The sun is known to be responsible during at least some solar flares for sharp increases in the cosmic-ray intensity on earth. In contrast to the flare pulses, the other principal changes in cosmicray intensity are all smaller in amplitude, less steep in energy dependence (extending perhaps to 30 Bev/c), much slower in time scale, and very much more isotropic (worldwide). These latter changes are not correlated sharply with any conspicuous solar event. A common cause for all of the nonflare cosmic-ray changes is here proposed: time (and energy) modulation of the incoming cosmic-ray beam by the random diffusion of the particles through turbulent clouds of magnetized plasma emitted from the sun. The sporadic cosmic-ray strom decreases (Forbush events) occur when the earth is immersed in a strong fresh cloud following a solar flare. The eleven-year variations in intensity and low-energy spectrum arise from the combined effect of much weaker and more diffuse clouds which travel across the solar system to the region of the outer planets. The recurrent 27-day effects depend on the presence near the earth of a region of longer diffusion mean free path, which punches a channel into the pre-existing clouds. The origin of the clouds in active centers and the channels in magnetic monopole regions of the sun is discussed, and parameters obtained which describe the clouds, and their motions, adequate to produce the observed effects and quantitatively, astrophysically plausible. The small but complex diurnal cosmic-ray variations are not discussed in detail, but appear to fit in naturally. The observed variation of all these effects throughout one solar cycle agrees with the picture presented.

• Received 20 October 1955

DOI:https://doi.org/10.1103/PhysRev.101.1397