Equatorial ionosphere–thermosphere system: Electrodynamics and irregularities

The equatorial ionosphere and thermosphere constitute a coupled system, with its electro dynamical and plasma physical processes being responsible for a variety of ionospheric phenomena peculiar to the equatorial region. The most important of these phenomena are: the equatorial electrojet (EEJ) current system and its instabilities, the equatorial ionization anomaly (EIA), and the plasma instabilities/irregularities of the night ionosphere (associated with the plasma bubble events – ESF). They constitute the major topics of investigations having both scientific and practical objectives. The tidal wind interaction with the geomagnetic field is responsible for the atmospheric dynamo electric fields, that together with the wind system, drives the major phenomena, under quiet conditions. Drastic modifications of these phenomena can occur due to magnetospheric forcing under solar-, interplanetary- and magnetospheric disturbances. They can also undergo significant modifications due to forcing by atmospheric waves (such as planetary- and atmospheric gravity waves) propagating upward or from extra tropics. This article will focus on the ambient conditions of the ionosphere–thermosphere system and the electro dynamics and plasma instability processes that govern the plasma irregularity generation. Major emphasis is given to problems related to the structuring of the equatorial night ionosphere through plasma bubble/ESF irregularity processes. Specific topics to be covered will include: equatorial electric fields, thermospheric winds, sunset electrodynamic processes, plasma drifts, EEJ plasma instability/irregularity generation, nighttime/post sunset plasma bubble irregularity generation, and very briefly, disturbance electric fields and winds and their effect on the ionization anomaly, the TEC and ESF/plasma bubble irregularities.