assumptions usually used to reconstruct wind vectors producing noisy and unphysical results. However, rather than disregarding these periods, it is possible to instead make use of the height variations to construct an altitude profile of wind. Using the prevailing F10.7 and Ap values for the night, the MSIS atmosphere model predicts the height profile of temperature. We can use this profile to assign a height to each 558 nm spectrum by comparing the fitted Doppler temperature to the MSIS model’s prediction. During times when these observations span a range of heights, it is possible to invert the line-of-sight wind components to create a height profile of the two-component horizontal vector wind. Examples of this process and a comparison with sounding rocket height profiles will be presented.