Some results referring to the long-term change of ionospheric parameters

Abstract

Mid latitude spread-F has extensively studied recently as compared to the earlier periods. In the course of these investigations two concepts have appeared in the literature concerning the origin of the mid-latitude spread-F. The first concept based on radar measurements attributes the development of spread-F to plasma instabilities (Fukao et al.\ 1988), while according to the other concept based on the vertical incidence sounding of the ionosphere the occurrence of mid-latitude spread-F is related to travelling ionospheric disturbances (Bowman 1981). Therefore, it seemed worthy to study mid-latitude spread-F from the point of view of its origin. This study has been carried out by analysing the connection between the occurrence of spread-F and the occurrence of sporadic E layers, keeping in mind that the development of sporadic E layers at mid-latitude is due to atmospheric gravity waves, which initiate plasma instabilities. On the other hand, the relation of the occurrence of spread-F to enhancements of the auroral electrojet index (AE) has been studied indicating the occurrence of large scale travelling ionospheric disturbances.