Abstract
New observations of the umbral limb-darkening are presented. We find a real and significant decrease in the umbra/photosphere intensity ratio towards the limb. This result contrasts the findings of previous authors and we believe this to be the first time such a decrease is reported. Our conclusion is based on broad band pinhole photometer intensity observations of 22 large sunspots covering the spectral region 0.387–2.35 μm. The data are selected from measurements on approximately 600 days during the last 15.5 yr. The application of the limb-darkening data to the study of the temperature stratification in the umbra is briefly discussed. The observations confirm the suggestion that the umbra/photosphere intensity ratio seems to be a linear function of the phase in the solar cycle.
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Albregtsen, F., Jorås, P.B. & Maltby, P. Limb-darkening and solar cycle variation of sunspot intensities. Sol Phys 90, 17–30 (1984). https://doi.org/10.1007/BF00153781
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DOI: https://doi.org/10.1007/BF00153781