Evidence for anomalous cosmic-ray hydrogen

Abstract

The High Energy Telescopes on the Voyager 1 and Voyager 2 spacecraft are used to measure the differential energy spectra of hydrogen and other elements. During the period of minimum solar modulation in 1987, changes in the shape of the hydrogen energy spectra are observed. It is shown that these changes are difficult to explain in the framework of current modulation theory, and are consistent with the emergence of an anomalous cosmic ray (ACR) hydrogen component. ACR hydrogen is predicted by the current theories of anomalous cosmic rays, but this is the first evidence that ACR hydrogen is actually present. Several different estimates of the contribution of ACR hydrogen are used to obtain peak fluxes of 0.33 ± 0.12 particles/m2s sr MeV and 0.67 ± 0.18 particles/m2s sr MeV for Voyager 1 and Voyager 2 respectively during the time period 1987/209-313. Using a model developed by Cummings and Stone (1987), we relate these fluxes of ACR hydrogen and the peak fluxes of ACR helium to the relative abundance of hydrogen and helium, n(H I)/n(He I), in the neutral gas flowing into the solar system from the local interstellar medium. For two different choices of parameters, we obtain values of 3 ± 1 and 5 ± 3 for n(H I)/n(He I), which should be compared to the cosmic relative abundance of ~ 10. Our values are consistent with previous results obtained from solar ultraviolet backscatter experiments, and support the hypothesis that hydrogen is substantially ionized in the very local interstellar medium.