Solar Wind

Abstract

A concise tutorial review is given of solar wind observations and theory with emphasis on the more recent findings from the Ulysses and SOHO missions, in particular about the plasma state and structure of the polar coronal holes, and on theoretical efforts to model the coronal hole flows and the fast solar wind emanating therefrom. The structure of the large-scale corona is discussed and the microstate of the solar wind in terms of particle and wave observations is analysed. Observational evidence is mounting that the magnetically open coronal holes are far away from a state of local thermodynamic equilibrium, as is the associated interplanetary solar wind, and that they differ substantially in their plasma parameters from the low-latitude streamers with closed magnetic fields, which only open intermittently to release the slow solar wind. The coronal sources and their plasma boundary conditions as well as the interplanetary constraints on the wind models are presented and discussed. Modern theories and models of the solar wind are reviewed. First the basic concept of the single-fluid Parker-type model is outlined, and then two-fluid models are described, in particular those incorporating strong heating of protons close to the Sun, which yields fast acceleration with the terminal wind speed being attained within 10 R _⊙. Finally, the most recent modelling efforts to generate the wind plasma through ionization in the chromospheric network are presented.