North-south anisotropy and radial density gradient of galactic cosmic rays.

Abstract

The cosmic ray north-south anisotropy arising from the heliocentric radial density gradient of cosmic rays and the interplanetary magnetic field has been studied using data for a wide range of rigidity (15-210GV in median primary rigidity) during the period 1969-1973. Two components of the anisotropy; the north-south asymmetry and the associated sidereal diurnal variation have been analyzed to reveal a three-dimensional structure of the anisotropy in space. By means of the least-squares method and using improved coupling coefficients, the direction and certain other parameters of the anisotropy have been determined.Definite evidence has been obtained for the existence of anisotropic flow perpendicular to the ecliptic plane. The magnitude of the flow is found to be 0.081±0.021% and 0.072±0.018% at 10GV for the periods 1969-1970 and 1971-1973, respectively. It is also found that its rigidity spectrum is slightly rigidity dependent and has an upper limiting rigidity of 200GV or more. Based on the derived parameters of the anisotropy, the heliocentric radial density gradient for high rigidity cosmic rays has been estimated as a function of rigidity for 1969-1973 as (3.0±1.1)×(P/10)-(0.7±0.2)%/AU for P≤200GV (P is rigidity in GV). The diffusion coefficient is also derived, which seems to be consistent with those so far determined.