In this note some properties of the nonconvolved brightness profile close to the extreme solar limb are described. In particular, at 6 cm and 11 cm wavelength relatively strong, narrow spikes should occur, noticeably affected by inhomogeneities (for example spicules) in the solar atmosphere.
In an earlier paper (Fiirst et al., 1979, here called Paper I) we reported on the brightness distribution across the solar disk and beyond the limb. We described a simple atmospheric model including inhomogeneities, which is capable of fitting the observed centre-to-limb variation and the radio radii at wavelengths of 1.2, 2.8, 6, and 11 cm. The temperature and density distribution with height above the photosphere was derived from network EUV-line intensities (model EUVN). However, Chambe (1978) first noticed that this model leads to a discrepancy between EUVline intensities and the centimeter radio brightness. In Paper I we have shown that the radio brightness is obtained when using an atmospheric model based on the cell's EUV-line intensities (model EUVc). Both models fit the observed centre-to-limb variation as well as the radio radii reasonably well. For the best fits, these models provide some information on the nonconvolved brightness profile close to the extreme solar limb, which might be tested by future high resolution observations. Some properties of these 'true profiles' in the equatorial plane shall be described in this paper.
In Figure 1 the true profile at 11 cm wavelength is plotted (model EUVN) with and without taking account of inhomogeneities (spicules). In the absence of spicules a strong narrow spike at the limb occurs, which is strongly depressed in the presence of inhomogeneities. At 11 cm wavelength a small spike remains outside the limb with about 180% of the central brightness Ic.
However, if we convolve this true profile to the 11 cm beam of the Bonn 100 m-telescope (4.5 arcmin half width), the spike becomes invisible.