Heat conduction and nonsteady phenomena in the solar wind

Abstract

The role of heat conduction in the solar wind is assessed by a dimensional analysis of the electron energy equation. The relaxation time for transient perturbations of electron temperature is shown to be ∼4×104 sec for reasonable solar wind parameters. For phenomena requiring longer times to propagate or develop (e.g., disturbances propagating near the solar wind speed) the electron energy equation can be approximated as time independent. Two such specific classes of solar wind variations, namely overtaking streams and shock waves, are considered as sources of electron temperature perturbations. In both cases heat conduction is found to be so efficient as to ‘smooth out’ the electron temperature, thus causing any increase to be much smaller than would be observed for protons. The few available pertinent observations of electron temperature are consistent with this analysis. A few effects of electron heat conduction on small-scale phenomena (e.g., the modification of the Rankine-Hugoniot relations applicable to interplanetary shocks) are briefly discussed.