KINETIC MODELS OF SOLAR AND POLAR WINDS JOSEPH LEMAIRE and VIVIANE PIERRARD Institut d’Aéronomie Spatiale de Belgique, Brussels, Belgium Abstract. The study of the solar corona has been strewn with great discoveries, surprises and controversies. The major steps since van de Hulst’s (1953) and Chapman’s (1957) early hydrostatic models of the extended corona, until the most recent generations of kinetic models of the coronal expansion and of the supersonic solar wind flows, are presented. These models are compared to in- situ observations. Progress in polar wind models went through a somewhat similar evolution that is outlined also. The advantages and limitations of the successive brands of solar wind and polar wind models are considered. 1. Introduction In this paper, a brief historical review of the theoretical models proposed to explain the main observational features of the solar and the polar wind, is given. We show how the solar wind and polar wind models have followed rather parallel develop- ment. This review is non exhaustive due to the limited space but just recounts the main steps in the discoveries and model development. In the often quoted review paper of Lemaire and Scherer (1973), it was shown that solar and polar wind models have followed similar initial evolution. Starting with hydrodynamic models of increasing complexity, exospheric kinetic models have completed the description of the phenomenon in the collisionless regions. Models based on the solution of the Fokker-Planck equation were recently developed to study the wind in any collisional regimes. Although current theoretical models of the radial expansion of the solar corona have improved step by step to provide a more or less comprehensive description of the solar wind, the heating mechanism of the solar corona remains controversial. Indeed, from a thermodynamical point of view, it is difficult to conceive that the corona is so much warmer (10 6 K) than the lower layers of the chromosphere and photosphere. These high coronal temperatures were eventually confirmed by Edlèn, who identified in 1942 coronal red and green lines in the emission spectrum of highly ionized iron ions. The large flow velocities observed in the fast solar wind are still a matter of debate. Indeed, in all current models, the velocity of the particles is related to their temperatures in the corona. But the fast solar wind originates from coronal holes, where, at least the electron temperature is lower than in the equatorial region of the corona. This leads most people to believe that an additional acceleration Astrophysics and Space Science 277: 169–180, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.