EFFECT OF INTERPLANETARY SHOCKS AND MAGNETIC CLOUDS ON ONSET OF COSMIC-RAY DECREASES

A detailed analysis has been carried out to study the onset times of cosmic-ray decreases occurring during 1978-1982 with respect to the arrival times of interplanetary shocks and magnetic clouds. The observations demonstrate that shocks, magnetic clouds and a combination of both could effectively trigger a cosmic-ray decrease when they are associated with turbulent sheaths of maximum thickness less-than-or-equal-to 15.0 hr (less-than-or-equal-to 0.15 AU). Further, the shocks associated with enhanced solar wind velocity produce a fast decrease and the magnetic clouds accompanied by extended and enhanced magnetic field produce a slow decrease. The decrease, non-correlated with the arrival times of shocks and magnetic clouds, represents a corotating cosmic-ray decrease produced by corotating streams.