Galactic cosmic-ray intensity to a heliocentric distance of 18 AU

The paper reports observations to heliocentric radial distances of 8.6 and 18.4 AU with Pioneer 11 and Pioneer 10 respectively. During a seven year period from March 1972 to March 1979, the galactic cosmic-ray intensity of greater than 80 MeV, as measured by detectors on Pioneers 10 and 11, exhibited aperiodic temporal variations by about a factor of 2 and on a time scale of the order of a year, and quasipersistent cyclic variations of a 26 day period and an amplitude of a few percent. For protons of an energy greater than 80 MeV, there is a fairly consistent heliocentric radial gradient of +2.1 (plus or minus 0.3%) per AU in integral intensity until 1978 April-May, at which time a substantial disruption of the distribution of cosmic rays in the heliosphere occurred.