Characteristics of Large Forbush Decreases Associated with Interplanetary Magnetic Clouds

Abstract

Abstract Three categories of magnetic clouds, i.e. clouds associated with shocks, with stream interfaces and with cold magnetic enhancements, have been utilized. Differences in amplitudes and profiles of cosmic ray decreases in association with these three types of clouds are found. Large Forbush-type decreases are associated with clouds preceded by a shock, whereas clouds followed by an interaction region produce smaller decreases in cosmic ray intensity. Those clouds associated with cold maanetic enhancement may produce a decrease of still smaller amplitude. Forbush-type decreases seem to be due to shocked plasma ahead of the maanetic cloud; the tangential discontinuitv and magnetic clouds may not be the main causes of these decreases. Using magnetic clouds identified durinq 1978-82 and their associated Forbush decreases we confirm our earlier result that the recovery time of Forbush decreases increases with the duration for which the magnetic cloud engulfs the earth and a simple empirical relation has also been obtained between these parameters. This relation may be used for computing the recovery time of Forbush decreases in the outer heliosphere as they propaaate away from the earth if we know the expansion rate of the clouds and acceleration/deceleration of these disturbances as they move away and as long as they retain their identity.