Azimuth‐dependent Es layer instability: A missing link found

Abstract

A number of features observed in association with nighttime midlatitude sporadic E (Es) have defied satisfactory explanation, until now. They include Es properties such as semitransparency, tilts, altitude modulation, plasma structure, and frontal structures with preferred alignment in azimuth. Other puzzling manifestations of Es include gigahertz scintillation and anomalously large electric fields (). All can now be interpreted self‐consistently in terms of a recently discovered, azimuth‐dependent Es layer instability [Cosgrove and Tsunoda, 2002b, 2003]. Zonal wind shear turns out to be responsible for both the initial formation of Es (wind shear theory) and its instability to vertical transport, the latter arising through an from Hall polarization. The meridional wind drives an important secondary polarization process that acts through conductivity perturbations to distort and produce a characteristic asymmetry in Es plasma structure, above and below the zonal wind shear node. Stability is restored through reduction in from the overlap of Es structures above and below the wind‐shear node; overlap is produced by antiparallel horizontal transport. Temporal evolution therefore is a cyclical process of Es layer distortion and structure development, followed by horizontal overlap and vertical reconvergence. In this paper we elucidate the underlying physics and discuss how hitherto puzzling observations now provide supporting evidence for this instability. We further endorse the findings that Hall polarization and the Es layer instability must be playing major roles in the coupled electrodynamics and instability of the nighttime midlatitude ionosphere [Tsunoda and Cosgrove, 2001; Cosgrove and Tsunoda, 2004; Cosgrove et al., 2004].