An investigation of the Forbush decreases in the cosmic radiation

Abstract

An analysis has been made of the large and rapid Forbush decreases in the cosmic radiation which occurred from 1954 to 1959. Data from the IGY network of cosmic-ray stations have been utilized to determine the changes in the primary rigidity spectrum, any significant differences in the onset times, and the existence of anisotropies during the decreases. To evaluate changes in the primary spectrum during a decrease it is necessary to know the spectrum before the event. It has been found that this long-term variation is adequately described by a modulation of the primary spectrum given by 1-C(t)P−1, where 1≤C(t)≤2.5 and C(t) is a function of time in the solar activity cycle. For individual decreases the modulation is 1-{C(tF)P−1+kF}, where 0≤kF≤4 per cent for nucleonic detectors at sea level and mountain elevations and 0.6≤C(tF≤2.2. No systematic variations of the onset times were evident, except for the smaller decreases where the effect of any superimposed daily variation is important. The primary radiation remained isotropic during the large decreases. These events were preceded within 3 hours by a sudden commencement geomagnetic storm. These results are discussed in terms of possible solar-modulating mechanisms.