ALBERT

Description: 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.

Description: In this chapter the application of the curlometer technique to various regions of the inner magnetosphere and upper ionosphere and for special circumstances of sampling is described. The basic technique is first outlined, together with the caveats of use, covering: the four-spacecraft technique, its quality factor and limitations; the lessons learnt from Cluster data, together with issues of implementation, scale size and stationarity, and description of the key regions covered by related methodology. Secondly, the application to the Earth’s ring current region is outlined, covering: the application of Cluster crossings to survey the ring current; the use of the MRA (magnetic rotation analysis) method for field curvature analysis; the use of THEMIS (Time History of Events and Macroscale Interactions during Sub-storms mission) three-spacecraft configurations to sample the ring current, and future use of MMS (Magnetospheric MultiScale mission) and Swarm data, i.e. the case of small separations. Thirdly, the application of the technique to the low altitude regions covered by Swarm is outlined, covering: the extension of the method to stationary signals; the use of special configurations and adjacent times to achieve 2, 3, 4, 5 point analysis; the use of the extended ‘curlometer’ with Swarm close configurations to compute 3-D current density, and a brief indication of the computation of current sheet orientation implied by 2-spacecraft correlations. Fourthly, the direct coordination of Cluster and Swarm to check the scaling and coherence of field-aligned currents (FACs) is outlined.