Forbush decreases and interplanetary magnetic field disturbances: Association with magnetic clouds

Magnetic clouds have been proposed as a mechanism to produce Forbush decreases in the cosmic radiation. We have examined the temporal association of magnetic clouds and Forbush decreases and find practically no association of the main phase of the Forbush decrease with the arrival of a magnetic cloud. On the other hand, Forbush decreases generally follow the strong interplanetary shocks which sometimes precede magnetic clouds. The main phase of the cosmic ray decrease occurs 2–5 hours after the shock and during the passage of the region in which the magnetic field is disturbed. It appears that a Forbush decrease is more likely to occur following a shock in which the magnetic field and plasma parameters are strongly enhanced. These results indicate that the decrease of the cosmic ray intensity may be produced by the smaller diffusion coefficient in the region behind the shock. The sweeping effect of the enhanced magnetic field associated with the fast shock also probably contributes to the rapid depression of the cosmic ray intensity seen in some decreases.