Fine structure and evershed motions in the sunspot penumbra

The observed inhomogeneity of the intensity and Evershed motions means any model of the penumbra must be essentially inhomogeneous. A simple model is put forward in which the interaction of convection rolls with an initially homogeneous magnetohydrostatic sunspot field causes a concentration of flux in the dark filaments. This process drives Evershed motions outwards in these regions; the motions are superficial and shear the lines of force, so that the field appears stronger and more horizontal in the dark filaments. This situation must be time-dependent to avoid rapid destruc tion of the whole spot.