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The galactic cosmic ray proton and helium nuclei spectra (j) have been measured aboard the Pioneer 8 spacecraft over a large energy range. These data are compared with spectra measured near solar minimum (j1965), and the modulation parameter M≡β ln (j1965/j) is calculated. A similar comparison is made for published electron spectra. The modulation parameters for all the components are observed to increase to a maximum and then to begin to decrease at low energies. At the same rigidity a splitting dependent on the charge‐to‐mass ratio (or, equivalently, the velocity) is observed for protons, helium nuclei, and electrons. This splitting is not in accord with the predictions of the simple diffusion‐convection model. Numerical solutions of the complete transport equation, which includes the process of adiabatic deceleration together with the processes of diffusion and convection, are considered. We discuss assumptions under which spectra and time variations can be generated that are in reasonable agreement with the observations.

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The functional form of the modulation for the 11‐year variation changed abruptly after the Forbush decrease of June 8, 1969. This change in the 11‐year modulation suggests that this Forbush decrease was an integral part of the 11‐year variation. Some details of the change as observed by neutron monitors and by spacecraft at low energies are discussed.