SPRG Seminars
September 3, 2013:
"Electron heat conduction in the solar wind: transition from Spitzer-Härm to the collisionless limit"
Stuart D. Bale, Space Sciences Laboratory, University of California at Berkeley
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-Harm collisional heat flux q_sh for temperature gradient scales larger than a few mean free paths (L_T >~ 3.5 lambda_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_fp/L_T >~0.28), the electron heat flux is limited to q_parallel/q_0 ~ 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 ~ 0.3 might be attributed to wave-particle interactions, an interplanetary electric potential, or inherent flux limitation. We also show a beta_e dependence to these results that is consistent with a local radial electron temperature profile T_e ~ r^(-alpha) that is a function of the thermal electron beta and that the beta 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.