Three-dimensional observations of Birkeland currents
The dominant coupling between the ionosphere and magnetosphere is now understood to be Birkeland currents. During substorms the Birkeland current system in the midnight sector is generally interpreted as a cross-tail current interruption and diversion along magnetic field lines to the conducting ionosphere, which carries the intensified auroral electrojet. In the present study, the equatorial current diversion, the Birkeland current intensification, and the auroral electrojet increase in activity were observed to be initiated simultaneously at the onset of substorms. The principal finding is that the field-aligned currents of the components of the current wedge have been observed simultaneously on the ground, at low-altitude satellite orbits, and at geosynchronous satellite orbits. Recent dynamic modeling work by Chen et al. (1982) appears to support our findings. These observations contribute significantly toward the confirmation of the substorm current wedge model and complement and extend the results obtained by Nagai (1982).