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Abstract Observations of 30‐MHz coherent backscatter from sporadic‐Eionization layers were obtained with a VHF imaging radar located in Ithaca, New York. The volume probed by the radar lies at relatively high magnetic latitudes, on the northern edge of the mid‐latitude region and underneath the ionospheric trough. Banded, quasi‐periodic (QP) echoes observed from Ithaca are similar to those found in lower midlatitude regions. The Doppler shifts observed are smaller and, so far, do not appear to reach the threshold for Farley‐Buneman instability. However, many of the echoes exhibit fine‐scale structure, with secondary bands or braids oriented obliquely to the primary bands. Secondary bands have been seen only rarely at lower middle latitudes. In previous observations, the QP scattering has been linked to unstable neutral wind shears. Neutral wind shear commonly found in the lower thermosphere could play a key role in the formation of these irregularities and explain some morphological features of the resulting plasma density irregularities and the radar echoes. We consider whether neutral instability and turbulence in the lower thermosphere is the likely cause for some of the structuring in the sporadic‐Elayers. Results of 3D numerical simulations of atmospheric dynamics in the mesosphere to lower thermosphere support the proposition. In particular, we focus on Ekman‐type instabilities that, like the more common Kelvin‐Helmholtz instabilities, are inflection point instabilities, although specifically associated with turning shears, and result in convective rolls aligned close to the mean wind direction, with smaller‐scale secondary waves aligned normal to the primary structures. 

Abstract Though the Kelvin‐Helmholtz instability (KHI) has been extensively observed in the mesosphere, where breaking gravity waves produce the conditions required for instability, little has been done to describe quantitatively this phenomenon in detail in the mesopause and lower thermosphere, which are associated with the long‐lived shears at the base of this statically stable region. Using trimethylaluminum (TMA) released from two sounding rockets launched on 26 January 2018, from Poker Flat Research Range in Alaska, the KHI was observed in great detail above 100 km. Two sets of rocket measurements, made 30 min apart, show strong winds (predominantly meridional and up to 150 ms⁻¹) and large total shears (90 ms⁻¹ km⁻¹). The geomagnetic activity was low in the hours before the launches, confirming that the enhanced shears that triggered the KHI are not a result of the E‐region auroral jets. The four‐dimensional (three‐dimensional plus time) estimation of KHI billow features resulted in a wavelength, eddy diameter, and vertical length scale of 9.8, 5.2, and 3.8 km, respectively, centered at 102‐km altitude. The vertical and horizontal root‐mean‐square velocities measured 29.2 and 42.5 ms⁻¹, respectively. Although the wind structure persisted, the KHI structure changed significantly with time over the interval separating the two launches, being present only in the first launch. The rapid dispersal of the TMA cloud in the instability region was evidence of enhanced turbulent mixing. The analysis of the Reynolds and Froude numbers (Re = 7.2 × 10³andFr = 0.29, respectively) illustrates the presence of turbulence and weak stratification of the flow.