The performance of the EDI instrument can be demonstrated with a few examples. Figure 1 shows, for a time interval of 0.5 seconds on 20 October 2000, the electron firing directions from both electron guns that returned the beam onto the associated detectors after one gyration in the magnetic field. The plane of the figure is perpendicular to the ambient magnetic field. As the satellite is oriented obliquely with respect to the field, its projection is an ellipse rather than a circle. During the 0.5 sec interval the spacecraft rotated by 45 degrees.
The quality of the measurements is documented by the fact that the firing directions cross over in nearly a single point. The vector d from the cross-over to the center of the spacecraft is proportional to the ambient electric field. The quantity d is proportional to the E/B2, wher E is the electric field and B the magneticm field.
Figure 2 shows a sequence of four such 0.5 sec snapshots, spaced 10 minutes apart. During the 40-minute interval the drift step increased from about 1 m to about 4 m.