Electromagnetic induction in the Earth by moving ionospheric current systems

Ground-based observations of magnetic disturbance fields at auroral latitudes, as well as at lower latitudes, have, for a number of years, led workers to infer the rapid spatial movement of electric current systems in the ionosphere. However, relatively little attention has been directed toward understanding the associated electromagnetic induction problem, namely the effects on electric- and magnetic-field components observed at the Earth's surface, caused by time-varying current systems moving above the earth of finite conductivity. The following discussion considers this problem and presents a method for determining the electromagnetic field components at the surface and within the interior of an n</it>-layered earth for a current system flowing east—west and moving with a uniform velocity to the north or south. The results for a line current source, which are given in detail, are used to derive expressions for the fields of a Gaussian electrojet, a ribbon electrojet and several asymmetric electrojets. This development differs substantially from previous work in that one can allow independent time variations of the source strength itself, which are essentially decoupled from the effects of its motion. Based on our own routine inspection of high-latitude magnetograms, we have found that moving sources are the rule and stationary sources are the exception. Theoretical calculations imply significant spectral broadening of temporal variations from moving sources relative to their stationary counterpart. Synthetic magnetograms visually demonstrate temporal contraction of ground-based observations relative to the true temporal morphology of the source itself. Moreover, there are significant phase lags and leads introduced between local field maxima observed on the ground and the time of the actual maximum strength of the source itself. Finally, magnetotelluric measurements are affected in a positive way by moving sources, in that the source appears to have a larger ‘effective width’ through averaging induction effects over a larger volume of the Earth during its lateral movement. For a harmonically time-varying source having a width of 500 km and a lateral movement of 500 m/s, the magnetotelluric relation appears valid to periods as long as 5 hr.