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The Composition of Anomalous Cosmic Rays from ACE, SAMPEX, and Voyager

The Composition of Anomalous Cosmic Rays from ACE, SAMPEX, and Voyager. R.A. Leske, A.C. Cummings, C.M.S. Cohen, R.A. Mewaldt, and E.C. Stone, California Institute of Technology, Pasadena, CA USA M.E. Wiedenbeck Jet Propulsion Laboratory, Pasadena, CA USA T.T. von Rosenvinge

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The Composition of Anomalous Cosmic Rays from ACE, SAMPEX, and Voyager

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  1. The Composition of Anomalous Cosmic Rays from ACE, SAMPEX, and Voyager R.A. Leske,A.C. Cummings, C.M.S. Cohen, R.A. Mewaldt, and E.C. Stone, California Institute of Technology, Pasadena, CA USA M.E. Wiedenbeck Jet Propulsion Laboratory, Pasadena, CA USA T.T. von Rosenvinge NASA/Goddard Space Flight Center, Greenbelt, MD USA

  2. ACR Measurements Using ACE/SIS at L1: Advantages: • Large collecting power • Good resolution, including isotopes • 100% duty cycle compared with LEO Disadvantages: • Deep in the heliosphere (near 1 AU) – ACRs are heavily modulated, subject to SEP and GCR background contamination

  3. ACR intensities at 1 AU are highly variable, and SEPs dominate near solar maximum

  4. He intensities at 2 different energies can be used to distinguish solar active periods from quiet times

  5. The selected “quiet” days cleanly show the modulated ACR and GCR background intensities

  6. ACRs at 1 AU have begun to recover in the present A<0 polarity cycle, and statistics should ~double by next solar minimum

  7. At 1 AU, abundant high-FIP elements such as N, O, Ne, and Ar show a clear ACR increase at low energies (below a few 10’s of MeV/nuc)…

  8. …while any increase in lower-FIP elements (e.g., C, Mg, Si, S, Fe) is much smaller. Some of these elements havebeen found to have a small ACR component in Voyager measurements in the outer heliosphere, but their source is probably not interstellar neutrals.

  9. ACE/SIS measures isotopic composition over an energy interval that includes both ACRs and GCRs

  10. Because of production of 15N by spallation during transport through the Galaxy, GCR N is ~50% 15N. Only 14N has a measurable ACR component, however, down to ~10 MeV/nucleon.

  11. Although it is small and statistically limited, 18O does seem to show an ACR component. The intensity is similar to that expected if the ACR 18O/16O ratio were the same as standard solar system (e.g., Anders and Grevesse) values.

  12. The measured (uncorrected) ACR 18O/16O ratio spans much of the range observed in the Galaxy, as summarized in the Wilson and Rood (1994) review.

  13. 22Ne shows a very clear ACR component, with an intensity much less than expected if the ACR source composition is the same as that of GCRs, and strongly resembling the solar wind composition. (GCR Source) (Neon A) (Solar Wind)

  14. The isotopic composition of ACR Ne is similar to several solar system components, and completely unlike that of the GCR source.

  15. GCR Ne is isotopically unusual. After correcting for propagation effects such as spallation, ~all other GCR elements with measured isotopic source abundances are remarkably similar in isotopic composition to that of the solar system. ACE/CRIS data from M. E. Wiedenbeck

  16. ApJ, to appear 20 Nov 2005 (also astro-ph/0508398)

  17. From the new Binns et al. study, after accounting for fractionation that depends on volatility rather than FIP, GCR source abundances are consistent with a source consisting of ~80% material with solar system composition with an admixture of ~20% Wolf-Rayet star material (from the models of Meynet & Maeder)

  18. ACR Measurements Using SAMPEX/MAST in Polar Earth Orbit: Advantages: • “Geomagnetic filter” removes GCR background, leaving pure ACRs for composition studies • Concentrated sample of trapped ACRs in radiation belt available for study • Geomagnetic filter enables additional ACR studies, such as charge state spectra Disadvantages: • Smaller instrument than ACE/SIS, with higher energy threshold • Limited duty cycle – ACRs not available at low magnetic latitudes

  19. In the polar Earth orbit of SAMPEX, particle access to the spacecraft is governed by the geomagnetic field, along with the particle energy and charge state. Filtered ACRs Mostly GCRs Trapped ACRs Mostly ACRs

  20. Low Earth orbit allows study of a unique, concentrated sample of ACRs that have become trapped in a radiation belt.

  21. Fully-stripped GCRs are filtered out of much of the magnetosphere, while singly (or lightly) ionized ACRs can gain access. (SAA removed)

  22. The effect of the geomagnetic filter allows ACRs to be observed completely free of GCR background. This has been used to study the upper extremes of the ACR energy spectrum…

  23. …and has also been used to determine ACR charge states, verifying that at low energies they are singly ionized like pickup ions, but are more stripped at higher energies. SAMPEX/HILT study by Klecker et al.

  24. The efficiency of the geomagnetic filter in rejecting GCRs is seen in these elemental abundance histograms. SAMPEX/MAST data, ~15-150 MeV/nucleon

  25. The isotopic composition of N, O, and Ne depends on position in the magnetosphere, reflecting the differences between ACRs and GCRs Pure ACRs

  26. Even using the pure ACR samples, statistics are too meager to detect ACR 15N. ACR 18O in the trapped belt seems similar to solar, and ACR 22Ne/20Ne is much less than in the GCR source. Leske, et al., Space Sci. Rev. 78, 149, 1996.

  27. ACE/SIS (at L1) can’t make use of the geomagnetic filter, but has better statistical accuracy and a lower energy threshold than SAMPEX/MAST. ACE/SIS 8/27/97-3/23/98 SAMPEX/MAST 1992-1997

  28. ACR Measurements Using Voyager in the Outer Heliosphere: Advantages: • Very high ACR intensity • Much less modulation, relatively less SEP and GCR background Disadvantages: • Much smaller instruments than available at 1 AU • Limited resolution, mostly used for elements rather than isotopes

  29. ACR intensities in the outer heliosphere are ~10-1000 times greater than at 1 AU, and much less modulated

  30. GCR intensities in the outer heliosphere are only ~20%-5x higher than at 1 AU

  31. Voyager spectra show small ACR increases in C, Na, Mg, Si, and S in addition to high intensities of ACR N, O, Ne, and Ar. Cummings et al., ApJ 578, 194, 2002

  32. Cummings et al looked in detail at the corrections needed to obtain ISM abundances from ACRs. It appears that differences in acceleration efficiency will be unimportant for N, O, and Ne isotopes.

  33. The minor ACRs are not likely due to interstellar neutrals or singly-charged ions, but may be due to an “outer source” of ions sputtered from material in the Kuiper belt.

  34. For high-FIP ACRs & PUIs, Cummings et al. and Gloeckler et al. found good agreement between the inferred neutral densities in the LISM and that expected from the Slavin & Frisch models.

  35. Summary

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