Voyager 2Measurements in the Outer Heliosphere of the Energy Spectra of Cosmic‐Ray Nuclei from Less than 100 MeV Nucleon−1to More than 1.0 GeV Nucleon−1

Abstract

We have made a new analysis of cosmic-ray data from the CRS experiment on the Voyager 2 (V2) spacecraft. This analysis, which includes both penetrating and stopping particles in the HET telescope, extends the energy range from an upper energy limit of 100-300 MeV nucleon-1 for stopping particles to over 1 GeV nucleon-1. This analysis provides almost continuous spectra over this broad energy range, including the peak in the differential spectra of the various nuclei. This enables us to obtain the relationship between the shape of the low-energy interstellar differential spectrum and the amount of solar modulation. Measurements at V2 at the modulation minimum in 1987 and again in 1997-1998 give nearly the same intensities and therefore the same solar modulation parameter, = 260 MV, which is much lower than the value of 400 MV at the Earth. In 1987 V2 was at 23 AU and in 1997-1998 at 54 AU. This implies that in 1997-1998 most of the modulation was occurring beyond 54 AU and also that the interplanetary part of the solar modulation out to ~54 AU in the two time periods of opposite solar magnetic polarity was greatly different. The measured secondary/primary ratios B/C and N/O can be fitted equally well using a simple leaky-box Galactic propagation model with an escape length γ ~ P-0.5 either with a break in the escape dependence at 1 GeV nucleon-1 or with no break in the escape-length dependence to the accuracy of the experimental data itself, ±3%, over the entire energy range from less than 100 MeV nucleon-1 to over 10 GeV nucleon-1 using a combination of both V2 data at the lower energies and HEAO data at the higher energies. This same escape-length dependence also predicts the measured higher Z secondary/primary ratio Z = 21-23/Fe well at all energies.