STUDY OF COSMIC-RAY H-ISOTOPE AND HE-ISOTOPE AT 23 AU

We have measured the spectra of H and He isotopes during the 1987 sola r minimum with the cosmic-ray detector system (CRS) on the Voyager 2 s pacecraft. By carrying out the measurement near solar minimum and at l arge heliospheric distances, the effects of solar modulation were redu ced. In particular, the adiabatic energy losses were smaller, and thes e results from 23 AU over the solar minimum period of cycle 21 represe nt observations at energies not accessible from previous measurements near 1 AU. The modulated spectra with the diffusion coefficient consta nt k(0) = 3.15 x 10(22) cm(2) s(-1) (which corresponds to a solar modu lation parameter of 360 MV at 23 AU and 500 MV at 1 AU) agree well wit h both our data at 23 AU and the previous solar minimum measurements a t 1 AU. The measured H-1 and H-2 spectra are both consistent with the calculated spectra, using standard Galactic and heliospheric propagati on models without invoking an anomalous hydrogen component. With the f ixed modulation parameter of 360 MV, the mean pathlengths, source spec tra, and cross sections were varied to study the effects of different input parameters on the spectra and relative abundances. At this stage of our work, we have not found any strong evidence from the low-energ y H-2 and He-3 data that H-1 and He-4 should have a different propagat ion history, or different types of source spectra from the heavier cos mic-ray nuclei.