Vanden Broeck, Grégory ; Bechet, Sabrina ; Rauw, Gregor ; Clette, Frédéric

Talk presented at Triennial Earth-Sun Summit, Dallas, United States on 2024-04-09

Abstract: The chromospheric emission in the Ca II K line is a primary tracer of magnetic activity in the Sun and other Sun-like stars. Our goal is to study the long-term brightness variation of the chromosphere in this line taking into account the different chromospheric structures. The Sun is observed from Earth from its Equator's point of view, whereas others stars are observed at all possible inclinations. The chromospheric plages, main contributors to the Ca II K emission, are distributed between mid-latitude and the Equator. Therefore, we suspect that the inclination angle of the solar rotation axis has an impact on the observed plages area fraction and intensity. Until now, the effect of such inclination on Ca II K plages areas has not been extensively studied through direct solar observations. For this study, we exploit the archives of synoptic full-disc images taken on-site at the Royal Observatory of Belgium with the Uccle Solar Equatorial Table (USET) since 2012. We first describe the methods to estimate the bright plages area fraction from the USET images. Then we explain how we reconstruct the solar images for multiple inclinations. Finally, we show the results obtained for the dependance of the inclination angle of view on the Ca II K plages area.

Keyword(s): Sun: activity ; Sun: chromosphere ; Sun: plages ; Sun: Sun-as-a-star ; Sun: rotation period