Sudden commencements of magnetic storms and atmospheric dynamo action

Abstract

A statistical investigation of world-wide sudden commencements of magnetic storms has been carried out using data from over 30 magnetic observatories distributed all over the world. An appreciable diurnal change in the amplitude of SC's has been found, and the average electric current system for the Ds field shows conspicuous current concentrations in the polar regions. The pattern of this current system is similar to that caused by an electric doublet centered on the highly conducting region near the geomagnetic-pole, and hence it is probable that the current system exists within the earth's atmosphere. On the other hand, the Dst field of SC's seems more likely to be of extraterrestrial origin. An atmospheric dynamo theory has been applied to interpret this Ds current system, on the assumption that the main source of electromotive force generation is due to the enhancement of electrical conductivity in the polar region. The change at a SC is so abrupt (within a few minutes) that it is reasonable to assume that the wind system in the ionosphere will not change. Thus, using the wind system estimated from the Sq field, the current system at the time of commencement of the storm has been computed, assuming an appropriate change in the electrical conductivity. Good agreement with observed results has been obtained, and it suggests that a dynamo action in the upper atmosphere is the dominant cause of geomagnetic variations during disturbances. Moreover, there has been found a consistent wind system which can produce the observed geomagnetic variations both for quiet, and disturbed conditions with a reasonable range of conductivity changes in the polar regions. This wind system consists of both a diurnal and a Semidiurnal term, and the estimated order of magnitude agrees with recent ionospheric measurements.