A Model Atmosphere for the Solar Limb Based on Continuum Observations

Abstract

The photospheric temperature distribution previously derived by the author is combined with the results for the low chromosphere obtained by the High Altitude Observatory group from observations of the continuum at the 1952 eclipse. The resulting model, assumed to be in hydrostatic equilibrium with negligible turbulence and a hydrogen-helium ratio 10 : 1, is used to calculate the intensities of continuous radiation at various heights near the limb and in the lowest 600 krn of the chromosphere at λλ 4700, 3650, and 3640, carrying out the numerical integrations with the aid of EDSAC. The gradient and absolute values of the emission at λ 4700 are in satisfactory agreement with observations in the lowest part of the chromosphere, as is the general behaviour of the calculated emission in the Balmer continuum. The darkening at the extreme limb is calculated, taking curvature into account, from the adopted model and from the non-grey radiative-equilibrium model calculated by Böhrn (1954 a). The observational results obtained at eclipses scatter too much among themselves for a convincing comparison to be carried out. The centre-limb variation in the Balmer discontinuity agrees well with observations for cos $$\theta \leqslant0\cdot 5$$ and satisfactorily for cos $$\theta \lt0\cdot 5$$ . This suggests equilibrium excitation of hydrogen to the second quantum level in those regions where Balmer continuum absorption is significant ( $${\tau }_{0} \geqslant0\cdot001$$ ).