Direct calculation of the ring current distribution and magnetic structure seen by Cluster during geomagnetic storms

Magnetic disturbances caused by the Earth's ring current, particularly during storm time activity, have a dominant effect on the geomagnetic field. Strong currents and large kinetic and magnetic energies can change considerably local field geometry and depress the ground geomagnetic field. The multispacecraft magnetic measurements of Cluster allow extensive in situ coverage of the ring current. We select 48 storm time Cluster crossing events to investigate the variation of the local current density distribution and magnetic configuration of the ring current. We find direct evidence for the existence of an inner, eastward flowing current in addition to the dominant westward current, in the ring plane. The radius of curvature of the magnetic field lines (MFLs) is found to be increasingly reduced at all local times during increasing storm activity, changing the resulting ring current magnetic geometry considerably, where the MFL configuration and the azimuthal current density distribution are asymmetric with the local time. During similar storm activity the radius of curvature of the local MFLs, R_c, is smallest on the nightside to duskside, medium on the dawnside, and largest on the dayside. This change in geometry may have significant influence on the spatial distribution of the particles with various energies in the plasmasphere, ring current, and radiation belts. Key Points First direct measurement of the storm ring current magnetic field geometry Direct evidence for the existence of an inner, eastward ring current MFLs configuration and the azimuthal current are both asymmetric with MLT