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Published December 1, 2001 | Published
Journal Article Open

Solar minimum spectra of galactic cosmic rays and their implications for models of the near-Earth radiation environment


The radiation dose from galactic cosmic rays during a manned mission to Mars is expected to be comparable to the allowable limit for space shuttle astronauts. Most of this dose would be due to galactic cosmic rays with energies < 1 GeV nucleon^(−1), with important contributions from heavy nuclei in spite of their low abundance relative to H and He. Using instruments on NASA's Advanced Composition Explorer (ACE) spacecraft, we have made the most statistically precise measurements to date of the solar minimum energy spectra of cosmic ray nuclei with charge Z = 4–28 in the energy range ∼ 40–500 MeV nucleon^(−1). We compare these measurements obtained during the 1997–1998 solar minimum period with measurements from previous solar minima and with models of the near-Earth radiation environment currently used to perform shielding and dose calculations. We find that the cosmic ray heavy-element spectra measured by ACE are as much as 20% higher than previously published solar minimum measurements. We also find significant differences between the ACE measurements and the predictions of available models of the near-Earth radiation environment, suggesting that these models need revision. We describe a cosmic ray interstellar propagation and solar modulation model that provides an improved fit to the ACE measurements compared to radiation environment models currently in use.

Additional Information

© 2001 The American Geophysical Union. Received August 24, 2000; Revised November 15, 2000; Accepted November 28, 2000. Paper number 2000JA000325. This research was supported by NASA at the California Institute of Technology (under grant NAG5-6912), the Jet Propulsion Laboratory, the Goddard Space Flight Center, and Washington University. We are grateful to the large group of dedicated and talented individuals who made the ACE mission possible and contributed to the development of the CRIS and SIS instruments( as acknowledged by Stone et al. [1998b,1 998c]. University of Chicago neutron monitor operations and research activities are supported by the National Science Foundation (grant ATM-9912341). IMP 8 GME helium data are provided on the IMP 8 GME web site by Robert E. McGuire, NASA Goddard Space Flight Center. We are grateful to the developers of CREME96 for presenting their programs on the Web for public use, and we are grateful to Gautam D. Badhwar at NASA Johnson Space Flight Center for providing GCR environment model data. Janet G. Luhmann thanks Lawrence W. Townsend and another referee for their assistance in evaluating this paper.

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