A Gaussian spread function for the solar aureole

An analytical expression for the limb profile and the aureole is derived for a Gaussian spread function with b 2. Power series with 3 terms are used as approximation to the limb darkening given by HSRA (Gingerich et al., 1971). For smaller values of the spread parameter b the expression given by Staveland (1972) gives a more correct result. The rms difference in the intensity is 0.005 between these analytical expressions and a numerical integration (Brahde, 1972) both inside and outside the limb. Mullan's (1973) expression gives too low intensities in the range ±2b around the limb.