A Discussion on solar studies with special reference to space observations - The production of solar and stellar chromospheres and coronae

Abstract

Knowledge of the detailed field of turbulence in the solar granulation, and of the consequent photospheric mechanical flux, is one of the basic elements for understanding the solar chromosphere and corona. The other element is constituted by the structure and magnetic fields of the supergranular network, since the coarse mottles at the supergranular boundaries seem to yield a mechanical flux nearly an order of magnitude larger than that of the supergranular cell regions. In the non-magnetic solar regions the upper photospheric mechanical flux is about 0.1 J cm-2 s-1. This flux is equal to that emerging from the low-chromospheric vibrations; it seems not to be related to the observed microturbulent motions. Above such regions the chromosphere may be fairly thin, not exceeding 1000 or 2000 km. The precise thickness and detailed structure of the chromospheric layers can only be determined from a discussion of the interplay of the downward conductive flux with the dissipational losses of the up- and downward mechanical fluxes, and the radiative losses of the chromospheric matter. The greater apparent thickness of the chromosphere at the limb is due to the accumulated influence of the spicules seen from the side. Lack of knowledge of stellar photospheric inhomogeneities and fields handicaps a reliable prediction of stellar chromospheres and coronae. Yet a few attempts have been made, and predicted X-ray fluxes from coronae of some nearby stars are given (table 3).