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  Modeling Stellar Ca ii H and K Emission Variations. I. Effect of Inclination on the S-index

Sowmya, K., Shapiro, A., Witzke, V., Nèmec, N.-E., Chatzistergos, T., Yeo, K. L., et al. (2021). Modeling Stellar Ca ii H and K Emission Variations. I. Effect of Inclination on the S-index. The Astrophysical Journal, 914(1): 21. doi:10.3847/1538-4357/abf247.

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 Creators:
Sowmya, K.1, Author
Shapiro, Alexander2, Author           
Witzke, V.3, Author           
Nèmec, Nina-Elisabeth3, Author           
Chatzistergos, T.3, Author           
Yeo, K. L.2, Author           
Krivova, N. A.2, Author           
Solanki, S. K.3, Author           
Affiliations:
1IMPRS on Physical Processes in the Solar System and Beyond, Max Planck Institute for Solar System Research, Max Planck Society, ou_1832290              
2Max Planck Research Group in Solar Variability and Climate, Max Planck Institute for Solar System Research, Max Planck Society, ou_2265637              
3Department Sun and Heliosphere, Max Planck Institute for Solar System Research, Max Planck Society, ou_1832289              

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 Abstract: The emission in the near-ultraviolet Ca ii H and K lines is modulated by stellar magnetic activity. Although this emission, quantified via the S-index, has been serving as a prime proxy of stellar magnetic activity for several decades, many aspects of the complex relation between stellar magnetism and Ca ii H and K emission are still unclear. The amount of measured Ca ii H and K emission is suspected to be affected not only by the stellar intrinsic properties but also by the inclination angle of the stellar rotation axis. Until now, such an inclination effect on the S-index has remained largely unexplored. To fill this gap, we develop a physics-based model to calculate S-index, focusing on the Sun. Using the distributions of solar magnetic features derived from observations together with Ca ii H and K spectra synthesized in non-local thermodynamic equilibrium, we validate our model by successfully reconstructing the observed variations of the solar S-index over four activity cycles. Further, using the distribution of magnetic features over the visible solar disk obtained from surface flux transport simulations, we obtain S-index time series dating back to 1700 and investigate the effect of inclination on S-index variability on both the magnetic activity cycle and the rotational timescales. We find that when going from an equatorial to a pole-on view, the amplitude of S-index variations decreases weakly on the activity cycle timescale and strongly on the rotational timescale (by about 22% and 81%, respectively, for a cycle of intermediate strength). The absolute value of the S-index depends only weakly on the inclination. We provide analytical expressions that model such dependencies.

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Language(s): eng - English
 Dates: 2021
 Publication Status: Published online
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 Table of Contents: -
 Rev. Type: Peer
 Identifiers: DOI: 10.3847/1538-4357/abf247
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Title: The Astrophysical Journal
Source Genre: Journal
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Publ. Info: Bristol; Vienna : IOP Publishing; IAEA
Pages: - Volume / Issue: 914 (1) Sequence Number: 21 Start / End Page: - Identifier: ISSN: 0004-637X
CoNE: https://pure.mpg.de/cone/journals/resource/954922828215_3