Global Solar Wind Structure: Effects of Pickup Protons

Abstract

The structure of the heliosphere is strongly affected by the pickup protons produced by the photoionization of the interstellar neutral hydrogen and by the charge exchange of the hydrogen atoms with solar wind protons. In the distant heliosphere (r>5–10 AU), the thermal pressure of pickup protons is higher than the solar‐wind proton and electron pressures, and the energy and momentum redistribution between solar‐wind and pickup protons leads to a deceleration of the solar wind flow and to an increase of average plasma temperature with heliocentric distance. The deceleration effect, combined with the additional plasma pressure from the pickup protons, acts to weaken the corotating interaction regions (CIRs). It causes, however, an overall compression of the (mostly azimuthal) interplanetary magnetic field. Using a three‐dimensional magnetohydrodynamic solar wind model that describes the pickup protons as a separate fluid, we have simulated the magnetic field and plasma distribution throughout the heliosphere to 100 AU. The source magnetic field on the Sun is assumed to be a tilted or non‐tilted dipole. Comparing the simulation runs with and without pickup protons, we quantitatively evaluate the effect of the solar wind deceleration on the weakening of the CIRs and on the increase of interplanetary magnetic field intensity. Unlike our earlier studies, the present simulations incorporate Hollweg’s “collisionless” heat flux approximation that obviates the need to use a non‐adiabatic polytropic index.