Steplike Changes in the Long-Term Modulation of Cosmic Rays

An investigation is reported of the rigidity dependence of the solar modulation of relativistic cosmic-ray particles at the orbit of Earth by comparison of the monthly averages of the neutron monitor at Deep River at geomagnetic cutoff rigidity 1 GV with those of the neutron monitor at Kula at cutoff 13 GV during the period of increasing solar activity which began in 1964-1965. The constancy with time of the efficiency of each monitor was confirmed to better than 0.5 percent by comparisons with other monitors near the same respective cutoffs. The regression curve expected between the monitor at Deep River and the monitor at Kula was calculated from the results of latitude survey by the method of coupling constants assuming that the modulating parameter (r, t) was independent of rigidity P and that the form of the modulation function wasf(P, P) = P . The monthly experimental points fitted the regression line calculated for a = 1 until December 1966; but thereafter they suddenly (between successive months) formed, for periods varying from 6 to 18 months, new regression lines which appeared to have about the same slope as the a = 1 line but were strongly displaced laterally. The a = I slope of the separate successive segments of the regression is attributed to the local modulation in the vicinity of the Sun associated with Forbush decreases and 27-day variations. The steplike long-lasting changes between the successive segments of the regression are believed to be demonstrated here for the first time. They are probably associated with the state of the interplanetary medium far beyond the orbit of Earth. Statements in the literature that the form of the modulation function is independent of time and that the modulating parameter is independent of rigidity appear to have been premature.