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

Abstract

Abstract. 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• i 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 •0 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 environme,nt, 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.

Near-Earth Space Radiation Models

“…The spectra for all of the ions from Lithium (Z=3) to Nickel (Z=28) are now very well defined by the NASA Advanced Composition Explorer (ACE) Cosmic Ray Isotope Spectrometer (CRIS) [40]. CRIS has continuously provided low energy (~50 -300 MeV/n) spectra daily for these ions since 1997.…”

An observation‐based GCR model of heavy nuclei: Measurements from CRIS onboard ACE spacecraft

“…[, Figure ] as well as that from our model, for elements C, O, Si, and Fe for the year 1997. In addition, following Davis et al [], we show the ACE/CRIS (circles) and SIS (squares) measurements for the elements studied during Sept. 1997 to March 1998. From the figure, we can see for the elements, time period and energy range studied, that our model reproduces ACE observations better than the other two models.…”

“…The data are thought to be the most statistically precise for both abundant and rare species to date. The residual systematic uncertainties in the absolute spectra measured by CRIS and SIS is conservatively estimated to be less than 10% (probably less than 5%) for both instruments [ Davis et al , ].…”

“…Furthermore, the Badhwar and O'Neill model and CRÈME96/Nymmik model are based on the balloon and spacecraft data of varying precision from about three decades before the work was done. Hence, the accuracy of these models is rather limited, and the solar minimum predictions is estimated to deviate from the ACE measurement by as much as 15–20% [ Davis et al , , Figure ], which suggests that both models need revision in light of the high‐quality data from ACE. Actually, O'Neill [, ] has updated the original Badhwar‐O'Neill Model with the new ACE data to produce more accurate GCR environment data.…”

Cosmic-Ray Spectra in Interstellar Space

“…For these interstellar spectra to be consistent with the observations, modulation in the heliosheath must be between ~1.5 and ~3 times that between 1 AU and 70 AU. Comparison of local interstellar carbon spectra used in several recent studies [11,12,13,14]. Also shown are solar minimum carbon data from ACE at 1 AU and Voyager-1 at ~70 AU [13].…”