The physics of chromospheres and coronae

Towards the goal to unravel the physical reasons for the existence of chromospheres and coronae significant progress has been made. Chromospheres and coronae are layers which are dominated by mechanical heating and usually by magnetic fields. The heating of chromospheres can be explained by an ordered sequence of different processes which systematically vary as function of height in the star and with the speed of its rotation. It seems now pretty certain that acoustic waves heat the low and middle chromosphere, and MHD waves the magnetic regions up to the high chromosphere. With faster rotation, the magnetic regions become more dominant. It seems that the highest chromosphere needs additional non-wave heating mechanisms and that there possibly reconnective microflare heating comes into play. For the corona many different heating processes occur which work in the various field geometries. Here more study is needed to identify the relevance of these various processes.