The evolution of the anomalous cosmic ray oxygen spectra from 1995 to 1998: Ulysses observations

Moraal and Steenberg (1999), showed that the peak energy in the anomalous c osmic ray spectra is independent of the radial distance up to a few AU away from the termination shock but dependent on the solar wind speed, the radi us of the termination shock and the scattering strength. In this paper we w ill discuss the variation of the cosmic ray oxygen energy spectrum as measu red by the Ulysses EPAC and the COSPIN/LET on board Ulysses. We found that the peak energy decreased from similar to5 MeV nucl(-1), when Ulysses was a t high northern heliographic latitudes embedded in the fast solar wind to s imilar to3.5 MeV n(-1), in the streamer belt. The shift towards lower energ y might also be caused by changing modulation although Voyager measurements indicate no variation of the ACR Oxygen spectrum at similar to 60 AU.