ELECTRON HEAT CONDUCTION IN THE SOLAR WIND: TRANSITION FROM SPITZER-HÄRM TO THE COLLISIONLESS LIMIT

We use a statistically significant set of measurements to show that the field-aligned electron heat flux q sub(||) in the solar wind at 1 AU is consistent with the Spitzer-Harm collisional heat flux q sub(sh) for temperature gradient scales larger than a few mean free paths L sub(T) [> ~] 3.5[lambda] sub(fp). This represents about 65% of the measured data and corresponds primarily to high beta , weakly collisional plasma ("slow solar wind"). In the more collisionless regime [lambda] sub(fp)/L sub(T) [> ~] 0.28, the electron heat flux is limited to q sub(||)/q sub(0) ~ 0.3, independent of mean free path, where q sub(0) is the "free-streaming" value; the measured q sub(||) does not achieve the full q sub(0). This constraint q sub(||)/q sub(0) ~ 0.3 might be attributed to wave-particle interactions, effects of an interplanetary electric potential, or inherent flux limitation. We also show a beta sub(e) dependence to these results that is consistent with a local radial electron temperature profile T sub(e) ~ r super(- alpha ) that is a function of the thermal electron beta alpha = alpha ( beta sub(e)) and that the beta dependence of the collisionless regulation constraint is not obviously consistent with a whistler heat flux instability. It may be that the observed saturation of the measured heat flux is a simply a feature of collisional transport. We discuss the results in a broader astrophysical context.