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World-wide changes in the geomagnetic field Nishida, Atsuhiro

Abstract

The geomagnetic field is found to change quite frequently on a world-wide scale. In the three months' period near sunspot maximum, such changes are found on 90 "per cent of all days. Most of these changes are not registered either as sudden commencements or as sudden impulses, and are tentatively called in this thesis 'worldwide changes'. The frequent occurrence of world-wide changes seems to be consistent with the idea that world-wide features of the geomagnetic field are always influenced by a permanently flowing corpuscular stream from the sun. The physical state of the corpuscular stream may be as variable as that of the solar atmosphere, and sudden changes in it will give rise to sudden, world-wide changes in the geomagnetic field. The morphology of world-wide changes is studied, and the form of the change, the distribution of magnitude and the mode of spreading over the earth are clarified. It is found that world-wide changes can be classified into two groups according to the sign of the main part of the change which appears all over the world. Those with an increase in the total force are called positive changes and those with a decrease are called negative changes. Except for the sign of the change, negative changes are morphologically identicalto positive changes. Since the morphology of sudden commencements and sudden impulses is the same as that of world-wide changes, they must be produced by a common mechanism, and an explanation of negative changes is a new, fundamental requirement imposed upon any theory of these changes. The observed change in the geomagnetic field may originate at the magnetospheric boundary where the solar corpuscular stream interacts with the geomagnetic field. The change may be modulated by the screening effect of the ionosphere before it is observed at ground level. Although this effect has been shown to be negligible for changes with a time scale of the order of world-wide changes, incorrect assumptions have been made in existing theories. More accurate calculations show that this effect is actually significant for a certain mode of the incident field. [ ... ]

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