Effects of Different Solar Wind Speed Profiles in the Heliosheath on the Modulation of Cosmic‐Ray Protons

Abstract

An asymmetric solar wind termination shock (TS) model is used to study the effects on the modulation of cosmic-ray protons for different scenarios of the solar wind speed (V) in the heliosheath. This two-dimensional model is applied using predictions for V in the heliosheath that were calculated with a time-dependent three-dimensional hydrodynamic model. Decreases stronger than the generally assumed V ∝ 1/r2 in the heliosheath are studied, as well as an extreme case, V ∝ r2, where r is the radial distance.The effect of the TS is enhanced under certain circumstances, and "barrier"-type modulation in the heliosheath also depends on the chosen V-profiles. Significant changes occur mostly for the A < 0 solar magnetic polarity cycle, at all distances in the equatorial plane, when the V is changed from an incompressible fluid (V ∝ 1/r2, ∇ V = 0) in the heliosheath, to V ∝ 1/r8, in a symmetrical model. For the asymmetrical case the TS is predicted to be more effective in the heliospheric nose than in the tail, especially for the A < 0 cycle during solar minimum conditions. The different profiles for V do not have a significant effect on the intensities inside the TS, but in the heliosheath the difference can be quite significant. It is found that V ∝ 1/r2 in the heliosheath is an oversimplification. The consequent effects of having ∇ V ≠ 0 in the heliosheath prove to be relevant for cosmic-ray modulation and acceleration, especially now that the Voyager 1 spacecraft encountered the TS and entered the heliosheath.