COMPARISON OF THE RIGIDITY DEPENDENCE OF THE 11-YEAR COSMIC-RAY VARIATION AT THE EARTH IN 2 SOLAR-CYCLES OF OPPOSITE MAGNETIC POLARITY

We have found that there is a significant difference in the rigidity d ependence of the 11-year variation of the galactic cosmic rays in the rigidity interval from 0.6 to 100 GV between the A > 0 modulation cycl e from 1977 to 1981 and the A < 0 cycle from 1987 to 1990. The overall intensity decrease from 1977-1981 is found to have a rigidity depende nce given by F(P) = exp (-1.46xP(-1.22+/-0.05)), where F (P) is the mo dulation function and P is the rigidity [GV] and for 1987-1990 F(P)= e xp (-1.75 x P--1.02+/-0.05). For the conventional diffusion-convection theory of modulation the rigidity dependence of the modulation is equ al to the rigidity dependence of the interplanetary diffusion coeffici ent. For a selected number of Forbush decreases we found that the rigi dity dependence of the decreases occurring near the period of minimum solar modulation was steeper than for those occurring later in the sol ar cycle. In no case was the rigidity dependence of any transient decr ease much different from that of the 11-year variation, however. The c rossover in the 1977 and 1987 primary proton energy spectra at E simil ar to 400 MeV observed near sunspot minimum shifts to similar to 650 M eV at the time of the maximum solar activity or minimum cosmic ray int ensity.