Long-term evolution of magnetospheric current systems during storms

We present a method to model the storm-time magnetospheric magnetic field using representations of the magnetic field arising from the various magnetospheric current systems. We incorporate the effects of magnetotail changes during substorms by introducing an additional localized thin current sheet into the Tsyganenko T89 model. To represent the storm-time ring current the T89 ring current is replaced by a bean-shaped current system, which has a cross section that is close to the observed distribution of trapped particles in the inner magnetosphere and has an eastward flowing inner and westward flowing outer components. In addition to the symmetric ring current, an asymmetric partial ring current is taken into account with closing Region 2 sense field-aligned currents. Magnetopause currents are varied in accordance with solar wind dynamic pressure variations. Three moderate geomagnetic storms when D, reached about - 150 nT and one big storm with D-st about -250 nT are modelled. The model free parameters are specified for each time step separately using observations from GOES 8 and 9. Polar, Interball and Geotail satellites and D-st measurements. The model gives a high time-resolution field representation of the large-scale magnetic field, and a very good reproduction of the D-st index. It is shown that the ring current is most important during intense storms, whereas the near-Earth tail currents contribute more to the D-st index than the ring current during moderate storms.