Estimate of electromagnetic quantities in space from ground magnetic records.

Magnetic field records at the earth's surface contain the effects of a variety of different source currents flowing in outer space. During the last decade, several computational techniques for the use of magnetometer data have been proposed to obtain the global pattern of the electric fields and currents and Joule heating in the ionosphere. The numerical algorithm has now been enormously improved, such that simultaneous more direct or in situ measurements by satellites and radars can be incorporated into the scheme. In particular, owing to recent progress in satellite-viewed auroral images, it is possible to estimate "instantaneous" patterns of the ionospheric electrodynamic quantities over the entire polar region with a time resolution of several minutes. This report demonstrates that, contrary to our conventional views, large ionospheric currents can flow in regions of low auroral activity.