Terrestrial Ring Current : A Review of Cluster Results Based on the Curlometer Technique
Résumé
The inner magnetosphere 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 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 energetic charged particles that are injected from the magnetotail toward the Earth during the magnetospheric storms and substorms. A method for directly computing currents in space is the multispacecraft curlometer technique, which is based on the Maxwell‐Ampère law application. This requires the use of four‐point simultaneous magnetic field measurements. The FGM (Fluxgate Magnetometer) experiment on board the four Cluster spacecraft allowed for the first time the instantaneous calculation of the magnetic field gradients and thus the 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 (Cluster Ion Spectrometry) 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, are reviewed in the light of recent progress on the accuracy of the method.