Data-driven Modeling of Substorm Growth Phase Magnetic Field

The plasma and magnetic field structure during the substorm growth phase is crucial to understanding development and initiation of substorms. In this study, we first established empirically the isotropic plasma sheet pressure distributions by fitting the Geotail and THEMIS pressure for the growth phase of isolated substorms (at least 240 min from previous onset). Nonlinear least square fitting was achieved with a high correlation coefficient of 0.82. We then computed the 3-D magnetic field in force balance with the fitted pressure. The pressure P shows strong dawn-dusk asymmetry with higher P in the post-midnight sector at r > ~10 RE, but P becomes higher in the pre-midnight sector inside r ~10 RE. These features are consistent with previous observational studies. The radial pressure gradient along midnight is at least a factor of 2 larger than the previously observed quiet-time gradient for Kp=1. The force-balanced magnetic field lines are more stretched compared with those of quiet-time and of T89 for Kp=3, and are twisted azimuthally toward dawn close to Earth and toward dusk further away from the Earth. The westward tail current peaks near midnight in the tail and the peak moves towards dusk with decreasing radial distance from the Earth inside r ~10 RE. The corresponding field-aligned current (FAC) pattern has region-1 sense FACs in the tail plasma sheet and region-2 FACs closer to the Earth. The region-2 FACs are rotated from their quiet-time picture, upward current dominating the midnight-to dawn MLTs and downward current from dusk to noon MLTs.