A New Fluid Scheme for Weakly Collisional Plasmas. II: The Case of Solar Wind

Abstract. In the previous paper of this series (Paper I) we investigated the possibility of extending to weakly collisional plasmas the new fluid scheme recently developed by Chen, Rao and Spiegel for neutral gases. In that pa- per we established the conditions under which such an extension is feasible and identified the solar wind as a good candidate to test the new theory. Such a test is performed in the present paper, where we actually build a model for the fast solar wind and compare it with the available data. We define the basic assumptions adopted: steadiness, spherical symmetry, neglect of mag- netic field and of minor ions. We limit our treatment to the range 2 −215R⊙ to cover the details of the transition from collisional to collisionless regimes, excluding the low corona and the chromosphere that appear to be far too complex to be described by our simple model. We present a detailed discus- sion concerning the choice of the parameters and boundary conditions en- tering the model. We then proceed to the actual construction of the model, whose main merit lies in its simplicity: it is a two-fluid model capable of ac- counting for both viscosity and thermal conduction, in spite of the very lim- ited number of equations to be solved. We then critically compare our re- sults with the observations and with other recent solar wind models. The level of agreement with the data is quite satisfactory and is comparable with that obtained by more sophisticated models.