A Transonic Collisionless Model of the Solar Wind

Because of the semi-collisional nature of the solar wind, the collisionlessor exospheric approach as well as the hydrodynamic one are both inaccurate.However, the advantage of simplicity makes them useful for enlightening somebasic mechanisms of solar wind acceleration. Previous exospheric models havebeen able to reproduce winds that were already nearly supersonic at theexobase, the altitude above which there are no collisions. In order to allowtransonic solutions, a lower exobase has to be considered, in which case theprotons are experiencing a non-monotonic potential energy profile. This is donein the present work. In this model, the electron velocity distribution in thecorona is assumed non-thermal. Parametric results are presented and show thatthe high acceleration obtained does not depend on the details of thenon-thermal distributions. This acceleration seems, therefore, to be a robustresult produced by the presence of a sufficient number of suprathermalelectrons. A method for improving the exospheric description is also given,which consists in mapping particle orbits in terms of their invariants ofmotion.Comment: 18 pages, 18 figures, accepted for publication in The Astrophysical Journal (1 May 2004