Statistical analysis of nightside geosynchronous magnetic field responses to interplanetary shocks

When an interplanetary (IP) shock passes over the Earth's magnetosphere, the geosynchronous magnetic field strength near the noon is always enhanced. Near the midnight, however, it increases or decreases. This indicates that the nightside magnetosphere is not always compressed by a sudden increase in the solar wind dynamic pressure. To understand the characteristics of nightside geosynchronous magnetic field responses to IP shocks, we statistically examined geosynchronous magnetic field perturbations observed near midnight between 2200 and 0200 magnetic local time during 120 sudden commencements (SC). We found that the SC-associated geosynchronous magnetic field perturbations in the BH (north--south/parallel to the dipole axis) component in the local dipole VDH coordinates are mostly positive in summer. However, the geosynchronous magnetic field perturbations in other seasons show no significant seasonal dependence in BH. Considering that the SC is accompanied by an increase in the cross-tail current (Jc) and that the SC-associated Jc is a main controlling factor of nightside geosynchronous magnetic field perturbations, it is suggested that the observation of the positive or negative response is due to the geometry of geosynchronous spacecraft relative to the SC-associated Jc. We also suggest that the SC-associated Jc is enhanced around geosynchronous orbit.