Helioradius dependence of interplanetary carbon and oxygen abundances during 1991 solar activity

A comparison of the heliocentric dependence of the relative abundance of low energy (similar to 1 MeV nucleon(-1)) carbon and oxygen on the Ulysses (similar to 3-4 AU) and Voyager 1 and 2 (similar to 35 and similar to 47 AU) spacecraft shows that the C/O abundance ratio is significantly smaller at the larger radial distances. This finding is attributed to a more abundant seed population for anomalous cosmic-ray oxygen than for carbon in the outer solar system, where the seed population can be accelerated by shocks in global merged interaction regions. The energy spectra of carbon and oxygen at the different locations are approximately the same.