Statistical Birkeland current distributions from magnetic field observations by the Iridium constellation

We present a statistical analysis of Birkeland currents derived from Iridium magnetometer data acquired in the Northern Hemisphere to determine the dependence of large-scale currents on the interplanetary magnetic field (IMF) direction. Because the Iridium data span nearly seven years, we can restrict analysis to only those intervals with stable currents. We used image comparison to quantify the consistency between successive one-hour current distributions and selected 1550 two-hour intervals, 5% of the data, for analysis. Results include: no statistically significant average currents are present poleward of 80 degrees during southward IMF; Region-2 currents are weak and confined to latitudes >65 degrees during northward IMF; there is marked contrast between currents for northward and southward IMF but the evolution of the patterns is continuous with IMF rotation. The directions of flows inferred from the most poleward currents are more consistent with theoretical expectations of transport away from magnetopause reconnection than previous results. We attribute the differences to the restriction in this analysis to intervals having relatively stable distributions of current so that the data set corresponds more nearly to pure states of the system.