Published February 26, 2021 | Version v1
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Characterization of Chromospheric Activity Based on Sun-as-a-star Spectral and Disk-resolved Observations

  • 1. Leibniz Institute for Astrophysics Potsdam
  • 2. {Ohio State University




The Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) is a state-of-the-art, thermally stabilized, fiber-fed, high-resolution spectrograph for the Large Binocular Telescope (LBT) at Mt. Graham, Arizona. It can be fed with sunlight from the Solar Disk-Integrated (SDI) telescope. Synoptic solar observations with PEPSI/SDI produce daily spectra with high signal-to-noise ratio, providing access to unprecedented, quasi-continuous, long-term, disk-integrated spectra of the Sun with high spectral and temporal resolution. The observed spectra contain a multitude of photospheric and chromospheric spectral lines in the wavelength range of 380 910 nm. Strong chromospheric absorption lines, such as the Ca II H & K lines, are powerful diagnostic tools for solar activity studies, since they trace the variations of the solar magnetic field. Derivation of activity indices, such as the Ca II H & K emission ratio S-index provides insight into the chromospheric magnetic field and its variability over the solar activity cycle. The well known relation between solar calcium indices and UV flux variations motivates us to compute an excess brightness indices from Ca II K full-disk images from of the Chromospheric Telescope (ChroTel) at the Observatory del Teide on Tenerife, Spain and UV data of the  Solar Dynamics Observatory (SDO). We present a set of indices representing magnetic activity at various heights in the solar atmosphere. In the present work, we carefully compare the indices computed from various datasets and discuss the differences in terms of physical and observational properties.



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