Electron heat conduction in the solar wind: Transition from spitzer-härm to the collisionless limit

Abstract

We use a statistically significant set of measurements to show that the field-aligned electron heat flux q ∥ in the solar wind at 1 AU is consistent with the Spitzer-Härm collisional heat flux q sh for temperature gradient scales larger than a few mean free paths LT ≳ 3.5λfp. This represents about 65% of the measured data and corresponds primarily to high β, weakly collisional plasma ("slow solar wind"). In the more collisionless regime λfp/L T ≳ 0.28, the electron heat flux is limited to q ∥/q 0 ∼ 0.3, independent of mean free path, where q 0 is the "free-streaming" value; the measured q ∥ does not achieve the full q 0. This constraint q ∥/q 0 ∼ 0.3 might be attributed to wave-particle interactions, effects of an interplanetary electric potential, or inherent flux limitation. We also show a βe dependence to these results that is consistent with a local radial electron temperature profile Te ∼ r -α that is a function of the thermal electron beta α = α(βe) and that the β 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. © 2013. The American Astronomical Society. All rights reserved.