SOLARNET Conference: The Many Scales of the Magnetic Sun

The solar ultraviolet (UV) and extreme ultraviolet (EUV) radiations which are major contributors to the solar spectral irradiance (SSI) below 4000 Å, significantly affect the composition and the thermal structure of the Earth's atmosphere. The Magnesium II core-to-wing ratio (also known as the Mg II index) is one of the best proxies for solar activity and UV-SSI variability. To date, the Mg II index has been studied, assuming Sun-as-a-star using low (11 Å) and medium spectral resolution (1 Å) data. On the other hand, Mg II h and k line pairs being optically thick, are highly sensitive to the thermodynamical properties of the line-forming region and the viewing angle. Therefore, in this study, we explore the center-to-limb variation (CLV) in the Mg II line and continuum intensities in various solar features like quiet Sun (QS), plage, inter-plage, sunspot umbra, and penumbra using the high spatial and spectral resolution data from the Interface Region Imaging Spectrometer (IRIS). Our study indicates that plages with the lowest magnetic field density show the highest limb darkening as opposed to the QS, whereas the umbra and penumbra show a combination of both limb darkening and brightening depending on their photospheric magnetic field densities. This would provide us an opportunity to understand the importance of the spatially resolved Mg II index on the SSI studies.