Observations of galactic cosmic-ray energy spectra between 1 and 9 AU

Detector systems onboard Pioneer 10, Pioneer 11, and Helios I have collected data on the variation of the 5-500 MeV per nucleon cosmic-ray helium component between 1 and 9 AU in order to evaluate the properties of low and medium-energy galactic cosmic rays. At 9 AU new spectral features are found for helium nuclei below 60 MeV per nucleon, while at energies above 100 MeV per nucleon the radial gradient is found to be much smaller than theoretically predicted. The conventional modulation theory can be made consistent with these observations. The study is applicable to investigations of particle acceleration, injection, and interstellar propagation.