Different B_z response regions in the nightside magnetosphere after the arrival of an interplanetary shock: Multipoint observations compared with MHD simulations

We present different magnetic field changes in the nightside magnetosphere in response to the interplanetary (IP) shock on 17 December 2007, using multiple spacecraft observations and global MHD simulations. The coexistence of two distinct B_z response regions in the nightside magnetosphere in a single event has been observationally identified for the first time. From the inner magnetosphere to the tail, they are the positive response (B_z increase) and the negative response (B_z decrease). This scenario reasonably agrees with the MHD model prediction. Moreover, the analysis of the response delay time shows that, for the three satellites which observed the negative responses of B_z, the one closest to Earth was the last to respond. This phenomenon can also be understood based on the model prediction that the negative response region develops toward Earth after its formation. In addition, the temporarily enhanced earthward flows in the negative response region, which were suggested to be responsible for the formation of this region by previous model studies, were also supported by the observation. At last, a global view of the B_z response processes in the nightside magnetosphere is presented based on MHD simulations.