Acceleration of weakly collisional solar-type winds

One of the basic properties of the solar wind, the high speed of the fast wind, is still not satisfactorily explained. This is mainly due to the theoretical difficulty of treating weakly collisional plasmas. The fluid approach implies that the medium is collision dominated and that the particle velocity distributions are close to Maxwellian. However, the electron velocity distributions observed in the solar wind depart significantly from Maxwellian. Recent kinetic collisionless models (called exospheric) using velocity distributions with a suprathermal tail have been able to reproduce the high speeds of the fast solar wind. In this Letter we present new developments of these models by generalizing them over a large range of corona conditions. We also present new results obtained by numerical simulations that include collisions. Both approaches calculate the heat flux self-consistently without any assumption on the energy transport. We show that both approaches-exospheric and collisional-yield a similar variation of the wind speed with the basic parameters of the problem; both produce a fast wind speed if the coronal electron distribution has a suprathermal tail. This suggests that exospheric models contain the necessary ingredients for powering a transonic stellar wind, including the fast solar wind.