The Interstellar Transport of Galactic Cosmic Rays

Using the Cosmic Ray Isotope Spectrometer: CRIS) onboard the Advanced Composition Explorer: ACE) spacecraft, new and improved high-precision measurements of the elemental composition and energy spectra of galactic cosmic rays with energies from 50-550 MeV/nucleon and nuclear charge 5<=Z<=28 are reported here. These results cover observations during two solar minimum periods of the solar cycle, the most recent of which exhibited very low levels of solar activity and the highest galactic cosmic-ray intensities of the space era. Observations of secondary cosmic-ray species, those produced primarily by spallation interactions of accelerated material with the interstellar medium, and their primary progenitors are used to test the models of the transport of cosmic rays in the Galaxy. Two principal models were tested: the simple leaky-box model and the nested leaky-box model. In the simple leaky-box model, cosmic rays accelerated at their sources are transported uniformly through the interstellar medium and escape in an energy-dependent fashion from the Galaxy. In the nested leaky-box model, cosmic rays are accelerated at their sources and briefly stored in surrounding cocoon regions before they escape into the Galaxy; escape from the cocoon regions is energy dependent while escape from the Galaxy is independent of energy. Our observed B/C and: Sc+Ti+V )/Fe ratios are compared with the results from these two models, and we find that the simple leaky-box model fits the data very well only when escape from the Galaxy has an unphysical energy dependence below 1 GeV/nucleon. Though the nested leaky-box model is considered to be more physically reasonable, we could not simultaneously fit the B/C and: Sc+Ti+V )/Fe ratios at all energies. A model that includes a small amount of reacceleration, a process that boosts low-energy cosmic rays up to higher energies, is found to agree well with results from the simple leaky-box model.