Coronal heating, nanoflares, and MHD turbulence

Abstract

Coronal heating is at the origin of the X-ray emission and mass loss from the Sun and many other stars. While different scenarios have been proposed to explain the heating of magnetically confined and open regions of the corona, they all rely on the transfer, storage and dissipation of the abundant energy present in photospheric motions. Here we focus on theories which rely on magnetic fields and electric currents both for the energy transfer and storage in the corona. The dissipation of this energy, whether in the form of reconnection in current sheets (nanoflares ?) or the dissipation of MHD waves, depends crucially on the development of extremely small scales in the coronal magnetic field, where kinetic effects are likely to be fundamental. The question of whether coronal heating and flares may be viewed respectively as the macroscopic, low-energy average and the high-energy, temporally intermittent aspect of the same underlying driven, dissipative, turbulent system is also addressed, with emphasis placed on the main observational and theoretical stumbling blocks in the way of a confirmation or disproof of such a conjecture.