The Thermal Force in Astrophysical Plasmas: Current Free Coulomb Friction

Abstract

The t hermal force (TF) is an exchange force mediated by Coulomb collisions between electrons and ions in a heat-conducting astrophysical plasma, is one of three, non-inertial, balancing terms in the parallel component of the generalized Ohm's law, and is magnetic field aligned with a size that scales with and is parallel to the dimensionless heat flux. The TF (i) increases the size of E _∥ above that implied by the electron pressure divergence; (ii) deepens the electrostatic trap for electrons about the Sun; (iii) strengthens the electron kurtosis and skewness, further levitating ions out of their gravitational well, (iv) constrains the heat flow in a plasma where parallel currents are preempted; and (v) is shown to be directly measurable using the full electron velocity distribution function above and below thermal energies. (vi) The usually ignored TF modifies all species internal energy equations; it enhances the rate of conduction cooling by the electrons, increases the ion entropy, and forestalls adiabatic behavior. Using estimates at 1 au this effect is especially strong in the higher speed wind U > 400 km s⁻¹ regime. (vii) On rather general grounds any physical heat transport is accompanied by an underlying TF; in almost all known cases of modeling astrophysical plasmas this dependence is ignored or demonstrably incorrect. It follows that attempts to predict species specific pressures without inclusion of the TF is futile.