The effect of a brief northward turning in IMF B z on solar wind‐magnetosphere coupling in a global MHD simulation

Abstract In this paper we examine the response of the magnetosphere‐ionopshere (M‐I) system to a transient northward excursion in the interplanetary magnetic field (IMF) using the Lyon‐Fedder‐Mobarry (LFM) global MHD simulation. The simulated IMF transitions hold from a steady southward IMF to a steady northward IMF before suddenly transitioning back to southward IMF after 20 min. Once the IMF returns southward, the M‐I system is in a state of reduced energy dissipation for approximately an hour as it reconfigures back into a standard southward IMF configuration. We find that the northward IMF excursion affects both the viscous and reconnection interactions with the solar wind. The flow of plasma in the magnetosphere is significantly disrupted by the reconnection cycle under northward IMF. This reduce the transfer of mechanical energy from the solar wind due to the viscous interaction, and the magnetosphere‐ionosphere system is in a mixed topological configuration containing elements produced by both of southward IMF reconnection and the Dungey cycle, as well as northward IMF reconnection and the presence of reverse cell convection at high latitudes. The effects of the transient northward IMF must be completely cleared out before the system can return to an optimal state of energy transfer characteristic of steady southward IMF. As a result, a simple 20 min excursion of northward IMF can put the magnetosphere‐ionosphere system into a reduced state of coupling to the solar wind for some time following the return to steady southward IMF; for LFM we saw a reduced state lasting an hour