Modulation of cosmic ray protons in the heliosheath

Abstract

A contemporary numerical model including a solar wind termination shock, a heliosheath, and drifts is used to study the features of cosmic ray proton modulation in the outer heliosphere. Voyager and Pioneer spacecraft observations over 22 years and out to ∼82 AU have shown markedly different behavior for minimum modulation conditions between the radial intensity profiles for periods of opposite magnetic polarities and that most of the residual modulation for these periods took place in the outer heliosphere, near and beyond where the termination shock is expected to be. The modeling results show that the inclusion of a termination shock in the modulation model for solar minimum conditions causes abrupt changes in the radial gradients, gr, at the termination shock, at almost all energies of interest to modulation studies. During A > 0 solar magnetic field polarity cycles this “barrier” effect contributes ∼60% to the overall modulation at 0.5 GeV. However, for A < 0 cycles the contribution at this energy is markedly smaller. For increased solar activity the modulation in the heliosheath is quite different from minimum activity. The heliosheath does not any longer play the role of a distinguished “barrier,” although abrupt changes in gr at the shock may still occur, especially for A > 0 cycles, surprisingly more so for higher energies.