Nightside geosynchronous magnetic field response to interplanetary shocks: Model results

Inspired by the fact that spacecraft at geosynchronous orbit may observe an increase or decrease in the magnetic field in the midnight sector caused by interplanetary fast forward shocks (FFS), we perform global MHD simulations of the nightside magnetospheric magnetic field response to interplanetary (IP) shocks. The model reveals that when a FFS sweeps over the magnetosphere, there exist mainly two regions: a positive response region caused by the compressive effect of the shock and a negative response region which is probably associated with the temporary enhancement of earthward convection in the nightside magnetosphere. IP shocks with larger upstream dynamic pressures have a higher probability of producing a decrease in B-z that can be observed in the midnight sector at geosynchronous orbit, and other solar wind parameters such as the interplanetary magnetic field (IMF) B-z and IP shock speed do not seem to increase this probability. Nevertheless, the southward IMF B-z leads to a stronger and larger negative response region, and a higher IP shock speed results in stronger negative and positive response regions. Finally, a statistical survey of nightside geosynchronous B-z response to IP shocks between 1998 and 2005 is conducted to examine these model predictions.