Cosmic ray decreases: 1964–1994

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Abstract
We have studied 30 years (1964–1994) of neutron monitor data in order to understand the principle mechanisms causing short‐term (< 20‐day duration) cosmic ray decreases seen at Earth. By examining the characteristics of associated low energy (<200 MeV) particle enhancements in combination with the neutron monitor data, we have determined the responsible solar wind disturbances for 153 of the 180 ≥ 4% decreases. The vast majority (86% of the 153 events) are caused by coronal mass ejections and the shocks that they generate. The ejecta is intercepted only when the solar event originates within 50° of the Sun's central meridian. For more distant events, only the shock is intercepted at Earth. We present a list 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 structures based on in situ solar wind data, if available. This list represents a compendium of major solar wind disturbances affecting a large section of the inner heliosphere over this time period. We also discuss enhanced daily variations in some events which are related to radial gradients caused by strong disturbances inside the Earth's orbit.