Simulating the earth's induction effects on substorm data recorded at mid-latitude stations: The three-dimensional problem.

A major problem encountered when modeling external current systems from ground-based magnetic observations is the evaluation of the earth's induction effects. Though it is quite difficult to represent accurately the effects of induced currents flowing inside the earth, they can be approximated relatively easily under certain circumstances (i. e. when global rather than local electrical properties of the earth are concerned). For this purpose, two general classes of earth models can be used, i. e. (1) the models which reduce the earth to a superconductor (either at the surface or at some depth) and (2) the models in which the earth is represented by a body of finite conductivity (either uniform or layered). The performances of these two types of earth models are tested with respect to substorm modeling from mid-latitude data (i. e. when neither the curvature of the earth nor the three-dimensional nature of the substorm current system can be neglected). The conclusions are almost identical to those obtained previously with high latitude data (i. e. when the induction problem can be reduced to the study of a flat electrojet flowing over a flat earth). They indicate that for most problems of substorm modeling from ground-based magnetic observations, it is sufficient to treat the earth as a superconductor (“perfect” conductor).