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

Talk presented at International Union of Geodesy and Geophysics, Berlin, Germany on 2023-07-13

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 (plages, network, sunspots). Those structures are the main contributors to the variability of the solar irradiance, which plays a important role in the chemistry of the Earth atmosphere. For this, we exploit the archives of synoptic full-disc images taken at the Royal Observatory of Belgium with the Uccle Solar Equatorial Table (USET). Ca II K images were recorded since 2012, allowing us to study the second half of the cycle 24 and the beginning of the cycle 25. We first describe the methods to determine the area fraction covered by bright plages, based on the USET images. In a Sun-as-a-star approach, we then compare our results with the S-index data from the TIGRE telescope, located in Guanajuato, Mexico, observing the integrated solar spectrum reflected by the Moon. The S-index measures the magnetic activity in stars and is calculated as the ratio of the flux in two spectral lines, Ca II H and K, which are sensitive to magnetic activity in stars. Our solar Ca II K images can thus serve as a "Rosetta stone" to study the similar mechanisms acting in other sun-like stars.

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