In situ measurements for the fast solar wind: (a) plasma temperatures from Helios (filled diamonds) and Ulysses (small points), with protons in red and electrons in blue. (b) Electron heat conduction flux from Helios (filled diamonds) and Ulysses (triangles). Also shown are least-squares fits (solid lines), the classical Spitzer–Härm heat flux (dashed line), and the Hollweg (1974, 1976) collisionless heat flux with α _e = 1.05 (dotted line).