Saturation of global field aligned currents observed during storms by the Iridium satellite constellation

We use Iridium constellation magnetometer data to characterize the global-scale storm-time Birkeland currents. For 11–12 August 2000 we find a strong storm main phase dawn–dusk asymmetry in the latitudes of the region 1–2 current systems and that the total current is not linearly related to the interplanetary electric field, Ey. Examination of 23 storms occurring for 1999–2003 shows clear evidence for a non-linear, saturation behavior in total current with Ey, with the total Birkeland current constrained to be below 20 MA.