In the introduction of the attached paper is mentioned that:
Electrons provide additional heating by carrying the bulk of the solar wind heat flux and through collisions with the protons.
Electron and proton heating by solar wind turbulence
What is meant by this phrase?
Electrons provide additional heating by carrying the bulk of the solar wind heat flux and through collisions with the protons.
First, the solar wind is a weakly collisional plasma at best (e.g., see https://physics.stackexchange.com/a/268594/59023). That is, collisions play an extremely minor role in nearly all processes in the solar wind. Second, the electron heat flux arises not due to collisions with protons but due the Lorentz force and differences in thermal speeds between the two populations.
That is, the electron thermal speed greatly exceeds the Sun's gravitational escape speed so they are free to go whenever they move in the anti-sunward direction. The Lorentz force limits their mobility to being mostly along the quasi-static magnetic field. Finally, the conservation of the first adiabatic invariant (i.e., magnetic moment of particle gyration) reduces any perpendicular (with respect to quasi-static magnetic field) velocity as the particle moves away from the Sun and the magnetic field strength decreases (e.g., see https://physics.stackexchange.com/a/670591/59023). These effects would take an isotropic, drifting Maxwellian and turn it into an anisotropic, skewed, narrow, magnetic field-aligned beam. This population of electrons is known as the strahl (German for beam). See the references below for more discussion.
It is currently thought that the strahl scatters as it propagates away from the sun and forms the other dominant suprathermal electon population in the solar wind called the halo. The cold, dense core of the electron population helps to balance the total electric current such that in the plasma rest frame, there is zero net current. In the core electron rest frame, the heat flux is dominated by the strahl electrons (sometimes the halo helps out too). In the plasma rest frame, the core electrons can have a rather large heat flux at times in the sunward direction. Though I don't think this is physically meaningful in the thermodynamic sense since the weakly collisional nature of the solar wind means the particles stream past each other and do not transfer significant energy through collisions via temperature gradients.
What is meant by this phrase?
This arXiv paper is based on a fluid approximation of the solar wind, which would automatically entail particle-particle collisions as a means of energy transfer, i.e., temperature gradients control the heat flux term.
