Midlatitude sporadic E-layer structuring from neutral instability and mixing in the lower thermosphere

Observations of 30-MHz coherent backscatter from sporadic Eionization layers were obtained with a VHF imaging radar located inIthaca, New York. The volume probed by the radar lies at relativelyhigh magnetic latitudes, on the northern edge of the mid-latituderegion and underneath the ionospheric trough. Banded, quasi-periodic(QP) echoes observed from Ithaca are similar to those found in lowermidlatitude regions. The Doppler shifts observed are smaller and, sofar, do not appear to reach the threshold for Farley-Bunemaninstability. However, many of the echoes exhibit fine-scale structure,with secondary bands or braids oriented obliquely to the primarybands. Secondary bands have been seen only rarely at lower middlelatitudes. In previous observations, the QP scattering has been linkedto unstable neutral wind shears. Neutral wind shear commonly found inthe lower thermosphere could play a key role in the formation of theseirregularities and explain some morphological features of theresulting plasma density irregularities and the radar echoes. Weconsider whether neutral instability and turbulence in the lowerthermosphere is the likely cause for some of the structuring in thesporadic-E layers. Results of 3D numerical simulations of atmosphericdynamics in the mesosphere to lower thermosphere support theproposition, coupled to plasma dynamics representative of the ionospheric E region. In particular, we focus on Ekman-type instabilities that,like the more common Kelvin-Helmholtz instabilities, are inflectionpoint instabilities, although specifically associated with turningshears and result in convective rolls aligned close to the mean winddirection, with smaller-scale secondary waves aligned normal to theprimary structures.