The Solar Granulation

Abstract

Convective motions play a significant role in determining the structure of the outer layers of many stars including the sun. Subsurface convective motions in the sun produce, at the visible surface, the well-known cellular pattern that is called the "solar granulation." The solar granulation has its origin in the convectively unstable hydrogen ionization zone which begins about 300 km below the visible photosphere. The instability results from a combination of high opacity, which causes a steep radiative temperature gradient, and a low γ (ratio of specific heats) associated with the ionization of hydrogen. An upward-displaced fluid element, expanding so as to remain in pressure-equilibrium with its surroundings, finds itself warmer (and hence lighter) than the surroundings, and thus tends to continue rising; the reverse holds for a downward-displaced element, which tends to sink farther. Thus convective motions develop, so that the outward flow of heat energy proceeds partly by radiative transport and partly by convective transport.