Birkeland current system key parameters derived from Iridium observations: method and initial validation results

The Iridium satellites in 780 km altitude, circular polar orbits provide continuous global monitoring of the Birkeland current system via engineering magnetometer data. These data have been used to characterize basic features of the global field-aligned currents (FACs) with a time window of 45 min and a time step of 15 min. The three sigma magnetometer data noise threshold is 93 nT on average. The fraction of measurements above the noise is used to provide one measure of the location of the auroral FACs. Measures are also presented for the mean latitude and equatorward/poleward extent of the region 1/region 2 FAC system. The equatorward latitude of region 1/region 2 FACs is anticorrelated with Kp, r = -0.68. Indices are presented for the net FAC intensity in terms of the eastward (westward) magnetic perturbation in the northern (southern) hemisphere by analogy with the AE, AU, and AL indices. The Iridium system indices show high correlation with the quick look auroral electrojet indices both in individual cases and statistically, r = +0.73 between their logarithms. Results are presented for two storms, 22–23 September 1999, Dst minimum approximately -160 nT, and 21–22 October 1999, Dst minimum approximately -230 nT, reflecting that intensification and equatorward expansion of the global FACs occur in response to southward IMF. Enhanced dynamic pressure promotes more rapid equatorial expansion, 10° in 1.5 hours for the September storm, for which the dynamic pressure was enhanced, 15–20 nPa, at southward IMF turning, as opposed to the October case, 13° over ~8 hours, for which the southward turning occurred during nominal dynamic pressure, ~5 nPa. In both storms the current intensity decreases to prestorm levels within an hour when the IMF turns northward or nearly horizontal, at the beginning of storm recovery. The key parameters are a useful means of accessing the Iridium system data for preliminary analyses, and the initial results provide motivation for future analyses to quantify the accuracy and reliability of products derived from the Iridium system data.