Studies of the Low-Energy Galactic Cosmic-Ray Composition near 28 AU at Sunspot Minimum: The Primary-to-Primary Ratios

The spectra and charge ratios of various primary nuclei such as He, C, O, Ne, Mg, Si, and Fe in Galactic cosmic rays were measured at about 100 MeV per nucleon on the basis of data from the High-Energy Telescopes of the CRS experiments on Voyagers 1 and 2 at average distance of 28 AU during 1986-1988. The source spectra of the above nuclei are found to be the same to within +/- 0.02 in the exponent over three orders of magnitude from 0.1 to 100 GeV per nucleon, with the exception of Ne, which is 0.05 flatter than Mg. Three of four primary/primary ratios at low energies are better fitted with a path length dependence on rigidity that continues to follow the P exp -0.60 dependence observed at higher rigidities without any flattening or turnover at low rigidities. The primary/primary ratios at low energies are very sensitive to reacceleration effects and show a different energy dependence than does the B/C ratio, for example.