A Tutorial Review on Sporadic E Layers

Abstract

The sporadic E layers (Es) form in the dynamo region of the ionosphere when metallic ions of meteoric origin are converged vertically in a wind shear. This paper provides a comprehensive update on sporadic E, a topic that has been studied for many years. The aim is to render useful information and physical understanding for both the general and specialized reader, and construct an integrated picture of sporadic E through a critical synthesis of recent findings. The basic aspects of the layer windshear theory are reviewed and then selected observations are presented which are tested against the theoretical predictions. The emphasis is placed on the tidal wind control of the diurnal and semidiurnal variability and altitude descent of sporadic E layers. There is now enough evidence to suggest that mid- and low-latitude sporadic E is not as “sporadic” as the name implies but a regularly occurring ionospheric phenomenon. This suggests that E layer physics could also be incorporated in existing atmosphere-ionosphere coupling models. Furthermore, the present review summarizes recent findings which provide physical insight into long-going problems and questions about the seasonal dependence and the global occurrence of Es. The experimental results, which are in favor of the windshear theory, imply that the key agents controlling sporadic E are: tidal wind atmospheric dynamics, the Earth’s horizontal magnetic field component, and the meteoric deposition of metallic material in the lower thermosphere.