Collisionless electron heat conduction in the solar wind

Abstract

The point of view that heat-conduction-driven plasma instabilities may not be capable of directly modifying the electron heat conduction flux in the solar wind is explored. The electron heat conduction flux is written either as the usual collision-dominated Spitzer-Harm flux, −KSH▽∥Te, or as the collisionless heat conduction flux (Hollweg, 1974a), 1.5nekTe(Vsw − ω × r)α. The factor α is of order unity but is only estimated. The former expression pertains close to the sun and far from the sun, where collisions are important, while the latter expression pertains in the intermediate region; the divisions between regions are taken to occur where the radial component of the mean free path equals the radial trapping distance, which is taken to be r/2. Perkins' (1973) term is omitted for three reasons: it is often smaller than the collisionless heat conduction flux; it can be reduced by plasma instabilities; and it is not observed by in situ measurements at 1 AU. The electron-proton coupling term is also omitted; this means that the electron temperature is overestimated and that the proton temperature must be specified ad hoc. In comparison with solar wind models which use the Spitzer-Harm flux throughout, the present computations yield the following new features: (1) The electron temperature is elevated in the collision-dominated region close to the sun. (2) The electron temperature falls off more rapidly in the region where the collisionless heat conduction flux is used, varying as ne2/3(1+α) if α = const. (3) The electron temperature and heat conduction flux at 1 AU are lower in the present computations than in models which use only the Spitzer-Harm flux. (4) The elevated electron temperature close to the sun results in higher solar wind flow speeds at 1 AU.