Cosmic ray density gradients related to north‐south asymmetry in activity on the Sun

Abstract

Data are presented from underground cosmic ray telescopes in both the northern and southern hemispheres from 1965 to 1983, analyzed as a function of interplanetary magnetic field (IMF) sense, to examine the long‐term properties of the cosmic ray density gradient perpendicular to the ecliptic plane, ▽Np. Yearly cosmic ray solar diurnal variation vectors for IMF (B) away from and toward the sun are obtained; these vectors include a contribution from the B × ▽Np anisotropy, from which the predominant direction of the perpendicular gradient, ▽Np, is inferred. These data are compared with the north‐south asymmetry in activity on the sun for the same period, based both on comprehensive flare index data and on solar events associated with type II radio emission. The cosmic ray data up to the 1969–1971 solar polar field reversal are consistent with a perpendicular cosmic ray gradient pointing southward, observed by telescopes in both hemispheres. The solar activity data indicate a statistically significant excess of activity in the sun's northern hemisphere from 1959 through 1970, which may well be the cause for this southward perpendicular cosmic ray gradient during this period. After the 1969–1971 reversal the solar activity data become reasonably symmetrically divided between the two solar hemispheres. The northern hemisphere cosmic ray data show some evidence of a northward pointing cosmic ray gradient after 1971, while data from the southern hemisphere telescopes continue to indicate a southward pointing gradient. These results can be accounted for if the resultant perpendicular cosmic ray gradient is the sum of two gradients—a north‐south symmetric gradient that reverses upon reversal of the sun's polar magnetic field, along with a larger north‐south asymmetrical gradient that is due to north‐south asymmetry in activity on the sun.