Resolving Vertical Variations of Horizontal Neutral Winds in Earth's High Latitude Space‐Atmosphere Interaction Region (SAIR)

• Published in: Journal of geophysical research. Space physics 2022-05, Vol.127 (5), p.e2021JA029805-n/a
• Main Authors: Branning, Kylee, Conde, Mark, Larsen, Miguel, Troyer, Riley
• Format: Article
• Language: English
• Subjects: Altitude; Atmospheric Composition and Structure; Atmospheric Processes; Atomic oxygen; Aurorae and Airglow; Auroral zone; Auroral zones; Cusp; Doppler effect; Doppler spectroscopy; Earth; Emissions; Functions (mathematics); geomagnetic cusp footprint; height profiling; Height variations; Ionosphere and Upper Atmosphere; Lower bounds; Magnetospheric Physics; Modelling; Oxygen; Planetary Sciences: Solid Surface Planets; Polar cusps; Polar Regions; Polynomials; Remote Sensing; Sky; Sounding rockets; space weather; space‐atmosphere interaction region; Spectroscopy; Thermosphere; Thermosphere: Energy Deposition; Thermospheric Dynamics; thermospheric neutral wind; Tracer techniques; Tracers; Wind; Wind measurement; Wind profiles
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2021JA029805

Few remote sensing or in‐situ techniques can measure winds in Earth's thermosphere between altitudes of 120 and 200 km. One possible approach within this region uses Doppler spectroscopy of the optical emission from atomic oxygen at 558 nm, although historical approaches have been hindered in the auroral zone because the emission altitude varies dramatically, both across the sky and over time, as a result of changing characteristic energy of auroral precipitation. Thus, a new approach is presented that instead uses this variation as an advantage, to resolve height profiles of the horizontal wind. Emission heights are estimated using the Doppler temperature derived from the 558 nm emission. During periods when the resulting estimates span a wide enough height interval, it is possible to use low order polynomial functions of altitude to model the Doppler shifts observed across the sky and over time, and thus reconstruct height profiles of the horizontal wind components. The technique introduced here is shown to work well provided there are no strong horizontal gradients in the wind field. Conditions satisfying these caveats do occur frequently and the resulting wind profiles validate well when compared to absolute in‐situ wind measurements from a rocket‐borne chemical release. While both the optical and chemical tracer techniques agreed with each other, they did not agree with the HWM‐14 horizontal wind model. Applying this technique to wind measurements near the geomagnetic cusp footprint indicated that cusp‐region forcing did not penetrate to atmospheric heights of 240 km or lower. Plain Language Summary We present a fundamentally new capability for measuring height variations in horizontal winds for altitudes between 100 and 150 km. It is difficult to measure these winds because space‐based platforms are unable to maintain such low orbits, whereas ground‐based remote sensing depends on an optical emission that behaves in ways that are difficult to account for. In particular, there are times when the emission height varies dramatically which, historically, made derivation of horizontal winds intractable. Here, we instead use this variation as a means to resolve how the wind changes with height. Not only does this allow height profiling, but we can now also make use of previously rejected data periods. Our results were validated by comparing to absolute wind measurements from a chemical tracer released by a sounding rocket. Comparing these profiles to the m