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Ann. Geophys., 26, 671-687, 2008
www.ann-geophys.net/26/671/2008/
© European Geosciences Union 2008
Statistical Birkeland current distributions from magnetic field observations by the Iridium constellation
B. J. Anderson1, H. Korth1, C. L. Waters2, D. L. Green2, and P. Stauning3
1The Johns Hopkins University, Applied Physics Laboratory, Laurel, MD, USA
2School of Mathematical and Physical Sciences, The University of Newcastle, Callaghan, NSW, Australia
3Solar-Terrestrial Physics Division, Danish Meteorological Institute, Copenhagen, Denmark
Abstract. 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° during southward IMF;
Region-2 currents are weak and confined to latitudes >65°
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.
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