Hydrodynamics of the Solar Photosphere: Model Calculations and Spectroscopic Observations

Abstract

A close-up view of the solar photosphere under good conditions of atmospheric seeing shows a characteristic cellular pattern covering the entire surface (except for sunspots), the so called solar granulation. Bright isolated elements, the granules, are surrounded by a network of interconnected dark lanes. In contrast to W. Herschel, who first reported the existence of some small-scale inhomogeneities on the solar surface in 1801, we now have a basic physical understanding of the origin of the solar granulation. The essential foundation was laid by Unsöld [1], who pointed out that because of the sharp increase with depth of opacity and ionization of hydrogen, the Schwarzschild criterion for convective instability should be satisfied in the deep photospheric layers. According to current mixing length models of the solar atmosphere (Kurucz [2]), the dividing level between stable (radiative) and unstable (convective) layers is located somewhere between τ ₅₀₀₀ = 0.8 and τ₅₀₀₀ = 1. Consequently, granulation is identified with convection cells in the uppermos0t layers of the hydrogen convection zone, forming the observed continually changing cellular pattern at the interface between optically thick and optically thin conditions. Granules represent hot rising gas elements while the intergranular lanes consist of cooler sinking gas. This picture has been verified spectroscopically, confirming the convective origin of the solar granulation.