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Heliospheric Modulation of Cosmic-Ray Positrons and Electrons: Effects of the Heliosheath and the Solar Wind Termination Shock

The importance of the solar wind termination shock (TS) and the modulation that may occur in the heliosheath have become most relevant, as emphasized by cosmic-ray observations of the Voyager spacecraft in the distant heliosphere close to the TS. The role of the TS and that of the heliosheath on cosmic-ray electrons and positrons are studied with a numerical model, including global drifts and a Jovian electron source. The consequent charge-sign dependence is computed using improved local interstellar spectra and new fundamentally derived diffusion coefficients. The importance of modulation in the heliosheath is studied for the two species, as well as the differences between a model with and one without a TS. We found that the modulation in the heliosheath depends on the particle species, is strongly dependent on the energy of the cosmic rays and the magnetic polarity cycle, and is enhanced by the inclusion of the TS. From the computations it is possible to estimate the ratio of modulation occurring in the heliosheath to the total modulation between the heliopause and Earth. For electrons the factor modulation in the heliosheath becomes comparable to the factor between the TS and Earth at low energies for both polarity epochs. During A > 0 cycles the heliosheath cannot really be considered a modulation "barrier" above ~150 MeV for electrons, while for positrons this occurs at somewhat higher energies during the A < 0 cycle. The presence of the TS is thus more pronounced for electron modulation during A > 0 polarity cycles but for positron modulation during A < 0 cycles. For positrons, with a completely differently shaped local interstellar spectrum, the modulated spectra have a very mild energy dependence below ~300 MeV, even at Earth, in contrast to electrons, protons, and antiprotons. These characteristic spectral features may be helpful for distinguishing between electron and positron spectra when measured near and at Earth. These simulations can be of use for future missions to the outer heliosphere and beyond.