STUDIES OF THE LOW-ENERGY GALACTIC COSMIC-RAY COMPOSITION NEAR 28-AU AT SUNSPOT MINIMUM - THE PRIMARY-TO-PRIMARY RATIOS

Data from the High-Energy Telescopes of the CRS experiments on Voyagers 1 and 2 at an average distance of 28 AU in 1986-1987-1988 have been used to measure the spectra and charge ratios of various primary nuclei such as He, C, 0, Ne, Mg, Si, and Fe in Galactic cosmic rays at approximately 100 MeV per nucleon. The solar modulation parameter relevant for this data is estimated to be phi = 320 +/- 100 MeV or about 0.6 of that applicable to previous measurements made at Earth. The results on these primary/primary ratios may thus be used to make sensitive tests of the source spectra of cosmic rays. We find the following: (1) The source spectra of the above seven nuclei appear 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. (2) 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-0.60 dependence observed at higher rigidities without any flattening or turnover at low rigidities. (3) 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. In order to fit both the observed He/C and C/Fe ratios at low energies using reasonable reacceleration models requires that reacceleration be less than 0.1 of the original acceleration or relatively unimportant.