Estimation of solar ultraviolet irradiance by examining spectroheliogram of Ca II K line

To evaluate the effects of 11-year solar cycle on the Earth’s climate, many studies have been operated as space climate. First thought may come with the total solar irradiance affecting climate change, but it changes only 0.1% in 11-yr activity cycle. Meanwhile, solar ultraviolet (UV) irradiance at wavelength ranging 1000-2000Å changes more than 10 % over the 11-year activity cycle. Most of UV is absorbed by the Earth’s middle atmosphere or thermosphere, so variation of UV irradiance could contribute on climate change. Due to the lack of observation with satellites before 1970s, we do not have direct observational data in UVs. On the other hand, the good correlation between solar images in UVs with those in chromospheric lines, such as Ca II K, has been also known. Because Ca II K line is observable on the ground, there are astronomical observatories around the world having Ca II K data for a century, at Kyoto University (from 1928), NAOJ (from 1917), Kodaikanal (from 1907), and Mt.Wilson (from 1915). By combining these 100-year long Ca II K observations and the correlation between UV and Ca II K line, we may find a hint on the long-term variation of the solar UV irradiance. . In this paper we examine correlations between a spectroheliogram of Ca II K line taken by Domeless Solar Telescope (DST) of Hida Observatory, Kyoto University and 1600Å emissions taken by Atmospheric Imaging Assembly (AIA) on board of Solar Dynamic Observatory (SDO) on May 5, 2018. By taking advantage of high wavelength resolution of DST, we search with various integration intervals of Ca II K line for a higher correlation with 1600Å emissions.