The heliocentric radial gradient in cosmic ray density and the ‘swinson’ sidereal time variation

Abstract

The sidereal time variation reported by Swinson depends on the existence of a heliocentric radial gradient of cosmic ray density in the rigidity range ≲ 100 GV and appears because of the inclination of the axis of rotation of the Earth to the normal to the ecliptic plane. It is sensitive to the polarity of the interplanetary magnetic field. Meson detectors on the surface of the Earth near the equator and at shallow underground depths can be used to measure this effect. In this paper results obtained at Makerere on the Earth's surface and at Kilembe (50 m.w.e.) in Uganda, East Africa, are compared with data previously reported by Swinson from Chacaltaya (25 m.w.e.) and Embudo (40 m.w.e.). From the relationship between the sidereal time variation and the heliocentric radial gradient it is concluded that the data are consistent with a local radial gradient of ( (41 ± 8) P) % A.U. −1 in the rigidity range 15 < P ≲ 100 GV during 1967–1971, a period including the most recent solar maximum. This estimate is consistent with modulation theory and the prevailing power spectrum in the interplanetary magnetic field. The Swinson effect is not appreciable at depths of more than 50 m.w.e. underground.