On calculating the electric and magnetic elds produced in technological systems at the Earth’s surface by a “wide” electrojet

Forecasting the geomagnetic e ects to technological systems on the ground requires rapid calculations of the electric and magnetic elds produced by the auroral electrojet. The electrojet is often modelled as a line current at a height of 100 km, but in reality it has a nite width and is typically spread over 5◦ or 6◦ of latitude. We show that the width of the electrojet can easily be included in electric and magnetic eld calculations by assuming that the ionospheric current density has a Cauchy distribution with a half-width a; j(x)=(I= )a=(x +a), at a height h. It is shown that the electric and magnetic elds produced at the surface of a layered earth by such a current are equivalent to the elds produced by a line current I at a height h+a. This equivalence, combined with the complex image method, leads to simple formulas that provide a method for fast calculation of the electric and magnetic elds that can a ect ground-based systems. Crown Copyright c © 2000 Published by Elsevier Science Ltd. All rights reserved.