Solar Wind Models from the Chromosphere to 1 AU

Recent models of the fast solar wind are characterized by low coronal electron temperatures while proton, α-particle, and minor ion temperatures are expected to be quite high and generally anisotropic, including large temperatures perpendicular to the magnetic field and parallel beams. This entails that the electric field should be relatively unimportant and that solar wind outflows with both high asymptotic flow speeds but maintaining a low mass flux should be a natural outcome of plasma expansion along open polar magnetic field lines. In this chapter we will explain why such changes with respect to the classical, electron thermally driven solar wind have come about and outline the most important remaining concerning the astrophysics of coronal winds. The progress we have seen in the last decade is largely due observations made with instruments onboard Ulysses (McComas et al. in Space Sci. Rev. 72:93, 1995) and SOHO (Fleck et al. in The SOHO Mission, Kluwer, Dordrecht, 1995). These observations have spawned a new understanding of solar wind energetics, and the consideration of the chromosphere, corona, and solar wind as a unified system. We will begin by giving our own, highly biased, “pocket history” of solar wind theory highlighting the problems that had to be resolved in order to make the original Parker formulation of thermally driven winds conform with observational results. Central to this discussion are questions of how the wind’s asymptotic flow speed and mass flux are set, but we will also touch upon higher order moments such as the ion and electron temperatures and heat fluxes as well as the possible role of Alfvén waves and particle effects in driving the solar wind outflow. Solar wind scaling laws will be discussed in the context of the origin of slow and fast wind streams.