A first comparison of POLAR magnetic field measurements and magnetohydrodynamic simulation results for field‐aligned currents

This letter presents comparisons between POLAR magnetic field measurements and results from a global numerical simulation of the Earth's magnetosphere using WIND measurements of the solar wind parameters as input. The comparison shows that the simulation provides a good representation for the magnetic perturbation associated with an upward Region 1 field‐aligned current (FAC) in the prenoon sector. This current system is caused by the interplanetary magnetic field (IMF) negative By component and also features downward current on open field lines at higher latitudes. The results also show effects caused by a gradual rotation of the IMF northward and a sudden return to its initial dawnward direction. The self‐consistent simulation Birkeland current pattern at ionospheric altitudes is also presented, and its evolution caused by the IMF rotations is discussed.