It is now possible to create mosaic 'images' of the global auroral zone electric currents in a small fraction of the time that had been needed previously.

Figure 8. Top panel shows location in magnetic latitude (MLAT) and local time (MLT) and coordinates for all data from March 9-10, 1999. The perturbation polarity is color coded, blue=Eastward, red=westward. Bottom panel shows the distribution of all data (green) and points flagged automatically as auroral zone detections (red). It is apparent that the vast majority of flagged points are at high latitudes where we expect them to be. Discrimination against spurious points is excellent. Moreover, the sense of the perturbations is consistent with statistical studies: eastward in the evening (north) and westward in the morning (north) and the reverse in the southern hemisphere. Opposite colors are observed poleward of the equatorward perturbations due to the polar cap signature mentioned above and observed in Figure 6. Together, these results show that the automated technique for picking out auroral current signatures works and yields results consistent with our expectations from previous knowledge.

Figure 8

Figure 9. Pattern of magnetic perturbations we expect derived from prior statistical analysis. Since the late 1970's we have known that the large scale auroral currents are roughly arranged as shown. They roughly appear as pairs of alternating in and out currents directed along the magnetic field. The corresponding magnetic perturbations are eastward (blue) in the afternoon and evening and westward (red) in the morning and pre-noon. In the southern hemisphere the sense of the magnetic perturbations is reversed. This pattern was established using months of data from single satellites. Whether this pattern is instantaneous and how it varies in time are important questions that we can only now address.

Figure 9

Figure 10. Iridium synoptic map of magnetic perturbations for July 31 at 2000 UT to August 1 at 0200 UT. Points are flagged as real signatures are plotted as blue (eastward) or red (westward). Data coverage is indicated by plotting satellite locations at the time of all magnetic data values by small gray dots. The color pattern of the dots in this figure agrees well with the qualitative pattern expected from statistical studies shown in Figure 8. The green circle is a best fit for the location of the maximum magnetic perturbation and the yellow annulus is a measure of the breadth of the currents. The processing to generate these plots and fits has been done on all of the data received.

Figure 10

Figure 11. First Iridium synoptic maps of magnetic perturbations for February 7, 1999. Each polar plot shows a six-hour span of data from either the northern or southern hemisphere in the same format as Figure 10. This series is the first synoptic map of large scale perturbations due to Birkeland currents ever producted. It shows the persistence of the pattern of Figures 9 and 10 and also shows a consistent polar cap signature. Differences in the latitude location of the perturbations are also apparent, especially between the last two pairs of maps. It is also clear that the distributions of points are organized by magnetic rather than geographic coordinates, further demonstrating that the signals are real and not noise. The time scale for generating such synoptic maps is determined by the number of satellites in each plane and by the time resolution with which the data are recorded. For Iridium the limiting factor is the recording time resolution. At sampling intervals of 185 s that are typically used, it takes at least 1 hour to accumulate enough detections to assemble a meaningful map. This is because one cannot be sure that a given pass will necessarily detect the auroral currents. In addition, one has to build up enough samples of the currents to develop a measure of the extent of the currents. Some of the fundamental processes in magnetospheric and ionospheric physics occur on time scales shorter than 1 hour, 20-30 minutes, so there is interest in achieving maps on time scales shorter than obtained with Iridium. In principal it would be possible to build a map every 10 minutes with 15 s resolution data. The longer time scale maps are still of great utility for studying geomagnetic storms, with time scales of hours to days, and comparing the patterns of large scale currents under different conditions when the system is not changing too rapidly.

Figure 11