COSMIC-RAY DECREASES AND PARTICLE-ACCELERATION IN 1978-1982 AND THE ASSOCIATED SOLAR-WIND STRUCTURES

A study has been made of the time histories of particles in the energy range 1 MeV to 1 GeV at the times of all >3% cosmic ray decreases in the years 1978 to 1982. Essentially all of the 59 decreases are associ ated with passages of interplanetary shocks, the majority of which acc elerate particles. We use the intensity-time profiles of the particles to separate the cosmic ray decreases into four classes which we subse quently associate with three types of solar wind structures. Decreases in class 1 (15 events) and class 2 (26 events) are associated with sh ocks driven by energetic coronal mass ejections. For class 1 events, t he ejecta is detected at 1 AU, whereas this is not usually the case fo r class 2 events. The shock must therefore play a dominant role in pro ducing the cosmic ray depression in class 2 events. In all class 1 and class 2 events (which comprise 69% of the total) the departure time o f the ejection from the Sun (and hence the location) can be determined from the rapid onset of energetic particles several days before the a ssociated shock passes the Earth. Class 1 events originate from within 50-degrees of central meridian while class 2 are predominantly from s ources beyond 20' from central meridian. Class 3 events (10 decreases) can be attributed to less energetic ejections which are directed towa rds the Earth. In these events the ejecta is more important than the s hock in causing the cosmic ray intensity depression. The remaining eve nts (14% of the total) can be attributed to solar wind periods with mu ltiple disturbances. All include ejecta material and at least half als o include corotating high-speed solar wind streams. It is argued that since energetic particles (from MeV to GeV energies) seen at Earth may respond to solar wind structures which are not detected at Earth, con sideration of particle observations over a wide range of energies is n ecessary for a full understanding of cosmic ray decreases.