ON 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 τ>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 (τS>0.2) of sunspot umbrae. © 1969 D. Reidel Publishing Company.