How important are the alpha‐proton relative drift and the electron heat flux for the proton heating of the solar wind in the inner heliosphere?

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. Key Points The energy in the alpha‐proton drift can account for half of the proton heating The solar wind heat flux can also account for half of the proton heating Mechanisms are identified for the energy transfer to proton heating