A fast solar wind model with anisotropic proton temperature

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Abstract

We explore the energy requirements for the fast solar wind when the anisotropy in the proton temperature is taken into account. Using a one‐dimensional, two‐fluid model with anisotropic proton temperature, we present high‐speed solar wind solutions which meet most of the empirical constraints currently available from in situ measurements in interplanetary space and very recent remote sensing observations of the inner corona. Included in the model is the momentum exerted on the flow by Alfvén waves, as well as heating due to their damping. However, to produce solutions consistent with these empirical constraints, additional heat input to both electrons and protons, as well as momentum addition to the protons, are found to be needed. These are described by ad hoc functions with adjustable parameters. While classical thermal conduction is adopted for both electrons and protons in the inner corona in the model computations, the corresponding heat fluxes in the outer corona are limited to values comparable to current observations. The fast 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 satisfy observational constraints, we find that the inclusion of a proton temperature anisotropy in the modeling of the solar wind requires that either the protons be highly anisotropic in the inner corona or that there exist a mechanism, in addition to adiabatic expansion, to cool them in the direction parallel to the magnetic field. Given these observational constraints and in the absence of knowledge of an efficient cooling mechanism, our model computations imply that the maximum temperature of the protons in the parallel direction has to be limited to 10 6 K in the corona. Furthermore, because of the strong coupling between 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 there. Although thermal anisotropy of the protons is found to have little influence on the dynamics of the fast solar wind, its inclusion imposes new requirements on the unknown coronal heating mechanisms.

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