Irregularities in the low- and mid-latitude E region:
A historical perspective

Roland T. Tsunoda
SRI International, Menlo Park, California, USA

Recent investigations of E region irregularities in the low- and mid-latitude E region have focused on the electrodynamics associated with larger-scale structures, in particular, wind shear and sporadic E in the nighttime ionosphere. Small-scale irregularities responsible for radar backscatter appear explainable in terms of the gradient-drift instability, operating locally without effects of field line mapping. A brief review will be presented that outlines the historical flow of research, the findings, and the status of our understanding. In particular, earlier findings are revisited to determine whether interpretation, at the time of discovery, remains reasonable, or whether a new interpretation emerges when viewed in light of current thinking. Topics of interest include Hall polarization, a sporadic-E layer instability, the effects of electrical coupling between the E and F regions, and neutral dynamics and the Kelvin-Helmholtz instability.