Space current around the earth obtained with Ampère’s law applied to the MAGSAT orbit and data

An application of Ampère’s law to the MAGSAT orbit and data enabled us to study the net space current (total intensity I) flowing through the plane enclosed by the satellite orbit, and its dependence on the ground magnetic disturbance revealed in the Kp- or AE-index. Even on magnetically quiet days, MAGSAT often (or sometimes persistently) detected minor or moderate disturbances in the polar regions, in particular inside the auroral oval, without increasing Kp- or AE-values. Such disturbances are attributable to field-aligned currents into or out of the ionosphere, which produce a magnetic field (of toroidal nature) detectable above the ionosphere associated with a weak magnetic field on the ground, resulting in an occasional poor correlation of I with Kp- or AE-indices. The calculated I-values on quiet days are shown to be the order of 10⁵ A, with a small-range UT variation. During magnetic storms or substorms the I-values become one order of magnitude greater, and the net space current is always antisunward, with its intensity roughly proportional to the AE-index values. The antisunward space current under the MAGSAT level is the Pedersen current in the ionosphere, and it constitutes the return current of the westward partial ring current (flowing at a distance of several earth radii in the dusk-side magnetosphere) along with the field-aligned currents between the partial ring current and the high-latitude ionosphere in the dayside and nightside hemispheres. The antisunward ionospheric current under the MAGSAT level contributes to a noticeable enhancement in the dawn-dusk asymmetry of the H-decrease at MAGSAT level in comparison with the ground data at the developing stage of magnetic storms or substorms.