The Role of Collisions in the Acceleration of the Solar Wind: Results From Kinetic Simulations.

The Knudsen number for a non uniform plasmas is generally defined as the ratio of the collisional mean free path of a typical particle of the plasma (electron, proton, ...) to the typical scale of variation of a macroscopic quantity of the plasma (density or temperature). For a solar wind electron in the the vicinity of the sonic point the Knudsen number K is likely to be of the order unity. As a consequence neither fluid nor collisionless models are appropriate models for the description of such a plasma. We present results from self-consistent time independent kinetic simulations which illustrate how fundamental collisions (not necessarily Coulombian collisions) are for the wind to be a transonic wind. As an example we show that if wave-particle interactions are neglected and non thermal distributions excluded a transonic wind is only compatible with a Knudsen number substantially smaller than unity, i.e. K=O(0.1). The conclusion is that for a transonic wind to exist the Knudsen number (measured by taking into account all type of collisions) near the sonic point is necessarily smaller than some limiting value of the order 0.1 unless non thermal velocity distribution functions are invoked.