An estimation of the fluctuations in the extreme limb of the Sun

Detailed computations of synthetic solar limb curves are carried out for the purpose of estimating the effects of inhomogeneities in the solar atmosphere upon the observed limb position. Methods of determining the limb position given a solar limb curve are compared. The method of finding the locus of a fixed intensity level with respect to the average disk-center intensity at a given wavelength seems to be the most tractable definition to use on noise free data. It is found that limb fluctuations due to the solar 5-min p-mode oscillations produce a fluctuation in the limb height of about 6 km (0.008 arc sec) rms. Limb fluctuations due to granulation and chromospheric structure are much smaller. The wavelength dependence of the solar H^? opacity causes the height of the limb to increase by about 35 km between 400 and 850 nm, thus leading to a ‘limb reddening’ at the extreme limb of the Sun.