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Published June 20, 2009 | Published
Journal Article Open

Elemental Composition and Energy Spectra of Galactic Cosmic Rays During Solar Cycle 23


We report improved measurements of elemental abundances and spectra for galactic cosmic-ray (GCR) nuclei obtained by the Cosmic Ray Isotope Spectrometer on board NASA's Advanced Composition Explorer (ACE) spacecraft during the minimum and maximum phases of solar cycle 23. We discuss results for particles with nuclear charge 5 ≤ Z ≤ 28 and typical energies between 50 and 500 MeV nucleon^(–1). We demonstrate that a detailed "leaky box" Galactic propagation model combined with a spherically symmetric solar modulation model gives a good (but not perfect) fit to the observed spectra by using a solar modulation parameter of φ = 325 MV at solar minimum and φ = 900 MV at solar maximum. Although our results are generally consistent with previous measurements from space-based and balloon-based missions, there are significant differences. The large geometrical acceptance and excellent charge resolution of the instrument result in the most detailed and statistically significant record of GCR composition to date in this energy range. The measurements reported here serve as a high-precision baseline for continued studies of GCR composition, solar modulation over the solar cycle, space radiation hazards, and other applications.

Additional Information

© 2009 American Astronomical Society. Print publication: Issue 2 (2009 June 20); received 2008 October 10; accepted for publication 2009 April 16; published 2009 June 3. This work is supported by NASA at the California Institute of Technology (under grant NNX08AI11G), the Jet Propulsion Laboratory, the Goddard Space Flight Center, and Washington University in St. Louis. We also acknowledge the use of the Climax neutron monitor data provided to the community by the University of New Hampshire under the National Science Foundation Grant ATM-0339527.

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