We investigate the effect of plasma pressure on the metic field in the near-Earth magnetosphere (2 to 6.5 RE) during the major magnetic storm of October 21-25, 2001. For this we obtain a time series of "snapshots", in each of which the magnetic forces are equilibrated by plasma pressure gradient forces. Each snapshot is computed using our 3-D equilibrium code, which is fed anisotropic pressure in the equatorial plane from a kinetic ring current model. As computational boundaries we use magnetic flux surfaces obtained from the T89 empirical model [Tsyganenko, 1989], parameterized by the appropriate Kp. We analyze the computed magnetic fields and electric currents at each stage of the storm. Our findings include significant (∼10) plasma β and large field depressions near Earth at the storm peak. The results clearly show the necessity of a magnetically self-consistent treatment of plasma transport in storm modeling.
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