On the annual variation of magnetic disturbance

Abstract
Statistical features of the annual incidence of magnetic disturbance, over a very wide range of disturbance intensity and latitude, are exhaustively investigated by means of theKindex and related ‘planetary’ indices. Two distinct and physically significant components are identified: (a) an annual component, with summer maximum and winter minimum; (b) a semi-annual component with equinoxial maxima. Soth components are found in all parts of the earth. The amplitude of the annual component increases markedly with latitude, while that of the semi-annual component changes little with latitude. The physical causes of the two types of variation are finally considered. The conclusions reached are (a) that the annual component is probably caused by an atmospheric dynamo effect; (b) that the semi-annual component arises because of a systematic annual variation of the angle between the earth's magnetic axis and the sun-earth line, along which travel the solar particles which cause magnetic disturbance.

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