Storm-time magnetic field variations observed by the ETS-VI satellite

To study the ring current structure in the inner magnetosphere, we have statistically examined the magnetic field data acquired by ETS-VI (the Engineering Test Satellite-VI). During a magnetic storm, the Dst index shows a rapid recovery of its amplitude for about 9 hours on average after the main phase and a subsequent long-lasting slow recovery. We have investigated this “two-step recovery” of the Dst index by obtaining magnetic field vectors and calculating the current structure in the inner magnetosphere for each magnetic storm phase determined by the Dst index. From this study, following results are obtained: (1) Throughout the storm-time, disturbed magnetic fields exhibit clear day-night asymmetry with strong peak in the nightside. (2) During the main phase, southward perturbed field components have a relative bump in the nightside region between ~2000 and ~0400 MLT and between ~4.0 and 6.4 R_E (geocentric distances in Earth radii). (3) The initial rapid recovery of the Dst index is considerably influenced by the nightside currents flowing between ~1800 and ~0600 MLT and between 5.6 and 7.2 R_E. These currents are thought to be mainly composed of the particles that escape the magnetosphere on the duskside flank, which are simulated in particle tracing in a realistic magnetosphere.