Radial gradient of cosmic ray intensity from a comparative study of data from Voyager 1 and 2 and IMP 8

The IMP 8 satellite and Voyager 1 and 2 space probes obtained cosmic ray measurements during the late 1977 to mid-1982 period. Comparisons of 27-day averages of the data show that a positive radial intensity gradient existed on the average during this period, and that the cosmic ray intensity decrease toward solar maximum in 1980-1981 proceeds in a stepwise fashion. The cosmic ray minimum reached in late 1980/early 1981 appears almost simultaneously at 1 AU and at 10 AU, with and without propagation time delay effects, between IMP 8 and the Voyager spacecraft. These data are generally consistent with a heliolatitudinal gradient of 0 + or - 1 percent/deg. Attention is given to the implications of these results in the overall context of cosmic ray modulation theory.