A FAST SOLAR-WIND MODEL WITH ANISOTROPIC PROTON TEMPERATURE

We explore the energy requirements for the fast solar wind when the an isotropy in the proton temperature is taken into account. Using a one- dimensional, two-fluid model with anisotropic proton temperature, we p resent high-speed solar wind solutions which meet most of the empirica l constraints currently available from in situ measurements in interpl anetary space and very recent; remote sensing observations of the inne r corona. Included in the model is the momentum exerted on the flow by Alfven waves, as well as heating due to their damping. However, to pr oduce solutions consistent with these empirical constraints, additiona l heat input to both electrons and protons, as well as momentum additi on to the protons, are found to be needed. These are described by ad h oc functions with adjustable parameters. While classical thermal condu ction is adopted for both electrons and protons in the inner corona in the model computations, the corresponding heat fluxes in the outer co rona are limited to, values comparable to current observations, The fa st solar wind solutions thus obtained differ from each other mainly in their thermal properties within 0.3 AU from the Sun, a region that is still poorly probed by in situ and remote sensing measurements. To sa tisfy observational constraints, we find that the inclusion of a proto n temperature anisotropy in the modeling of the solar wind requires th at either the protons be highly anisotropic in the inner corona or tha t there exist a mechanism, in addition td adiabatic expansion, to cool them in the direction parallel to the magnetic field. Given these obs ervational constraints and in the absence of knowledge of an efficient cooling mechanism, sur model computations imply that the maximum temp erature of the protons in the parallel direction has to be limited to 10(6) K in the corona. Furthermore, because of the strong coupling bet ween electrons and protons, and between the parallel and perpendicular motions, at the coronal base, the electron temperature as well as the perpendicular proton temperature cannot be much higher than 10(6) K t here. Although thermal anisotropy of the protons is found to have litt le influence on the dynamics of the fast solar wind, its inclusion imp oses new requirements on the unknown coronal heating mechanisms.