Abstract

This report explores the feasibility of explaining the observed proton heating in the inner heliosphere (1) by tapping the field-aligned relative drift between alpha particles and protons in the solar wind plasma and (2) by tapping the strahl-electron heat flux from the Sun. The observed reduction of the alpha-proton drift kinetic energy from 0.3 to 1 AU and the observed reduction of electron heat flux from 0.3 to 1 AU are each about half of the energy needed to account for the observed heating of protons from 0.3 to 1 AU. A mechanism is identified to transfer the free energy of the alpha-proton relative drift into proton thermal energy: the alpha-proton magnetosonic instability. A mechanism is identified to transfer kinetic energy from the strahl-electron heat flux into proton thermal energy: weak double layers. At the current state of knowledge, the plausibility of heating the solar wind protons via the alpha-proton magnetosonic instability is high. The properties of the weak double layers that have been observed in the solar wind are not well known; more data analysis and plasma simulations are needed before the plausibility of heating the solar wind protons by the double-layer mechanism can be evaluated.