We report on data from the LECP (Low Energy Charged Particle) instruments on Voyagers 1 and 2. Our focus is on observations in the outer heliosphere of low-energy oxygen ions with energies in the ranges (1) ~ 4-16 keV/nuc and (2) ~130-250 keV/nuc. The lower energy component (1) is detected as a result of being boosted by solar wind convection into the sunward looking sector of thin solid-state detector. The higher energy component (2) is identified by careful corrections of rate channel data. We concentrate on the solar inactive periods ~1984-1986 and ~1995-1998. It is during these periods that component (1) is most easily detected because during solar minima the solar wind speed tends to be higher and interplanetary proton intensities, which reduce instrumental sensitivity for detecting the convected oxygen ions, tend to be lower. We have interpreted component (1) as pickup oxygen ions having speeds up to few times the solar wind speed, i.e., in the low-energy portion of the high-energy tail of the pickup ion distribution. During 1996-1997, the three-point intensity spectrum at Voyager 1, which covers 6.3-16.3 keV/nuc oxygen, is consistent with a model wherein pickup oxygen ions are stochastically accelerated to form the high-energy tail, which deceases exponentially with energy. However, component (2), which is at an energy well below the low-energy turnover of anomalous cosmic ray oxygen, has an intensity that lies far above the model-predicted tail. In fact, a simple linear fit to components (1) and (2) is consistent with an oxygen ion intensity that decreases with energy as a power law with a power law index ~2. This result is remarkably similar in form to the high-energy tails observed over a broad energy range at ACE. We will report on our progress in enlarging our data sets and in interpreting the results and their implications for acceleration processes in the solar wind.

The spatial gradients of daily-averaged fluxes of protons and alpha particles have been derived in several energy-nucleon passbands from simultaneous observations at 1 AU (with the IMP 8 CPME instrument) and in the ecliptic 1-5 AU with the Ulysses HISCALE instrument and Voyager 1 and 2 (with the LECP instrument). A positive radial gradient of the 0.3 to 0.5 MeV proton fluxes suggests that interplanetary acceleration, probably associated with shocks, becomes effective in the 2-3 AU region. Protons in the 2-4 MeV energy interval, by contrast, exhibit a negative radial gradient and suggest less efficient shock acceleration in this region. However, the 2-4 MeV/nucleon helium nuclei fluxes show a positive radial gradient for all the Ulysses and Voyager observations, implying that the He/proton abundance ratio increases strongly with heliocentric distance. Values of He/H<5 at 0.35-1 MeV/nuc have been observed directly in individual CIR events at Ulysses (Simnett et al., GRL, in press). Interstellar He atoms are ionized to He+ ions which are picked up by the solar wind and can be accelerated by interplanetary shocks, both flare and CIR-generated, in the 1-5 AU region via shock drift acceleration. Interstellar H is much less abundant than He within 3 AU and may not be significantly injected into this process at this distance. Results of the evaluation of the gradients by several approaches will be shown along with theoretical arguments concerning the injection process. These observations extend to higher energies the argument of Gloeckler et al. (JGR 99, 17637, 1995) that the acceleration of interstellar helium that is proposed as a source of anomalous cosmic rays begins in the 1-5 AU region.