Solar wind in the distant heliosphere

Abstract

This paper presents a quantitative MHD model to study the effects of pickup protons on the solar wind. The model includes the kinetic theory solution of interstellar neutral hydrogen, the ionization process for the production of pickup protons, the spiral interplanetary magnetic field, and the full MHD equations. In order to study the effects of pickup protons, we first obtain the kinetic theory solutions of interstellar neutral hydrogen. This solution is then used to calculate the flow of interstellar hydrogen and the production of pickup protons. We study a one‐dimensional, steady state model of the pickup proton solar wind in the upwind direction. The solar wind proton parameters predicted by this solution agree reasonably well with observational data from Voyager 2. Throughout the heliosphere the temperature of pickup protons is higher than the temperature of the solar wind protons by two orders of magnitude. Outside the ionization radius, the solar wind temperature and the fast speed begin to increase and the bulk speed and the Mach number of the wind begin to decrease. The component of the equation of motion along the streamline direction can be written in the form of a Bernoulli equation; it can clearly explain the deceleration of the solar wind. In the distant heliosphere the sum of the kinetic energy and the thermal energy decreases with the heliocentric distance due to the charge exchange process; at the same time, the thermal energy of the solar wind plasma increases due to heating by the pickup process. The combined action causes a significant radial decrease in the kinetic energy of the wind.