On the Semiannual Variation of Geomagnetic Activity and its Relation to the Solar Corpuscular Radiation

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Abstract
The semiannual variation of geomagnetic activity has been in the past tentatively explained by two different theories, the so-called “equinoctial hypothesis” and the “axial hypothesis.” The latter relates the effect to the inclination of the sun's axis with respect to the sun-earth line, i.e., to the annual variation of the heliographic latitude of the earth. From a reëxamination of the data for geomagnetic activity calculated by Bartels (1932, 1940) we have found a strong modulation of the amplitude of the semiannual variation during the eleven year period of the solar cycle, which is apparently related to the heliographic latitudes of the sunspot-zones and provides indirect support for the axial hypothesis. The amplitude of the modulation is 13 per cent during the increasing phase of solar activity, when the mean sunspot latitude is 20°; it reaches 27 per cent during the late decreasing phase when the mean sunspot latitude is 10°. Three axial-symmetric models, are derived, which give the statis... Abstract The semiannual variation of geomagnetic activity has been in the past tentatively explained by two different theories, the so-called “equinoctial hypothesis” and the “axial hypothesis.” The latter relates the effect to the inclination of the sun's axis with respect to the sun-earth line, i.e., to the annual variation of the heliographic latitude of the earth. From a reëxamination of the data for geomagnetic activity calculated by Bartels (1932, 1940) we have found a strong modulation of the amplitude of the semiannual variation during the eleven year period of the solar cycle, which is apparently related to the heliographic latitudes of the sunspot-zones and provides indirect support for the axial hypothesis. The amplitude of the modulation is 13 per cent during the increasing phase of solar activity, when the mean sunspot latitude is 20°; it reaches 27 per cent during the late decreasing phase when the mean sunspot latitude is 10°. Three axial-symmetric models, are derived, which give the statis...

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