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Session 72 - Cosmic Rays and Abundances.
Oral session, Thursday, June 11

[72.01] Measurements of the Isotopic Composition of Anomalous Cosmic Rays from the ACE Solar Isotope Spectrometer

R. A. Leske, C. M. S. Cohen, A. C. Cummings, R. A. Mewaldt, E. C. Stone (Caltech), B. L. Dougherty, M. E. Wiedenbeck (JPL), E. R. Christian, T. T. von Rosenvinge (NASA/GSFC)

Anomalous cosmic rays (ACRs) are believed to represent a sample of the neutral interstellar medium that has been swept into the heliosphere, ionized, picked up by the solar wind, and accelerated to energies of \sim 1 to \sim 50 MeV/nucleon at the solar wind termination shock. Measurements of the isotopic composition of ACRs can, in principle, provide information on the composition of the nearby interstellar medium and how it may have evolved over the last 4.6 billion years since the formation of the solar system. We present new measurements of the elemental and isotopic composition of ACR oxygen and neon with energies >8 MeV/nucleon, made with the Solar Isotope Spectrometer (SIS) on the Advanced Composition Explorer (ACE). In addition to the well--known low--energy ACR enhancements in the intensity of ^16O and ^20Ne, we also observe similar enhancements in ^18O and ^22Ne, with relative abundances of ^18O/^16O \sim 0.002 and ^22Ne/^20Ne \sim 0.1. We compare these abundances to those measured in solar system material and in galactic cosmic rays, and discuss the implications for the isotopic composition of the local interstellar medium and the nature of the galactic cosmic ray source material.

This research was supported by NASA at the California Institute of Technology (under grant NAG5-6912), the Jet Propulsion Laboratory, and the Goddard Space Flight Center.

Program listing for Thursday