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 ∥ 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 L T ≳ 3.5λ fp . 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 λ fp / 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 β e dependence to these results that is consistent with a local radial electron temperature profile T e ∼ r −α that is a function of the thermal electron beta α = α(β e ) 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.