The Importance of Suprathermal Electrons in the Solar Wind

Abstract:

Non-Gaussian distributions are ubiquitous in systems having long-range interactions, from real-world networks to astrophysical plasmas. The corona and solar wind are no exception. In this review, we concentrate on the corona and solar wind electrons, whose suprathermal tail governs heat transport and plays a crucial role in the temperature structure and wind production, as first suggested thirty years ago by Olbert and confirmed by a large number of subsequent studies. These non-thermal electrons have been measured in both the corona and solar wind, and are a direct consequence of the fast increase with speed of the Coulomb free-path, compared to the pressure scale-height. This situation has four important consequences: (1) the fluid description, on which the vast majority of solar wind models are based is inadequate; (2) the heat flux is NOT given by the classical Spitzer-Härm expression in the corona and solar wind; (3) for most non-thermal distributions (except the convenient and fashionable Kappa distribution), the fraction of supra-thermal electrons increases with altitude in the corona because of velocity filtration; for example, with a sum of Maxwellians, the hotter the population, the larger the increase with altitude of its fractional contribution; (4) ad-hoc heat addition – assumed in most models, is not necessarily required to produce the observed variation in temperature and the wind acceleration. We will shortly review the observed electron velocity distributions together with the theoretical expectations, the major role of the electric field and the consequences on the heat flux, the temperature structure and the wind acceleration.