THERMALLY DRIVEN ONE-FLUID ELECTRON-PROTON SOLAR-WIND - 8-MOMENT APPROXIMATION

In an effort to improve the ''classical'' solar wind model, we study a n eight-moment approximation hydrodynamic solar wind model, in which t he full conservation equation for the heat conductive flux is solved t ogether with the conservation equations for mass, momentum, and energy . We consider two different cases: In one model the energy flux needed to drive the solar wind is supplied as heat fur from a hot coronal ba se, where both the density and temperature are specified. In the other model, the corona is heated. In that model, the coronal base density and temperature are also specified, but the temperature increases outw ard from the coronal base due to a specified energy flux that is dissi pated in the corona. The eight-moment approximation solutions are comp ared with the results from a ''classical'' solar wind model in which t he collision-dominated gas expression for the heat conductive flux is used. It is shown that the ''classical'' expression for the heat condu ctive flux is generally not valid in the solar wind. In collisionless regions of the how, the eight-moment approximation gives a larger ther malization of the heat conductive flux than the models using the colli sion-dominated gas approximation for the heat flux, but the heat flux is still larger than the ''saturation heat flux,'' This leads to a bre akdown of the electron distribution function, which turns negative in the collisionless region of the flow. By increasing the interaction be tween the electrons, the heat flux is reduced, and a reasonable shape is obtained on the distribution function. By solving the full set of e quations consistent with the eight-moment distribution function for th e electrons, we are thus able to draw inferences about the validity of the eight-moment description of the solar wind as well as the validit y of the very commonly used collision-dominated gas approximation for the heat conductive flux in the solar wind.