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

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 statistical distribution of the solar particle flux for three different 3-yr periods during the solar cycle according to mean sunspot-latitudes of 20° 15° and 10°. The distribution in heliographic latitude is given by two Gaussian functions centered at the heliographic latitudes of the active zones, as defined by the mean latitudes of the sunspots. These models explain the observed semiannual geomagnetic variations, except for a phase-lag of 20 to 30 days. We suggest that a semiannual density variation of charged solar particles which are trapped in the earth's magnetosphere can provide the required phase-lag mechanism. This implies a lifetime for these particles of the order of one month. The models can also provide a quantitative explanation of the observed delay of geomagnetic activity behind solar activity during the 11-yr cycle. Furthermore, they are in reasonable agreement with preliminary results on the statistical latitude distribution of cornets with ionized tails. A space probe in a heliocentric orbit is proposed in order to obtain further evidence on the latitude distribution of solar particle streams.