Simulation studies of ionospheric electric fields and currents in relation to field‐aligned currents, 1. Quiet periods

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Abstract
Computer simulation studies of the electric fields and currents in the global ionosphere produced by field‐aligned electric currents for quiet periods are conducted. The steady state equations for current conservation are solved numerically by assuming (1) several divided regions of the global earth (such as the polar cap, auroral zone, and middle‐low latitudes), (2) exponentially distributed anisotropic electric conductivities for each zone with a continuous change at the boundaries of the regions, and (3) exponentially distributed downward and upward field‐aligned current intensities in the auroral region, assumptions based on our current knowledge of auroral phenomena and geomagnetic variations as well as rocket and satellite measurements of field‐aligned currents. Resultant computer‐plotted diagrams include equipotential contours of the electric fields, vector distributions of the electric fields and currents, and electric current patterns equivalent to the magnetic field effect produced by the field‐aligned and real ionospheric currents. One of the merits of this simulation method is that the three‐dimensional current system can roughly be estimated from the equivalent current system obtained from ground‐based geomagnetic data alone. This paper also provides a foundation for a similar study of substorms. The following main results are obtained: (1) Conductivity inhomogeneity alters considerably the electric field pattern that has previously been obtained by assuming the uniform conductivity distribution. (2) Even a slight conductivity enhancement along the nightside auroral belt results in a large modification of the electric field. (3) The existence of the strong conductivity gradients and the field‐aligned currents in the equatorward half of the auroral oval reduces the electric field in the middle and low latitudes. This corresponds to the ‘shielding’ effect of the electric field inside the Alfvén layer in the magnetotail. (4) Seasonal changes in the polar cap conductivities cause surprisingly large effects on the electric fields and currents. (5) The equivalent ionospheric currents differ significantly from real ionospheric currents in both intensity and direction.