Estimation of ionospheric electric fields, ionospheric currents, and field‐aligned currents from ground magnetic records

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Abstract
An approximate method of separating the effects of ionospheric currents from those of field‐aligned currents in ground magnetic perturbations observed in high latitudes is developed. The distribution of ionospheric electric fields can also be estimated. The procedure includes the following steps: (1) the calculation of the equivalent ionospheric current function on the basis of magnetic H and D component records on the earth's surface, (2) the computation of the electric potential distribution from the equivalent ionospheric current function by using a simple model of the ionospheric conductivity, (3) the derivation of ionospheric current vectors as well as electric fields and, (4) the derivation of the field‐aligned current intensity by taking the divergence of the ionospheric currents. Several examples for both quiet and disturbed conditions are utilized to demonstrate how our method is successful in estimating the intensities of the electric fields and the ionospheric and field‐aligned currents in the sense that the estimated values are in good agreement with those observed recently by radar and satellite methods. Significant portions of the H component in nightside auroral latitudes appear to result from the east‐west ionospheric currents, called the auroral electrojets, while both the north‐south ionospheric current and field‐aligned current are almost equally important in producing the D component excursions. It is found that the ionospheric and field‐aligned current distributions obtained are not very sensitive to the choice of the ionospheric conductivity model, unless the auroral enhancement is not given in an appropriate place. This indicates that even a simple conductivity distribution inferred from the distribution of the magnetic perturbations can make it possible to estimate the three‐dimensional current system with a reasonable accuracy.