Sudden Commencements of Geomagnetic Storms and Their Local Irregularities

Abstract

The vertical components of geomagnetic variations at times of sudden commencements of magnetic storms apparently show an irregular distribution over the world, in contrast to the simultaneous horizontal field-change. The geomagnetic vertical field-change depends on the topographic condition at magnetic observatories, and this seems to be explained from the viewpoint of electromagnetic induction within the earth's outermost layer. An earth model with uniform electric conductivities is the zero-order approximation, with which the behavior of sudden commencements of geomagnetic storms at continental stations can be explained. The earth seems to act as a perfect conductor to screen the geomagnetic field-change under the earth surface (I₁⁰/E₁⁰→1/2) for sudden commencements of extremely short rise-time, so far as the data of continental stations are concerned. In a higher-order approximation for the electrical conductivity-distribution of the earth's outermost layer, the conductivity is non-uniform owing to the sea-land contrast over the world. When the non-uniformity of electric conductivity in the earth's outermost layer is simply taken into account, the observed tendency of the vertical geomagnetic variation at most non-continental stations can also be reasonably explained by the electromagnetic induction, except in special areas such as the Japan Islands and the Antarctic coast.