COMPLEX IMAGE METHOD FOR CALCULATING ELECTRIC AND MAGNETIC-FIELDS PRODUCED BY AN AURORAL ELECTROJET OF FINITE-LENGTH

The electromagnetic field due to ionospheric currents has to be known when evaluating space weather effects at the earth's surface. Forecast ing methods of these effects, which include geomagnetically induced cu rrents in technological systems, are being developed. Such application s are time-critical, so the calculation techniques of the electromagne tic field have to be fast but still accurate. The contribution of seco ndary sources induced within the earth leads to complicated integral f ormulas for the field at the earth's surface with a time-consuming com putation. An approximate method of calculation based on replacing the earth contribution by an image source having mathematically a complex location results in closed-form expressions and in a much faster compu tation. In this paper we extend the complex image method (CIM) to the case of a more realistic electrojet system consisting of a horizontal line current filament with vertical currents at its ends above a layer ed earth. To be able to utilize previous CIM results, we prove that th e current system can be replaced by a purely horizontal current distri bution which is equivalent regarding the total (= primary + induced) m agnetic field and the total horizontal electric field at the earth's s urface. The latter result is new. Numerical calculations demonstrate t hat CIM is very accurate and several magnitudes faster than the exact conventional approach.