April 15, 1998

ACE News #13: Evidence for the Rare Isotopes 22Ne and 18O in Anomalous Cosmic Rays


Anomalous Cosmic Rays (ACRs) are a sample of the neutral interstellar medium (ISM) that has been swept into the heliosphere, ionized, picked up by the solar wind, and accelerated to energies of ~1 to ~50 MeV/nuc at the solar wind termination shock. To date seven elements have been unambiguously identified in ACRs: H, He, C, N, O, Ne, and Ar, all of which are partly neutral in the ISM because of their relatively high first ionization potentials. Isotope measurements of these elements can provide information on the present-day composition of the nearby interstellar medium and its evolution over the last 4.6 billion years since the formation of the solar system.

Plotted above are spectra for oxygen and neon isotopes measured by the Solar Isotope Spectrometer (SIS) during quiet-time days from 9/97 through 3/98. Above ~40 MeV/nuc these spectra are dominated by galactic cosmic rays (GCRs) which have spectra roughly proportional to kinetic energy below ~100 MeV/nuc (solid lines). At lower energies the ACR component exceeds the GCR component and sudden turn-ups are evident for all but 21Ne. The ACR spectra of the abundant isotopes 16O and 20Ne have been studied before, but these are the first reported spectra for the rare isotopes 18O and 22Ne in ACRs.

Quantitative interpretation of these ACR observations will require corrections for GCR contributions (solid lines above) and for small differences in the acceleration efficiency, but qualitative conclusions are already possible. We observe about one 18O for every 500 16O, generally consistent with the terrestrial 18O/16O ratio of 0.0020, but apparently less than the 18O/16O ratio of ~0.004 reported for the Galactic Center and based on spectroscopic studies.

The solar system includes several Ne components, including solar wind (with 22Ne/20Ne ~ 0.073) and the lunar/meteoritic components Ne-A (22Ne/20Ne ~ 0.12) and Ne-C (22Ne/20Ne ~0.09). Galactic cosmic rays are unusually rich in 22Ne; to produce the observed ratio of ~0.6 requires 22Ne/20Ne ~ 0.4 in GCR sources. While it is premature to compare the above data to the solar system Ne components, the observed ACR ratio of ~0.1 or less is clearly well below the GCR source ratio of ~0.4. This (as well as earlier SAMPEX and Voyager data) provides evidence that GCRs are not simply a sample of interstellar matter (as has been suggested), and supports models that include contributions from sources rich in 22Ne, such as Wolf-Rayet stars.

....contributed by Richard Mewaldt and Rick Leske of Caltech

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Last modified 15 April 1998, Rachael Kubly
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