A study of the long‐term variation and radial gradient of cosmic rays out to 23 AU

An examination of the long‐term modulation of cosmic rays with E > 60 MeV from 1972–1980 using data from the Pioneer, Voyager and IMP spacecraft has revealed that (1) the long‐term modulation effects are propagated outward radially from the sun with a typical speed of 350–500 km/s, (2) the average integral radial gradient during this period between 2 and 23 AU is (2.85±0.5)%/AU, and (3) the boundary of the heliocentric modulation region in the ecliptic plane as derived from simple conventional modulation theory is >65 AU during this time period. The implications of these results to cosmic ray modulation theories are discussed.