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Abstract Observations of coherent scatter from patchy sporadicElayers in the subauroral zone made with a 30‐MHz coherent scatter radar imager are presented. The quasiperiodic (QP) echoes are similar to what has been observed at middle latitudes but with some differences. The echoes arise from bands of scatterers aligned mainly northwest to southeast and propagating to the southwest. A notable difference from observations at middle latitudes is the appearance of secondary irregularities or braids oriented obliquely to the primary bands and propagating mainly northward along them. We present a spectral simulation of the patchy layers that describes neutral atmospheric dynamics with the incompressible Navier Stokes equations and plasma dynamics with an extended MHD model. The simulation is initialized with turning shears in the form of an Ekman spiral. Ekman‐type instability deforms the sporadicElayer through compressible and incompressible motion. The layer ultimately exhibits both the QP bands and the braids, consequences mainly of primary and secondary neutral dynamic instability. Vorticity due to dynamic instability is an important source of structuring in the sporadicElayer. 

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. 

Abstract The Pedersen component of the Lorentz force produces an acceleration that is generally in the zonal direction in much of the dawn and dusk sectors in the auroral oval. During geomagnetically disturbed conditions, as the neutral flow begins to accelerate through the ion drag force and the flow speeds increase, a balance develops in the meridional direction between the Coriolis, curvature, and pressure gradient forces, which are dominant in the lower thermosphere. The gradient wind equation that describes this balance predicts that the cyclonic flow on the dawn side is limited to the so‐called regular solution, which has a maximum value of twice the geostrophic wind speed. The anticyclonic flow on the dusk side, on the other hand, can satisfy either the regular or anomalous solution with a transition at twice the geostrophic wind speed. The anomalous flow solutions have wind speeds significantly greater than the transition value, but are limited by the inertial wind value, that is, the value that corresponds to a balance between the curvature and Coriolis forces. The analysis is carried out to show this result, which indicates that a significant quantitative asymmetry is expected between the dawn‐ and dusk‐side flow, as is observed and has been shown in both observations and a number of numerical modeling studies. Implications for the wind distribution of perturbed pressure gradients and inertial instability are discussed. 

Abstract Observations of backscatter from field‐aligned plasma density irregularities in sporadicE(E_s) layers made with a 30‐MHz coherent scatter radar imager in Ithaca, New York are presented and analyzed. The volume probed by the radar lies at approximately 54° geomagnetic latitude, under the midlatitude trough and at the extreme northern edge of the zone whereE_slayers are prevalent. Nonetheless, the irregularities exhibit many of the characteristics of quasiperiodic echoes observed commonly at lower middle latitudes. These include a tendency to occur in elongated bands stretching from the northwest to southeast in the Northern hemisphere separated by tens of kilometers and propagating to the southwest. In addition, the irregularities were found to exhibit finer‐scale structures with secondary bands oriented nearly normally to the primary bands. We investigate the proposition that the primary bands are telltale ofE_s‐layer structuring caused by neutral Kelvin Helmholtz (KH) instability in the lower thermosphere and that the secondary bands signify secondary KH instability. Results from a 3D numerical simulation of KH support this proposition. 

We quantify the short-term (<30 day) variability of column O/N 2 measured by GOLD from January 2019 to August 2022 for various geomagnetic activity conditions. We find enhanced variabilities at high latitudes during active (Kp ≥ 3.0) times and weak but statistically significant variabilities at low latitudes. For active times, the largest absolute variability of O/N 2 ratio is 0.14 and the largest relative variability is 20.6% at ∼60.0°N in Fall, which are about twice those of quiet times. The variability at higher latitudes can be larger than that of lower latitudes by a factor of 5–8. We further quantify contributions of magnetospheric forcing to O/N 2 variability in the Ionosphere-Thermosphere region by correlating O/N 2 perturbations with Dst. During geomagnetic active times, positive correlations as large as +0.66 and negative correlations as large as −0.65 are found at high and low latitudes, respectively, indicative of storm-induced O and N 2 upwelling at high latitudes and down welling at low latitudes. During quiet times, correlations between O/N 2 perturbations and Dst become insignificant at all latitudes, implying a more substantial contribution from below. O/N 2 variabilities maximize in Fall and decrease towards Summer, while correlations maximize in Spring/Summer and decrease in Winter/Spring, which may be related to seasonal variations of geomagnetic activity and mean circulation.