COSMIC-RAY DECREASES - 1964-1994

We have studied 30 years (1964-1994) of neutron monitor data in order to understand the principle mechanisms causing short-term (< 20-day du ration) cosmic ray decreases seen at Earth. By examining the character istics of associated low energy (<200 MeV) particle enhancements in co mbination with the neutron monitor data, we have determined the respon sible solar wind disturbances for 153 of the 180 greater than or equal to 4% decreases. The vast majority (86% of the 153 events) are caused by coronal mass ejections and the shocks that they generate. The ejec ta is intercepted only when the solar event originates within 50 degre es of the Sun's central meridian. For more distant events, only the sh ock is intercepted at Earth. We present a fist of all 180 events seen in the years 1964-1994 together with the associated solar event, when this can be determined, and some details about the solar wind structur es based on in situ solar wind data, if available. This list represent s a compendium of major solar wind disturbances affecting a large sect ion of the inner heliosphere over this time period. We also discuss en hanced daily variations in some events which are related to radial gra dients caused by strong disturbances inside the Earth's orbit.