THE RELATION BETWEEN GEOMAGNETIC-VARIATIONS AND THEIR TIME DERIVATIVES AND IMPLICATIONS FOR ESTIMATION OF INDUCTION RISKS

By using the IMAGE magnetometer network data, we compare the geomagnet ic variation field B and its time derivative dB/dt in and near the aur oral region. We show that although the auroral electrojet is the main reason for large B's, it cannot alone produce large dB/dt's, so smalle r-scale current systems are important. This is evidenced by the horizo ntal field: the geographic north component (X) is clearly larger than the eastward one (Y) in the auroral region, but dX/dt and dY/dt are ne arly equal. This result is important when geomagnetic induction risks on man-made conductors are estimated. The key quantity is the horizont al geoelectric field, and contrary to what is often assumed, it can ha ve large values in any direction, not just parallel to the electrojet.