Relation between the ring current and the tail current during magnetic storms

We study the dynamics of the magnetospheric large-scale current systems during storms by using three different magnetospheric magnetic field models: the paraboloid, event-oriented, and Tsyganenko T01 models. We have modelled two storm events, one moderate storm on 25-26 June 1998, when D-st reached - 120 nT and one intense storm on 21-23 October 1999, when D-st dropped to -250nT. We compare the observed magnetic field from GOES 8, GOES 9, and GOES 10, Polar and Geotail satellites with the magnetic field given by the three models to estimate their reliability. All models demonstrated quite good agreement with observations. Since it is difficult to measure exactly the relative contributions from different current systems to the D-st index, we compute the contributions from ring, tail and magnetopause currents given by the three magnetic field models. We discuss the dependence of the obtained contributions to the D-st index in relation to the methods used in constructing the models. All models show a significant tail current contribution to the D-st index, comparable to the ring current contribution during moderate storms. The ring current becomes the major D-st source during intense storms.