Photo-electric observations of the intensity profile of a sunspot at a heliocentric angle of 76 ~ at four wavelengths are reported. An intensity peak at the centre-side penumbra-photosphere border predicted by Jensen et al. (1969) has been observed.
Several papers have recently been published in this journal describing theoretical and observational work on sunspot intensity profiles (Jensen et aL, 1969; Wilson and McIntosh, 1969; SchriSter and Wittmann, 1969). A considerable amount of interest has been concentrated on the centre-limb changes in the profiles. We wish to report photo-electric observations of the intensity profile of a sunspot at a heliocentric angle of 76 ~ .
The observations were obtained under conditions of good (but not excellent) seeing at Gornergrat, Switzerland. The spot was allowed to drift at the diurnal rate over a 4"-diameter aperture. The intensity was sampled for 70 microsec every 80 microsec and 1280 samples lumped together to give a 2'.'3 sampling interval. Observations were made at 224416, 5216, 6376, and 7100, with a bandwidth varying between 1 and 1.5 β’. Seeing conditions were fairly stable during most of the observing period but deteriorated noticeably when the longest-wavelength drift scans were being made. For this reason we shall not discuss these scans in detail.
Some of the scans are shown in Figure 1. They exhibit the following features: (i) At 2 6376 there is a clear intensity peak at the centre-side penumbra-photosphere boundary. This enhancement is visible on all the scans at this wavelength, though not with uniform clarity possibly because of seeing variations. The 2 7170 drift scans also show the same feature.
(ii) At shorter wavelengths this peak becomes much less definite. It is barely present at 2 4416.
(iii) On the limb side the limb-darkened contour is disturbed, there being a definite inflexion at the penumbra-photosphere border. Interpolation of the undisturbed intensity distribution over the region of the spot (see Figure 2) shows that an intensity peak is present on the limb side of the spot as well as on the centre side. The distinctness of this feature seems to be wavelength-dependent, though perhaps not as strongly as the centre-side enhancement.
(iv) There is a marked asymmetry in the profiles, the rise from umbra to photosphere being more gradual on the limb side.
Figure 2 shows a single scan at each of three wavelengths after correction for the finite scanning aperture and a short-range (blurring) function of gaussian shape and Solar Physics 11 (1970) 319-322.