Study of Dst/ring current recovery times using the WINDMI model

摘要

We use the WINDMI model of the nightside magnetosphere to investigate the contributions of ring current, magnetotail current, and magnetopause current on the observed two-phase decay of the Dst index. For the analysis, several geomagnetic events in the period 2000-2007 were identified, during which the interplanetary magnetic field (IMF B_z) turns northward during the early recovery phase of the storm. The Dst recovery rate for these events were first estimated for either of two possible periods: by assuming an initial fast decay phase or by assuming an overall decay for the entire duration of storm. The recovery rates were estimated by matching Dst and Dst~* data against WINDMI model predictions. We consistently found an increase in the Dst recovery times when a shorter initial decay phase was chosen as compared to an overall decay phase, thus, confirming the observations of two-phase decay and indicating the possibility of contributions from faster initial decay mechanisms. We then modified the Dst index as estimated by the WINDMI model to include contributions from the cross-tail current and magnetopause currents. The modified Dst was then optimized for all the events. The optimized results correlate very well to the Dst dynamics and indicate that under northward IMF B_z conditions and during the early recovery phase of a storm; contributions from the geotail currents to the fast initial decay of the Dst index are important, while the slower recovery of Dst in the later phases of the storm are due to the charge exchange dominated ring current decay.