The Equatorward Boundary of the Auroral Current System During Magnetic Storms

Our current knowledge of the geomagnetic poleward and equatorward boundary dynamics is limited, particularly, how deep those two latitudinal boundaries can extend into lower geomagnetic latitudes during magnetic storms. We want to understand the motion of the boundary because it is important in terms of the location and magnitude of the effects of geomagnetic disturbances associated with storms on the ground. In this study we derive spherical elementary ionospheric currents from ground magnetometer arrays covering North America and Greenland during six magnetic storms in 2015 and 2018. With two dimensional maps of the auroral region current, we select the equatorward boundary of the region 2 currents by-eye and fit the boundary with an ellipse to derive the location of the equatorward boundary at magnetic midnight. We have obtained over 500 boundaries and find that the midnight boundary location varies between 45 degrees and 66 degrees magnetic latitude. We examine the influence of the interplanetary magnetic field (IMF), solar wind plasma, and geomagnetic indices on the location of the magnetic midnight equatorial boundary and find that the equatorial boundary location is best correlated with the IMF Bz, VBz, and the Sym-H index. We demonstrate that as the Bz component becomes more negative, the magnitude of VBz increases, and the magnitude of the Sym-H index increases, the magnetic midnight equatorial boundary shifts equatorward during periods of moderate to high geomagnetic activity.