A study of the long-term modulation of galactic cosmic ray intensity

It is shown that the 11-yr variation of the galactic cosmic-ray intensity is linearly related to the λ5303 coronal intensity close to the solar equator (± 5° or ± 10°). The available data on the temperature of the inner corona is used to calculate the annual average solar wind velocity at low heliolatitudes (± 5°) for each year during the period 1957-1967. The 11-yr variation of the computed velocity from the solar data is linearly related to the variation of cosmic-ray intensity. In terms of the theory of diffusion-convection, the empirical results are used to estimate the cosmic-ray intensity in the nearby interstellar space and hence the residual modulation at solar minimum. The following are the important conclusions: 1. (1) The size of the modulating region (R) varies with solar cycle such that for a given rigidity P, R - r/K = const. where r = 1 a.u. and K is the isotropic diffusion coefficient. 2. (2) The residual modulation of the cosmic-ray intensity is roughly proportional to P-1. 3. (3) The modulating parameter η for 1965 is found to be slightly higher than the upper limit of 1 GV set by Gloeckler and Jokipii (1967)