Modulation of anomalous protons: Effects of different solar wind speed profiles in the heliosheath

Abstract

Two termination shock acceleration modulation models are used to study the modulation of anomalous protons, in particular the effects of different scenarios for global solar wind speed (V) variations in the heliosheath. The first numerical model simulates a symmetric heliosphere and the second simulates an asymmetric heliosphere with respect to the Sun. The modulation differences between these models are illustrated and discussed. The geometry of the heliosphere in the latter model is deduced from a time‐dependent three‐dimensional hydrodynamic model of the heliosphere which provides the different scenarios for the V‐profiles in the heliosheath. The modulation models include the solar wind termination shock, global drifts, adiabatic energy changes, diffusion, convection, and a heliosheath. The anomalous protons are kinetically described using the Parker transport equation. A solar wind speed decreasing stronger than the generally assumed V ∝ 1/r2 dependence, with r the radial distance from the Sun, is studied as well as an extreme scenario with V ∝ r2. The stronger decrease produces a compressive flow in the heliosheath which results in additional acceleration of anomalous protons in the heliosheath. The solutions are shown for solar minimum and moderate maximum modulation conditions for both heliospheric magnetic field polarity cycles. Significant modulation differences are found to occur between these different scenarios for V in the heliosheath. If the stronger than V ∝ 1/r2 scenarios in the heliosheath are real, the anomalous intensities should increase beyond the TS, which should be measurable by the Voyager 1 spacecraft in the near future.