Heating of the Upper Atmosphere

Abstract

It was Biermann (1946) and Schwarzschild (1948) who first suggested the heating of the upper atmosphere (the chromosphere and corona) by sound waves that are generated from turbulence in the convection zone and then steepen to form shock waves as they propagate upwards. Until relatively recently this was universally accepted but now it is thought to be important only for the low chromosphere. This chapter first gives a summary of some energy-balance models that have been proposed for the upper atmosphere (assuming a simple form for the heating), and then it proceeds to discuss the processes that may produce the heating. Qualitatively, it is clear that a source of heat is needed to balance not only the energy radiated away in the chromosphere but also the energy removed by conduction from the temperature maximum. Quantitatively, however, it is still uncertain how the heating varies with altitude, and the detailed nature of the heating mechanism is highly controversial. It is also probable that the heating mechanism in the outer corona is collisionless and so beyond the scope of this book.