Coronal heating, solar wind acceleration and fast reconnection. (Invited)

We review the questions of coronal heating and solar wind acceleration focusing on basic physical mechanisms invoked in the dissipation of free energy in the corona. After a general discussion of the problem of the Poynting flux source and energy balance in the corona, the Parker nanoflare scenario and its generalization to wave-reflection generated turbulence in the open corona will be discussed within the framework of reduced magnetohydrodynamics. We will then move on to the types of current sheets which form, the thicknesses which may be reached and the possibilities for fast reconnection: we will show that a connection between turbulence evolution and linear stability may be be established by studying the growth rate of the tearing mode for sheets with aspect ratios scaling as S^-α. The growth becomes independent of S when the aspect ratio scales as a/L ∼ S^-1/3 which is far thicker than Sweet-Parker current sheets for the huge Reynolds numbers of the solar corona. In addition, this argues in favor of a "fluid" trigger for energetic events, so that kinetic effects become important in current sheets which are already turbulent. Observational implications are discussed in the conclusions.