Response of the magnetosphere‐ionosphere system to a sudden southward turning of interplanetary magnetic field

A sudden southward turning of interplanetary magnetic field (IMF) is simulated by the University of Michigan's BATS‐R‐US model. The main goal of this study is to determine the time delay as well as physical processes between when an IMF discontinuity reaches the bow shock, when reconnection is initiated at the magnetopause, and when the ionosphere starts to react. While observations or empirical models might give an estimate of the time delay for the propagation of the discontinuity from the bow shock to the magnetopause, the global MHD simulation provides a more comprehensive insight of responses of the magnetosphere‐ionosphere system. An idealized north‐to‐south IMF transition is modeled, using a solar wind velocity of 400 km/s. After the southward IMF encounters the bow shock, it takes about 6 min for the north‐to‐south IMF transition front to arrive at the subsolar geomagnetic field. The ionospheric response to this sudden southward IMF turning is delayed by another ∼4 min, during which the magnetosphere undergoes a conversion from cusp reconnection to subsolar reconnection and the Alfvén wave propagation to the ionosphere takes place. Thereafter, changes in the ionosphere and ground magnetic perturbations associated with the southward IMF are observed. These responses appear to be globally onset as described in many other studies. The time it takes from the encounter of the IMF transition with the bow shock to when the ionospheric reaction takes place varies with the solar wind speed, ranging from nearly 15 min for a solar wind speed of 300 km/s to just over 6 min for solar wind speeds of 600 km/s.