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[1] 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 = À0.68. 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 À160 nT, and 21-22 October 1999, Dst minimum approximately À230 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.

Abstract. The Kp index indicates geomagnetic disturbances in a simple manner. The index is derived from magnetic field data acquired at 11 ground stations distributed worldwide at subaurora! latitudes (49ø-62ø), has values ranging from 00 to 90 in 28 steps, and is given at 3-hour intervals. Kp is widely used to study the dynamic relationship between the solar wind and the magnetosphere, to empirically specify the location of the plasmapause and other plasma regions/boundaries, and also as input to various models of the magnetosphere and ionosphere. Currently, the official Kp index is delivered with a delay of many days, so it is not useful for near-real-time monitoring of the state of the magnetosphere. We have developed an algorithm to derive an estimated Kp, denoted Kpest, using magnetometer data from nine ground stations that can transmit data in near real time. The algorithm is fully automated and includes a data-cleaning routine, a quiet-day-curve routine, and a routine to convert magnetic field deviations to Kpest. We have evaluated the performance of Kpest using archived magnetometer data and the official Kp. When data from all of the nine stations are available, the linear correlation coefficient between gpest and Kp is 0.93. In addition, we find that a similarly high correlation between Kpest and Kp can occur when data from only one to three stations are used. We conclude that an automated procedure using data from a small number of ground stations can generate Kp estimates that are reliable in the context of near-real-time monitoring of space weather.

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