American Geophysical Union Become an AGU Member
Subscribe to AGU Journals		 AGU Home AGU Publications

Read Full Article    Cited by

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. A11, PAGES 21,429–21,441, 1994

Cosmic Ray Decreases and Shock Structure: A Multispacecraft Study

H. V. Cane

Also at Physics Department, University of Tasmania, Hobart, Australia.


I. G. Richardson

Also at Department of Astronomy, University of Maryland, College Park, Maryland.


T. T. von Rosenvinge

Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, Greenbelt, Maryland


G. Wibberenz

Institut für Reine and Angewandte Kernphysik, University of Kiel, Kiel, Germany


Abstract

We examine >60-MeV/amu ion data from three spacecraft (IMP 8 and Helios 1 and 2) at the time of a number of short term (<20-day duration) cosmic ray decreases (>1 GeV) detected by ground-based neutron monitors in the years 1976 to 1979. The multispacecraft data allow us to investigate the structure of the modulation region and in particular the relative importance, as a function of location, of the shock and shock driver (ejecta) in causing the reduction in particle densities. Although 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 shock-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 intercepted, is usually evident at all energies. Thus the overall shape of a decrease at >60 MeV/amu depends primarily on whether the ejecta is intercepted. We find that the particle density inside ejecta increases with increasing radial distance from the Sun. In many of the events in this study, entry and exit of ejecta are accompanied by abrupt changes in the decrease and recovery rates which indicate that the effect of the ejecta is local. In contrast, the effect of the shock lasts many days after the shock has passed by and is evident at large angular distances from the longitude of the solar source, i.e., the effect of the shock 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.

Received 20 December 1993; accepted 13 June 1994.

Read Full Article    Cited by

Citation: Cane, H. V., I. G. Richardson, T. T. von Rosenvinge, and G. Wibberenz (1994), Cosmic Ray Decreases and Shock Structure: A Multispacecraft Study, J. Geophys. Res., 99(A11), 21,429–21,441.