Modeling magnetospheric current response to solar wind dynamic pressure enhancements during magnetic storms: 1. Methodology and results of the 25 September 1998 peak main phase case

[1] We present a methodology for using the modular Tsyganenko storm magnetic field model (TS04) as a tool to investigate the response of magnetospheric currents to solar wind dynamic pressure enhancements during magnetic storms. We demonstrate the technique by examining the contribution of each model current to the observed dawn-dusk asymmetric ground H perturbation during a peak storm main phase event. We add present pressure terms to the parameterizations of several individual model currents and fit them to the observed low-latitude and midlatitude H perturbations. We find that the asymmetric H perturbation for this case is primarily due to a net field-aligned current (FAC) system, with upward FACs peaking in the evening and downward FACs peaking in the morning, and the equatorial portion of the partial ring current (PRC). This net FAC system includes the region 1 and region 2 FACs and the closure FAC of the PRC, with the largest contribution from the PRC closure FACs. The model results show that in this main phase case, the PRC played a more important role in causing the dawn-dusk asymmetric H perturbation than did the other currents owing to its strength and asymmetry. The model results do not show significant contributions from the symmetric ring current and tail current, but the magnetopause current gives a significant positive perturbation at all MLTs. In the companion paper, our methodology is applied to obtain a more complete picture of the storm time current response to pressure enhancements by examining pressure increases occurring during different storm phases.