Hydromagnetic emissions consisting of a series of overlapping wavetrains of rising frequency are explained by hydromagnetic waves of anisotropic mode propagating along the field‐aligned paths in the magnetosphere. The rising frequency of emission is attributed to the dispersive nature of the velocity of waves; the repetitive period of wavetrains is associated with the transit time of the wave packet bouncing between geomagnetic conjugate points along the line of force. It is suggested that an initiating wave is triggered by spontaneously injected high energy particles of superluminal motion and that the wave is subsequently amplified by plasma beams through the mechanism of cyclotron instabilities.

Hydromagnetic oscillations of the Earth's ionized outer atmosphere along the geomagnetic lines of force are considered. The observational evidence of world-wide geomagnetic pulsations yields the distribution of ionic density in the outer atmosphere extending beyond the ionosphere. It is found that the ion density is about 103 per cm3 at a distance of a few earth radii decreasing exponentially to a value of the order of 5 per cm3 in interplanetary space. A theoretical consideration of the temperature of the outer ionosphere is also discussed.

No 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.