Cosmic ray decreases and particle acceleration in 1978‐1982 and the associated solar wind structures

Abstract

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 associated with passages of interplanetary shocks, the majority of which accelerate particles. We use the intensity‐time profiles of the particles to separate the cosmic ray decreases into four classes which we subsequently associate with three types of solar wind structures. Decreases in class 1 (15 events) and class 2 (26 events) are associated with shocks driven by energetic coronal mass ejections. For class 1 events, the ejecta is detected at 1 AU, whereas this is not usually the case for class 2 events. The shock must therefore play a dominant role in producing 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 of the ejection from the Sun (and hence the location) can be determined from the rapid onset of energetic particles several days before the associated shock passes the Earth. Class 1 events originate from within 50° of central meridian while class 2 are predominantly from sources beyond 20° from central meridian. Class 3 events (10 decreases) can be attributed to less energetic ejections which are directed towards the Earth. In these events the ejecta is more important than the shock in causing the cosmic ray intensity depression. The remaining events (14% of the total) can be attributed to solar wind periods with multiple disturbances. All include ejecta material and at least half also 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, consideration of particle observations over a wide range of energies is necessary for a full understanding of cosmic ray decreases.