The Sun as the Ultimate Challenge to Astrophysics: The Vital Phase of Solar Physics

Abstract

Observations indicate that the various active phenomena observed on the Sun are the principal constituents of the activity of other solitary late main sequence stars although in different proportions and prominence. But our ability to provide quantitative descriptions of the activity of other stars is limited observationally by our inability to resolve their disks and theoretically by the absence of clear ideas and quantitative theory for the phenomena in the Sun. Thus, for instance, it is not clear why the Sun is obliged to form spots, so it is not clear how to interpret the enormous spot areas that appear on some M-dwarfs. Similary, it is not clear why some M-stars produce flares 10³ times more energetic than on the Sun. The same holds for the structure of the X-ray coronas of other stars, the strength of their stellar winds, the nature of the stellar dynamos, and the luminosity variations in step with their general level of activity. None of these phenomena are properly understood for the Sun, yet there is reason to expect that the crucial observational studies of the Sun (solar microscopy and spectrometry in visible, UV, and X-rays, helioseismology) as well as critical attention to theoretical possibilities, promise progress with several aspects of the problem. The essential point is that the magnetohydrodynamics of the high Reynolds and Lundquist numbers characterizing the convective zone of the Sun is qualitatively different from the familiar concepts of hydrodynamics and the plasma physics laboratory. The fibril state of the magnetic field at the visible surface of the Sun is the direct indication of that fact, and it appears that the dynamo and the several consequences of the dynamo fields can be understood only in terms of the complicated dynamics of magnetic fibrils. We shall be able to estimate the properties of the fibril fields of other stars only when we better understand the physics of fibrils in the Sun.