Effect of the northward turning of the interplanetary magnetic field on the dipolarization

An outstanding question in magnetospheric physics is whether the association between the northward turning of the interplanetary magnetic field (IMF) and the substorm onset is a random relation or a causal relation. In this work, we investigate the effect of the northward turning of the IMF on the dipolarization in the magnetotail by using the global PPMLR-MHD simulation model and observations. Four simulation runs are carried out: (1) a dipolarization under steady IMF conditions, (2) a dipolarization associated with a northward turning of the IMF, (3) an observed event under steady IMF conditions, and finally (4) an observed event associated with IMF northward turning. Our results show that (1) the northward turning of the IMF can enhance and prolong the dipolarization initiated by internal trigger, which is consistent with the statistical studies, and (2) the event simulations are similar to the observations. All these indicate that the northward turning of the IMF could play an important role in substorm triggering; however, it does not mean that it is necessary.