Galactic cosmic ray intensity from 1 to 9 A.U.

The galactic cosmic ray intensity at Pioneer 10 has been compared to that at Pioneer 11 over the respective ranges of heliocentric radial distances 3.9 < r 10 < 9.0 and 1.0 < r 11 < 5.0 astronomical units (A.U.) during the period April 1973 — January 1976. The dominant variation is a coherent temporal one with a peak‐to‐peak amplitude of about 35 percent of the mean. A further cyclic variation of about 8 percent peak‐to‐peak has a 26‐day period and is attributed to solar‐corotating interplanetary magnetic field structures. After detailed time‐lag analysis the ratios of intensities at the two spacecraft yield a radial gradient G = +2.0 (± 0.5) percent per A. U., integral above a particle energy E = 80 MeV/nucleon and over the differential radial distance range 1.2 < (r 10 ‐ r 11 ) < 5.3 A.U.