Evolution of Anomalous Cosmic-ray Oxygen and Helium Energy Spectra during the Solar Cycle 22 Recovery Phase in the Outer Heliosphere

We present annual 0.3–40 MeV nucleon 1 anomalous cosmic-ray (ACR) oxygen and helium energy spectra from the 1991–1999 cosmic-ray recovery phase of solar cycle 22. These observations were made with the Low Energy Charged Particle instruments aboard the Voyager 1 and Voyager 2 spacecraft while at helioradial positions ranging from 34 to 76 AU. The peak intensities of both species increased by 2 orders of magnitude during this period, while the energies of peak O and He intensity decreased from ∼9 to 1.3 MeV nucleon 1 and from ∼35 to 6 MeV nucleon , respectively. Using these observations along with published O measurements from the Solar Anomalous Magnetospheric Particle Explorer at 1 AU, we investigate ACR transport phenomena. We make estimates related to transport parameters such as the relative change in the scattering mean free path over time, the rigidity dependence of the mean free path, and the distance between the Sun and the solar wind termination shock. Subject headings: convection — cosmic rays — diffusion — interplanetary medium — Sun: activity