Abstract.We discuss the well known observation that during the storm recovery phase the Dst-variation shows a two-stage decay pattern, decaying first quickly and then decaying with a larger decay parameter. This finding is discussed in the context of the magnetic storm which was observed on November 25-27, 1986. Contrary to frequently used interpretations as two spatially separated ion populations or two different, atomic ion components. we propose an alternative explanation for this feature. We argue that during the recovery phase of the magnetic storm the D st decay is controlled by the decay of a two current system' the ring current (DR) and the magnetospheric tail current (DT).

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[1] A new self-consistent version of a time-dependent magnetospheric paraboloid model is presented and tested on the 24-27 September 1998 magnetic storm interval (minimum Dst = À207 nT). The model uses DMSP satellite data to identify the location of the inner boundary of the magnetotail current sheet and the magnetic flux in the lobes and their variations with time. These inputs plus upstream solar wind dynamic pressure and IMF B z values are used to iteratively model the Earth's field during the storm. Several interesting results with important consequences are obtained: (1) the model tail field strength at the Earth's surface (DT = À134 nT) is a significant fraction of the ring current value (DR = À167 nT); (2) the movement of the tail current sheet inward to L = 3.5-4.0 at storm maximum is consistent with geosynchronous magnetic field data; (3) at the Earth's surface the Chapman-Ferraro magnetopause current field (DCF = 117 nT) is almost equal at storm maximum to the value from the tail current, thus the fields from the two systems nearly cancel; (4) the magnetic flux from the polar cap in the course of the magnetic storm main phase approximately doubles in comparison with the magneto-quiet interval just before the storm onset; this fact shows that the driven processes prevail over dissipation processes throughout the storm main phase; (5) the large-scale internal currents in the magnetosphere (ring current, field-aligned currents, and magnetotail current) have significant influence on the shape and size of the magnetosphere; the location of the magnetopause subsolar point is different from that obtained by extrapolation of empirical results taken during high geomagnetic activity intervals and from magnetospheric models that do not include feedback from internal magnetospheric currents.