[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.

Wavelet analysis is suitable for investigating waves, such as Pi 2 pulsations, which are limited in both time and frequency. We have developed an algorithm to detect Pi 2 pulsations by wavelet analysis. We tested the algorithm and found that the results of Pi 2 detection are consistent with those obtained by visual inspection. The algorithm is applied in a project which aims at the nowcasting of substorm onsets. In this project we use real-time geomagnetic field data, with a sampling rate of 1 second, obtained at mid-and low-latitude stations (Mineyama in Japan, the York SAMNET station in the U.K., and Boulder in the U.S.). These stations are each separated by about 120• in longitude, so at least one station is on the nightside at all times. We plan to analyze the real-time data at each station using the Pi 2 detection algorithm, and to exchange the detection results among these stations via the Internet. Therefore we can obtain information about substorm onsets in real-time, even if we are on the dayside. We have constructed a system to detect Pi 2 pulsations automatically at Mineyama observatory. The detection results for the period of February to August 1996 showed that the rate of successful detection of Pi 2 pulsations was 83.4% for the nightside (18-06MLT) and 26.5% for the dayside (06-18MLT). The detection results near local midnight (20-02MLT) give the rate of successful detection of 93.2%.

An interesting variation in the geomagnetic D‐component has been found by analysis of the Magsat data. Characteristics of this variation are: that it appears everyday on the low‐latitude duskside and is antisymmetric about the dip equator (negative in the northern hemisphere); that the magnitude (from 5 to 25 nT) depends upon the geographic longitude and altitude; and that no corresponding change can be seen on the ground magnetic data. The change suggests a meridional current system in the equatorial ionosphere, produced in association with the electrojet current along the dip equator. This effect should be taken into account in the study of geomagnetic anomaly of internal origin with the Magsat data.

We present a statistical study of the diurnal variation of the occurrence frequency of geomagnetic sudden commencements (SCs) observed at Kakioka (geomagnetic latitude, θ = 27.4• ). SCs with an H -component amplitude ( H ) larger than 40 nT occur more frequently in the nighttime than the daytime, while those with smaller amplitudes ( H < 39 nT) occur more frequently in the daytime. Three large amplitude SCs ( H = 85, 117 and 145 nT at Kakioka) were analyzed in detail. All three exhibited larger amplitudes during the nighttime at all low latitudes except those near the dayside equator. A statistical study reveals that the averaged amplitudes are slightly larger in the daytime at Alibag (θ = 10.2• ) but considerably larger in the nighttime at three higherlatitude Japanese stations, Kanoya (θ = 21.9• ), Kakioka and Memambetsu (θ = 35.4• ). Case studies of two moderate amplitude SCs which occurred at the same UT indicate that nighttime SC amplitudes at low latitudes are slightly (considerably) larger than daytime amplitudes when the interplanetary magnetic field (IMF) points northward (southward). We suggest that the diurnal variation of SC amplitudes can be explained by a combination of field aligned and resultant ionospheric currents produced during the main impulse of SCs.

The impulse response of the geomagnetic indices (Dst, AL, AU and AE) to the interplanetary magnetic field southward component (IMF Bz) is calculated on the assumption that the magnetosphere acts as a linear system. Hourly data for 300 days are used in this analysis. The main results obtained are as follows: (1) Using interplanetary data, a fairly large portion of geomagnetic disturbances may be predicted with the assumption of a linear system. (2) The response of the Dst index and the AL index develops at two steps or has an oscillating property with a period of several hours. (3) The response of the AU index is rather different from that of the other indices. These results may suggest that the disturbances represented by the Dst and AL indices have a common origin associated with the interplanetary magnetic or electric field, whereas those indicated by the AU index have an origin somewhat different from it.