Empirical modeling of a CIR-driven magnetic storm

The empirical analysis of structure and evolution of the geomagnetic field and underlying electric currents during the 8-11 March 2008 magnetospheric storm, driven by a corotating interaction region, is presented. It is based on the high-resolution geomagnetic field model TS07D (http://geomag_field.jhuapl.edu/model/) and the low-altitude mapping of field-aligned currents obtained using Iridium satellites. Compared to storms, driven by coronal mass ejections, the equatorial currents are found to be overall more dawn-dusk symmetric. Only in the early main phase a moderate hook-like westward current flowing from the Region-2 inflow area on the dawn side to the dusk/afternoon magnetopause is detected, along with an eastward current near the pre-noon magnetopause. New tail-type currents are found to dominate the storm-time magnetosphere in early main and recovery phases at the moments of strong peaks of the solar wind dynamic pressure. Similar effects, found in CME-driven storms, coincide in time with the plasma sheet density bursts. A clear distinction between the periods dominated by the partial ring current and the tail-type currents seen in the model agrees with the Iridium data that indicate a significant reduction and contraction of the field-aligned current pattern in the latter case. Overall small magnitudes of the total field-aligned currents, with rather transient enhancements, appear to be the most distinctive feature of CIR-driven storms. Comparison between the model and observations made by five THEMIS probes shows good agreement on storm timescales. Moreover, every deviation arising during a substorm reveals characteristic signatures of the tail current sheet thinning and dipolarization.