Observations and Fully Coupled Simulations of Nighttime Ionospheric Irregularities in the Subauroral E Region

Observations and theory of plasma density irregularities in subauroral sporadic E layers are presented. The irregularities produce so‐called quasiperiodic (QP) coherent scatter radar echoes, and imagery of the echoes reveals large‐scale structuring which normally takes the form of bands of backscatter elongated from northwest to southeast and propagating to the southwest in the northern hemisphere. We associate the large‐scale structuring with neutral dynamics in general and Ekman‐type instability in particular. This is an inflection point instability, a generalization of Kelvin‐Helmholtz instability in turning shears, the same shears that form sporadic E ionization layers in the first place. A key parameter for the instability is , the asymptotic direction of the neutral winds at the top of the shear spiral relative to magnetic east. The parameter controls the alignment of the rolls produced by inflection point instability. Numerical simulations involving fully coupled neutral and ionospheric dynamics reproduce many of the characteristics of the QP echoes. We argue that only angles within the first two quadrants are consistent with sporadic E layer formation, and only angles in the second quadrant are consistent with the production of strong field‐aligned currents which may be necessary for small‐scale field‐aligned plasma density irregularity creation. This may explain the alignment of QP‐echo striations observed by imaging radar.