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 $q_\parallel$ 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 $L_T \gtrsim 3.5 ~λ_{fp}$. 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 $λ_{fp}/L_T \gtrsim 0.28$, the electron heat flux is limited to $q_\parallel/q_0 \sim 0.3$, independent of mean free path, where $q_0$ is the 'free-streaming' value; the measured $q_\parallel$ does not achieve the full $q_0$. This constraint $q_\parallel/q_0 \sim 0.3$ might be attributed to wave-particle interactions, 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 $T_e \sim r^{-α}$ 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. We discuss the results in a broader astrophysical context.

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