5.3.3 Limb Darkening of Solar Photosphere
The brightness of the solar surface is not uniform. It gradually decreases from the
disk center toward the limb. This is called solar limb darkening. In the early 1940s,
the solar atmosphere was widely assumed to be in radiative equilibrium.
Hitotuyanagi pointed out that the limb darkening derived on this assumption clearly
deviates from the observed darkening. He considered the existence of a convection
layer under the solar atmosphere in radiative equilibrium. In the convection layer,
since a part of radiative energy is consumed by the kinetic energy of convection
flow, the temperature declines faster than the case of pure radiative equilibrium. The
existence of a convection layer was first recognized in 1930 by A. Unsöld, and its
effects on the Fraunhofer lines and continuous spectrum were investigated by E. A.
Milne in 1930 and by P. C. Keenan in 1938.
Hitotuyanagi examined the effects of the convection layer in detail, and he
derived theoretical limb darkening. The results of his numerical calculations in the
Fig. 5.9 Theoretical limb
darkening and comparison
with observations. The
dotted and real curves
represent the first and
second approximation,
respectively. (Hitotuyanagi
1941a)
132
5 Astronomy in Early Showa. III. Sendai 1926–1945
The brightness of the solar surface is not uniform. It gradually decreases from the
disk center toward the limb. This is called solar limb darkening. In the early 1940s,
the solar atmosphere was widely assumed to be in radiative equilibrium.
Hitotuyanagi pointed out that the limb darkening derived on this assumption clearly
deviates from the observed darkening. He considered the existence of a convection
layer under the solar atmosphere in radiative equilibrium. In the convection layer,
since a part of radiative energy is consumed by the kinetic energy of convection
flow, the temperature declines faster than the case of pure radiative equilibrium. The
existence of a convection layer was first recognized in 1930 by A. Unsöld, and its
effects on the Fraunhofer lines and continuous spectrum were investigated by E. A.
Milne in 1930 and by P. C. Keenan in 1938.
Hitotuyanagi examined the effects of the convection layer in detail, and he
derived theoretical limb darkening. The results of his numerical calculations in the
Fig. 5.9 Theoretical limb
darkening and comparison
with observations. The
dotted and real curves
represent the first and
second approximation,
respectively. (Hitotuyanagi
1941a)
132
5 Astronomy in Early Showa. III. Sendai 1926–1945