Forbush decreases produced by diffusive deceleration mechanism in interplanetary space

Transient decreases in the intensity of galactic cosmic radiation are known to be associated with the arrival at the earth of large clouds of magnetized solar plasma. The large scale of the phenomenon indicates that the clouds expand significantly while crossing the solar system. An interpretation is given based on the assumption that the plasma is magnetically turbulent. Cosmic rays enter the cloud, then execute a random-walk motion in space. While within the expanding cloud they must lose energy in an inverse Fermi mechanism and by betatron action in the weakening magnetic field. The energy spectrum within the cloud is depressed below the galactic spectrum. The spectrum is calculated in the isotropic approximation using age-diffusion theory, considering only the effects of spatial diffusion and energy loss. Convection is neglected. Several prominent features of observed decreases are thereby accounted for, i.e., the time profile, the magnitude, and the approximate rigidity dependence.