COSMIC-RAY DECREASES AND SHOCK STRUCTURE - A MULTISPACECRAFT STUDY

We examine >60-MeV/amu ion data from three spacecraft (IMP 8 and Hello s 1 and 2) at the time of a number of shortterm (<20-day duration) cos mic ray 1 decreases (>1 GeV) detected by ground-based neutron monitors in the years 1976 to 1979. The multispacecraft data allow us to inves tigate the structure of the modulation region and in particular the re lative importance, as a function of location, of the shock and shock d river (ejecta) in causing the reduction in particle densities. Althoug h the shocks contributing to cosmic ray decreases often have particle enhancements associated with them in the >60-MeV/amu data, this is not the case for three of the events discussed in this paper where a shoc k-associated decrease is also evident. Whereas the shock can cause an increase or decrease at low (i.e., less than neutron monitor) energies , the reduction of particle densities in the driver, if it is intercep ted, is usually evident at all energies. Thus the overall shape of a d ecrease at >60 MeV/amu depends primarily on whether the ejecta is inte rcepted. We find that the particle density inside ejecta increases wit h increasing radial distance from the Sun. In many of the events in th is study, entry and exit of ejecta are accompanied by abrupt changes i n the decrease and recovery rates which indicate that the effect of th e ejecta is local. In contrast, the effect of the shock lasts many day s after the shock has passed by and is evident at large angular distan ces from the longitude of the solar source, i.e., the effect of the sh ock is nonlocal. Within 1 AU there seems to be no radial dependence of the shock effect. One cosmic ray decrease seen at Earth, which had an unusual profile, can be understood if the median plane of the ejecta was inclined to the ecliptic.