On calculating the electric and magnetic fields produced in technological systems at the Earth's surface by a wide electrojet

Forecasting the geomagnetic effects to technological systems on the ground requires rapid calculations of the electric and magnetic fields 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 finite width and is typically spread over 5 degrees or 6 degrees of latitude. We show that the width of the electrojet can easily be included in electric and magnetic field calculations by assuming that the ionospheric current density has a Cauchy distribution with a half-width a, j(x)=(I/pi )a/(chi (2) + a(2)), at a height h. It is shown that the electric and magnetic fields produced at the surface of a layered earth by such a current are equivalent to the fields 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 fields that can affect ground-based systems. Crown Copyright (C) 2000 Published by Elsevier Science Ltd. All rights reserved.