Asymmetries in the Forbush decreases of the cosmic radiation: 1. Differences in onset times

Abstract

The cosmic-ray neutron intensity at a large number of stations has been analyzed to determine the differences in onset times of the large Forbush decreases occurring from 1957 to 1961. From the directional response characteristics of the neutron detectors, these differences in onset times are related to directions in space beyond the geomagnetic field. It is observed that the onset of the Forbush decrease was always earlier from west of the earth-sun line, regardless of the location on the sun of the solar flare, which was the source of the plasma cloud causing the decrease. The onset differences from the directions west and east of the earth-sun line varied from the order of minutes to a few hours. For Forbush decreases whose onset at some stations occurred before the sudden commencement of a magnetic storm, the solar plasma cloud responsible for the decrease generally originated from the western hemisphere of the solar disk. The stations recording the earlier decrease were, in general, sampling particles from west of the earth-sun line at the time of the decrease. These results are explained in terms of the configuration of the solar plasma cloud, and conclusions are drawn about the plasma velocity and the magnitude of the associated magnetic field.