The sun is not equally bright over the whole sphere, but rather is darkened toward the limb. This effect is well-known as limb darkening. The limb darkening coefficient is defined by the ratio of the center intensity to limb intensity. In this study, we calculate the limb darkening coefficient using the photospheric intensity estimated from solar images taken by solar and helispheric observatory (SOHO) and solar dynamics observatory (SDO). The photospheric intensity data cover almost two solar cycles from May 1996 to December 2016. The limb darkening coefficient for a size of 0.9 diameter is about 0.69 and this value is consistent with solar limb darkening. The limb darkening coefficient estimated from SOHO shows a temporal increase at solar maximum and a gradual increase since the solar minimum of 2008. The limb darkening coefficient estimated from SDO shows a constant value of about 0.65 and a decreasing trend since 2014. The increase in the coefficient reflects the effect of weakened solar activity. However, the decrease since 2014 is caused by the aging effect.

• Byeongha Moon(Department of Earth Science Education, Chonnam National University, Gwangju 61186, Korea)
• Dong-Gwon Jeong(Department of Earth Science Education, Chonnam National University, Gwangju 61186, Korea)
• Suyeon Oh(Department of Earth Science Education, Chonnam National University, Gwangju 61186, Korea) Corresponding author
• Jongdae Sohn(Korea Astronomoy and Space Science Institute, Daejeon 34055, Korea)