On the center-to-limb variation of sunspot brightness

A preliminary analysis of recent measurements of the brightness ratio spot-photosphere of various authors leads to the result, that the limb-darkening of the umbra is considerably less than that of the photosphere. The small value of the center-to-limb variation implies a very small temperature gradient, which is, for optical depths >0.2, about one order of magnitude smaller than previously thought (Figure 3). This result would seem to rule out radiative equilibrium in the deeper layers (\S>0.2) of sunspot umbrae.Mitteilung aus dem Fraunhofer Institut Nr. 83.