Limb-darkening and solar cycle variation of sunspot intensities

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 m. 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.     

The wavelength dependence of granulation (0.38–2.4 μm)

Using 2 pinhole photometers the intensity of the undisturbed photosphere was recorded simultaneously in 6 and in 4 wavelength regions. The rms value of the intensity variation in each of the 10 wavelength regions decreases slightly with increasing value of the heliocentric angle; this result confirms recent observations by other authors and supports the critique of the results given by Edmonds (1964).We report the detection of a secondary maximum in the wavelength dependence of the intensity variation at 1.5 m.     

The effect of spatial smearing on solar Doppler measurements

A mathematical method for calculating the influence of scattered light on solar Doppler measurements is developed and presented.It is shown that the main contribution to the error signal is caused by the long range scattering component of the stray-light.The method is applied on Doppler measurements of solar rotation, and the results are compared with the observations from Stanford and Mt. Wilson that hitherto have only been crudely corrected for the effects to straylight. Some of the large scatter in the published results on the solar rotation rate is shown to be caused by a too simple parameterization of the straylight.Within the uncertainties of the current measurements it is shown that the Doppler rotation rate of the solar photosphere is equal to the sunspot tracer rotation rate.     

Solar cycle variation of sunspot intensity

Broad band pinhole photometer intensity observations of 15 large sunspots covering the spectral region 0.387–2.35 m are presented. The data are based on measurements on approximately 500 days during the period June, 1967 to December, 1979.We have found real and significant intensity differences between large sunspots. These differences may be explained by a systematic variation in the umbral temperature throughout the solar cycle. A connection between umbra intensity and heliographic latitude is discussed.No center-limb variation in the umbra/photosphere intensity ratio is detected. We have searched for possible connections between umbra intensity and a number of other sunspot parameters, like the spot size, without detecting any significant correlation. We conclude that the umbra/photosphere intensity ratio seems to be a unique function of epoch for large sunspots.     

On the need for space observations of the umbra/photosphere intensity ratio

We draw attention to the possibility of distinguishing between different sunspot theories by observing: (i) The umbra/photosphere intensity ratio as a function of spot size and (ii) the morphology and time evolution of sunspot inhomogeneities such as umbral dots. In arguing the need for space observations of sunspot intensities we discuss the corrections for stray light for ground based and space observations.The opportunity to use the November 13, 1986 Mercury transit as an in situ calibration event is pointed out.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.