Comparison with magnetic field data from the Defense Meteorology Satellite Program (DMSP) illustrates how coarse time sampling will make the Birkeland currents look like spikes (Figure 5 below).

Because there are multiple Iridium satellites in each orbit plane one can check that the spikes occur in the same place in space on subsquent satellites (Figure 6 below). One can also use periods of time when one or more of the Iridium satellites is returning data at a higher rate and see whether the expected signatures are obtained (Figure 7 below). Both checks are successful.

Figure 5. Two orbits of DMSP F7 magnetic field data, also cross track component, showing the full time resolution, 1-s, 15-s, 45-s and 135-s data. The large scale currents are faithfully resolved at 15-s sampling. At 45-s sampling one usually has at least one data point in the electrical current region, but detection is only marginally certain. At 135-s sampling one will have roughly one sample within the current region out of every two crossings. Thus, one detects the presence of the auroral currents roughly half of the time. Thus, although very coarse time resolution data makes it impossible to set a lower limit on the current intensity (since it is likely that one misses the signature completely), one can compile a large number of crossings to get an average sense of the currents. Although not ideal this works if the currents do not change a great deal during the accumulation interval.

Figure 5

Figure 6. This shows the first Iridium data processed to look for Birkeland current signatures. At this time data from one orbit plane were used. The figure shows two orbits of data from three satellites. The spikes in the cross track magnetic field are marked by dots (bottom panel). The upper two panels show the magnetic local time and magnetic latitude of the three satellites and dots are placed on these traces where the spikes occur. It is clear that the spikes occur at a given satellite when it crosses through the same location in space as where the other satllites detect spikes, consistent with a Birkeland current signature.

Figure 6

Figure 7. Top panel shows magnetic latitude for 6 Iridium satellites for a 3 hour 45 minute period on March 9- 10, 1999. At this time vehicles 81 and 80 recorded data at fairly high rates, 15s, whereas the other vehicles recorded data at 185s resolution. All vehicles are in the same orbit plane. Bottom panel shows the cross track magnetic perturbations for all six satellites where the time series have been shifted so that the MLAT time series exactly overlap. The fact that the time series from 81 and 80 yield nearly identical signatures indicates that the auroral currents were fairly stable at this time. The signature bears a striking similarity to those in the previous figure recorded by DMSP showing that the Iridium satellites do indeed detect the auroral current signatures. In addition, the satellites sampled very coarsely in time yield perturbations just as one expects from the previous figure, when in the current region, the points follow the perturbations recorded by 80 and 81. (There is one outlier at low latitudes from 41.) Also remarkable is the DC offset apparent over the polar cap, 2200-2210 UT and 2250-2300 UT. This is a real feature which I was not expecting to be able to detect since it is at the lower limit of the sensor sensitivity. However, it looks like we are doing better than I expected and this feature is being observed.

Figure 7