The Equatorial Electrojet

The equatorial electrojet (EEJ) represents a ribbon of intense electric current flowing in the ionospheric E region on the dayside along the dip-equator. The primary reason for the high current density is the geometry of the geomagnetic field with its horizontal field lines at these latitudes. This enables a greatly enhanced electrical conductivity in the ionospheric E layer over a latitudinal width of about 600 km. Any low-latitude electric field in that region will give rise to significant currents in that channel. In this chapter we will first present some historical observations that lead to the discovery and early characterization of the EEJ. In those years, most of the studies were based on magnetic signatures observed on ground. Significant progress in understanding the EEJ could be made when measurements from low-Earth orbiting satellites became routinely available. Subsequently, we describe the electrodynamics that governs the EEJ properties. These can be used for predicting important EEJ features. Besides the physics-based models, an empirical model based on a large observational data set is presented. From this model, the main climatological characteristics of the EEJ can be deduced, such as diurnal, annual and longitudinal variations, as well as dependencies on solar and magnetic activities. The tidal modulation plays an important role for the temporal and spatial variation of the EEJ intensity. We describe both the influences of solar and lunar tides. At times the commonly eastward EEJ current reverses to westward. Here the most important processes are described that cause the counter equatorial electrojet. In the end, the prime features of the EEJ are summarized and remaining open issues are presented.