Cosmic-ray gradients from Pioneer-10 and Pioneer-11

The University of California at San Diego Cerenkov counters on the Pioneer-10 and Pioneer-11 spacecraft en route from Earth to Jupiter detect cosmic-ray protons and ..cap alpha..-particles with kinetic energy T>480 MeV per nucleon and cosmic-ray electrons with T>6 MeV. Using simultaneous data from the two spacecraft, we have determined the cosmic-ray integral intensity gradient in the time interval 1973 April 16--August 31. During this period Pioneer-10 moved from 4.06 to 4.66 AU and Pioneer-11 from 1.02 to 2.11 AU. We have used data in different channels to determine the counting rate from Jovian electrons and to subtract this from the cosmic-ray counting rate on Pioneer-10. During April and May there was a solar event followed by a Forbush decrease; we have examined the effect of this event on the measured gradient. It appears that disturbances of this kind can cause large changes in the gradient values. The radial gradient is found to be 0.15 +- 2.3 percent per AU, and we consider this value to be basically consistent with zero. We have calculated the theoretical gradient in this integral energy range using the force-field approximation and expect a value of about 8 percent per AU. Suggestions advanced to explain the discrepancy between theoretical and observed values include: (1) that the accepted diffusion coefficients are too low; (2) that spherically symmetric models are inadequate; (3) that temporal variations of the interplanetary medium are important; and (4) that another transport mechanism is required. (AIP)