The isotopic composition of the anomalous low-energy cosmic rays

Abstract

We present new observations of the isotopic composition of low-energy He, C, N, O, and Ne nuclei. The observations cover the energy interval from ~5 to 30 MeV per nucleon, the energy range where the "anomalous cosmic ray" fluxes of He, N, O, and Ne have been observed. We compare the isotopic composition of the anomalous cosmic rays with that of higher-energy galactic cosmic rays in an effort to understand the origin of these two components. For three of the elements studied (He, N, and Ne) we find significant variations in the isotopic composition at essentially the same energy where the elemental composition changes (~30-50 MeV per nucleon), in agreement with earlier studies. Thus, fragmentation products such as ^3He and ^(15)N that are relatively abundant in galactic cosmic rays appear to be essentially absent from the anomalous cosmic ray component. In addition, we find a difference between the ^(22)Ne/^(20)Ne ratios in the anomalous and galactic cosmic rays, implying that the nucleosynthesis of these two components has differed. Although limited by their statistical accuracy, our low-energy isotope results are generally consistent with the proposed interstellar-neutral origin for the anomalous cosmic rays. If correct, this model suggests that future measurements of anomalous cosmic rays can be used to analyze the isotopic structure and evolution of the local interstellar medium.

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