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

Abstract

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.

Wavelike Oscillations in High Latitude Thermospheric Doppler Temperature and Line‐Of‐Sight Wind Observed Using All‐Sky Imaging Fabry‐Perot Spectrometers

“…Thus, although oscillations were seen in E‐region temperatures, we presume that these mostly would have been due to changes in the aurora rather than the actual fluctuations in the background temperature at a constant height. In the case of 558 nm LOS wind measurements, the vertical gradients of horizontal wind in the E‐region can also be strong (Branning et al., 2022; Larsen, 2002), which means that height variations of the emission layer may add artifacts to the high pass filtered LOS wind time series that could, at times, dominate over other signals, such as those that might indicate the presence of atmospheric waves. Periods when such artifacts appear can be easily identified as “bursts” of noise in the wind time series correlated with periods when the 558 nm Doppler temperature is changing rapidly.…”

High Spatial and Temporal Resolution Studies of Thermospheric Vector Wind Fields Derived From a Ground‐Based Network of All‐Sky Fabry‐Perot Interferometers

“…However, such artifacts only arise at times when there is substantial altitude variation across the instrument array's field of regard. Many years of Fabry‐Perot observations from Alaska indicate that such time periods are relatively limited and most commonly encountered during a brief window spanning several hours in the pre‐midnight sector (Branning et al., 2022).…”

“…We have assumed the spectral emissions we observe originate from a height of 120 km for 558 nm, and 240 km for 630 nm. The centroid altitude for both emissions is known to vary as a consequence of varying characteristic energy of the auroral precipitation (Branning et al., 2022; R. Sica et al., 1986). This height variation is not important for the 630 nm emission because vertical gradients in the wind field are expected to be small at F‐region heights.…”

4D-Imaging of Earth’s Space-Atmosphere Interaction Region

“…Unfortunately, there are few techniques that can provide routine monitoring of height profiles for winds in this region. However, Branning et al [2022] described a new technique, in which the naturally occurring height variation of the aurorally excited optical emission from atomic oxygen at 558 nm can be used to essentially "paint in" the Doppler shift information for a range of altitudes. Under appropriate conditions, this information can be used infer the height profile of the background wind field.…”