Evolution of the Energy Spectra of Anomalous Cosmic Rays in the Outer Heliosphere

We present energy spectra of anomalous cosmic ray hydrogen, helium, and oxygen derived from data collected from experiments on the Voyager and Pioneer spacecraft from 1993 through 1996. The sequence of energy spectra show the effects of decreasing modulation as the solar cycle approaches and reaches solar minimum conditions. We suggest a number of factors that may be responsible for the spectral evolution, including the approach of the spacecraft to the source region of the particles (the solar wind termination shock), possible changes in the source strength, decreasing tilt of the neutral current sheet, and changes in the diffusion mean free path. We find that the data are inconsistent with source strength changes, but changes due to the other factors are possible and will require more complete 2 and 3-dimensional propagation models to make quantitative assessments. We find evidence for anomalous cosmic ray hydrogen up to energies as high as 400 MeV/nuc.