Modelling the electric and magnetic fields at the Earths surface due to an auroral electrojet

Abstract

A calculation of the electric and magnetic fields at the Earths surface due to an ionospheric electrojet system is required for the estimation of geomagnetically induced currents (GIC) in a technological network, which are a manifestation of space weather effects on the ground. Two models of the auroral electrojet for calculating the geoelectromagnetic field are compared in this paper: an infinitely long line current and a more realistic system consisting of an electrojet of a finite length and of vertical (field-aligned) currents at its ends. The Earth is described by a two-layer half-space, and a small and a large period of oscillation in time are considered. In the case of an infinite line current the field at the Earths surface is independent of the space coordinate parallel to the electrojet, and in the case of the finite electrojet two profiles perpendicular to the electrojet and located at the centre of the electrojet and near the edge of the electrojet are considered. The most significant shortcoming of the infinite model is the neglect of the horizontal electric component perpendicular to the electrojet as well as the horizontal magnetic component parallel to the electrojet. It is shown in this paper that these components may in reality get values in the order of 0.1–1 V⧹km and 100–200 nT, respectively. The parallel electric component and the perpendicular horizontal magnetic component are overestimated by the infinite model by about 10–50% at their maxima achieved beneath the electrojet. At large distances the infinite model leads to an incorrect sign of these components. The vertical magnetic component is accurately obtained by the infinite model.