Outstanding problems in the equatorial ionosphere-thermosphere electrodynamics relevant to spread F

Dynamic coupling of the atmospheric regions involving upward energy transport coupled with the locally active thermal tidal modes establish the wind system of the thermosphere whose interaction with the magnetized conducting ionospheric layers produces the dynamo electric fields and currents that control the quiet time electrodynamic processes of the equatorial ionosphere-thermosphere system. The plasma fountain responsible for the ionization anomaly, the vertical and zonal plasma drifts of the post-sunset hours leading to plasma bubble/spread F irregularity generation, and the Hall electric field that drives the electrojet current system are among the most notable manifestations of the these processes. The electrodynamic processes to be discussed in this paper will concern mainly the equatorial spread F (ESF) irregularity generation and their variabilities. The key factors that control the ESF generation by generalized Rayleigh-Taylor instability process, such as the the prereversal enhancement electric field (vertical drift) that is controlled by post-sunset zonal wind and longitudinal conductivity gradients, meridional/transequatorial winds, flux tube integrated conducitvities, and the seed perturbations are discussed in some detail, focussing attention on the aspects of their seasonal, logitudinal and day-to-day variabilities. The role of the evening F layer vertical drift in the instability process leading to ESF has been the most extensively investigated experimentally so far. Recent theoretical and computational results have advanced greatly our understanding of the importance of the key factors in the development process of spread F. However, observational identification of the relative importance of some of the key control factors in a given spread F event or in its day-to-day variablity is still lacking, the main examples being that of the seed perturbation, meridional/transequatorial winds and integrated conductivities. However, some progress on understanding the seasonal pattern has been achieved. Enhanced conductivity below F region and in E layer in the longitude sector of the South Atlantic Magnetic Anomaly seems to influence the equatorial spread F electrodynamics. (C) 2001 Elsevier Science Ltd. All rights reserved.