Electron Heat Flux and Thermal Force in the Solar Wind at 1

The origin and evolution of non-equilibrium characteristics of electron velocity distribution functions (eVDFs) in the solar wind are still not well understood. They are key in understanding heat conduction and energy transport in weakly collisional plasma, as well as in the scenario at the origin of the solar wind. Due to low collision rates in the solar wind, the electron populations develop temperature anisotropies and velocity drifts in the proton frame, as well as suprathermal tails and heat fluxes along the local magnetic field direction. These nonthermal characteristics are however not as strong as predicted by collisionless models.

Non-thermal eVDFs have a frictional coupling to the ions due to the speed dependence of unavoidable Coulomb collisions despite there being no slippage between electrons and ions. This coupling is classically known as the thermal force (TF). It enhances the parallel electric field and plays an important, but usually neglected, role in two fluid energy transfers between electrons and ions.

The TF is directly measurable using in-situ eVDFs, allowing a new, local, quantitative measure of Coulomb coupling that maybe important with possibly other microphysical processes to locally control non-thermal properties. The heat flux and the TF are expected to be positively and strongly correlated. The TF role in understanding the observed heat flux remains to be explored.

We present here recent work on solar wind electrons from enhanced electron measurements of eVDFs from Wind. These accurate measurements are based on a sophisticated technique that combines measurements of eVDFs and of Quasi-Thermal Noise. We determine total electron as well as core, halo and strahl parameters, including heat flux and the TF. We discuss the properties of the parallel electron heat flux and the contribution from core, halo and strahl, as well as the properties of the newly determined TF. We also discuss the collisional vs collisonless characteristics of the electron heat flux, particularly its correlation with the size of the TF.