3D ANALYSIS OF THE RING CURRENT, FOR THE 20 APRIL 2002 EVENT, USING ENAsIMAGE INVERSIONS (IMAGE/HENA) AND THE CURLOMETER TECHNIQUE(CLUSTER/FGM DATA)

摘要

The inner magnetosphere's electric currentsrnconfiguration and mapping is one of the keys forrnunderstanding current loop closure inside the wholernmagnetosphere. Development of the ring current as arnfunction of geomagnetic activity is still one of the mostrncrucial aspects about this region.rnA method to estimate the current density vector is thernmulti-spacecraft curlometer technique, which is based onrnMaxwell-Ampere's law application. This requires the usernof four point magnetic field high-resolutionrnmeasurements. The FGM experiment on board the fourrnCluster spacecraft allows for the first time anrninstantaneous calculation of the magnetic field gradientsrnand thus a measurement of the local current density.rnMoreover, inversions of Energetic Neutral Atom (ENA)rnimages obtained by IMAGE/HENA can provide the globalrnion distribution in the equatorial plane, and thus anrnestimate of global partial pressures distribution. By using arnrealistic magnetic field model, a part of the globalrnpressure-driven current system can be computed and usedrnto put the local results of the curlometer technique intorncontext. Using these two complementary methodsrn(curlometer technique using in-situ measurements andrnneutral atom image inversion using remote sensing data),rnan instantaneous 3D mapping of the current can be made.rnThe ring current distribution has been studied for the 20rnApril 2002 storm event. During the early recovery phasernof this storm (Dst～ - 101 nT; Kp=6), Cluster was crossingrnthe perigee at R～4 RE in the evening sector (MLT～21).rnThe Cluster constellation provided a direct measure of thernlocal current density, by applying the curlometerrntechnique. Simultaneously, Cluster was situated inside thernHENA/IMAGE field-of view, allowing a validation of thernion fluxes obtained from HENA/IMAGE imagerninversions. Our analysis reveals the existence of anrnimportant local current density (～ 30 nA/m2), even if the Cluster spacecraft are situated slightly duskward of thernring current bulk, as shown by the HENA image inversion.rnThe curlometer results reveal also the existence of anrnasymmetry between the two hemispheres, characterized byrna southward orientation of the current at the equator.