Stochastic Fluctuations of Low-Energy Cosmic Rays and the Interpretation of Voyager Data

Abstract

Data from the Voyager probes have provided us with the first measurement of cosmic ray intensities at MeV energies, an energy range which had previously not been explored. Simple extrapolations of models that fit data at GeV energies, e.g. from AMS-02, however, fail to reproduce the Voyager data in that the predicted intensities are too high. Oftentimes, this discrepancy is addressed by adding a break to the source spectrum or the diffusion coefficient in an ad hoc fashion, with a convincing physical explanation yet to be provided. Here, we argue that the discrete nature of cosmic ray sources, which is usually ignored, is instead a more likely explanation. We model the distribution of intensities expected from a statistical model of discrete sources and show that its expectation value is not representative, but has a spectral shape different from that for a typical configuration of sources. The Voyager proton and electron data are however compatible with the median of the intensity distribution. We stress that this model can explain the Voyager data without requiring any unphysical breaks.