Terrestrial Ring Current: a Review of Some Cluster Results Based on the Curlometer Technique

Abstract:

The inner magnetospheres electric currents configuration and mapping is one of the key elements for understanding current loop closure inside the entire magnetosphere. The ring current is toroidal-shaped and flows in the near-Earth region, where the magnetic field is dipole-like. This current system is driven by the pressure gradients, and it is formed by the drift of the charged particles that are injected from the magnetotail towards the Earth during the magnetospheric storms and substorms. A method for directly computing current is the multi-spacecraft curlometer technique, which is based on Maxwell-Amperes law application. This requires the use of four point simultaneous magnetic field measurements. The FGM experiment on board the four Cluster spacecraft allowed for the first time an instantaneous calculation of the magnetic field gradients and thus a measurement of the local current density. This technique requires however a careful analysis concerning all the factors that can affect the accuracy of the current density calculation. The CIS experiment, on board these spacecraft, provides the ion distribution functions of the current carriers within its energy range. Earlier and more recent results, based on Cluster data acquired during passes in the ring current region at different perigee altitudes, will be reviewed in the light of recent progress on the accuracy of the method.