Limits on the interstellar cosmic ray electron spectrum below ∼1–2 GeV derived from the galactic polar radio spectrum and constrained by new Voyager 1 measurements

Abstract

Using a Monte Carlo Diffusion Model for electron propagation in the Galaxy we have calculated new electron spectra below 1–2 GeV that reproduce the observed polar galactic nonthermal radio synchrotron spectrum above ∼4 MHz where absorption effects in the disk of the galaxy are small. A full convolution procedure is used for the first time in these calculations. These interstellar electron intensities, which require a rapidly increasing diffusion coefficient at low rigidities, are significantly reduced from previous calculations made assuming the diffusion coefficient is a constant below 1–2 GV. This increasing diffusion coefficient is consistent with the recent suggestions that the magnetohydrodynamic waves that provide the scattering of cosmic rays may themselves be dissipated by resonant scattering from the cosmic rays (protons) but at a lower rigidity than originally suggested. The new interstellar electron spectra we calculate are more consistent with the electron spectra below ∼100 MeV being measured by the Voyager spacecraft, now at >105 AU and well beyond the heliosphere termination shock. Significant solar modulation effects of factors ∼5–10 are still required to explain the electron spectrum being measured at Voyager at 2008.0, but these are much less than the modulation factors ∼102–103 required to agree with typical earlier propagation calculations.