A hydrostatic model atmosphere of a typical umbral core is constructed on the basis of both Na I D 2 line profile and umbral core-to disk continuum intensity ratio. The observations were obtained by Mullan and Wyller with photoelectric means over the range from 4000 |AA a n d 6500 |AA . The computed line profile of Na I D 2 and the emergent continuum intensity distribution are compared with the observations. The temperature of umbral cores is found to be lower by 300 ˚ K t o 500 ˚ K than that of their ambient umbral region. The detailed physical structure of a working model of umbral cores is proposed.

ABSTRACT
Ⅰ. INTRODUCTION
Ⅱ. PHOTOELECTRIC OBSERVATIONS OF UMBRAL CORES
Ⅲ. MODEL ATMOSPHERES OF UMBRAL CORE
Ⅳ. COMPARISON OF THEORETICAL RESULTS WITH OBSERVATIONS
Ⅴ. CONCLUSION
ACKNOWLEDGEMNTS
REFERENCES

• HONG SIK YUN(Department of Astronomy, College of Natural Sciences, Seoul National University)