Poster Open Access

# Modelling Solar Ca II H&K Emission Variations

Sowmya, Krishnamurthy; Shapiro, Alexander I.; Witzke, Veronika; Nèmec, Nina-E.; Chatzistergos, Theodosis; Yeo, Kok Leng; Krivova, Natalie A.; Solanki, Sami K.

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"description": "<p>The emission in the near ultraviolet Ca II H&amp;K lines, often quantified via the S-index, has been serving as a prime proxy of solar and stellar magnetic activity. Despite the broad usage of the S-index, the link between the coverage of a stellar disk by magnetic features and Ca II&nbsp;H&amp;K emission is not fully understood. In order to fill this gap we developed a physics-based model to calculate the solar S-index. To this end, we made use of the distributions of the solar magnetic features derived from the simulations of magnetic flux emergence and surface transport, together with the Ca II&nbsp;H&amp;K spectra synthesized using a non-local thermodynamic equilibrium (non-LTE) radiative transfer code.</p>\n\n<p>We show that the value of the solar S-index is influenced by the inclination angle between the solar rotation axis and the observer&rsquo;s line-of-sight, i.e. the solar S-index values obtained by an out-of-ecliptic observer are different from those obtained by an ecliptic-bound observer. This is important for comparing the magnetic activity of the Sun to other stars. We computed time series of the S-index as they would be observed at various inclinations dating back to 1700. We find that depending on the inclination and period of observations, the activity cycle in solar S-index can appear weaker or stronger than in stars with a solar-like level of magnetic activity. We show that there is nothing unusual about the solar chromospheric emission variations in the context of stars with near-solar magnetic activity.</p>",
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"affiliation": "Max Planck Institute for Solar System Research",
"@id": "https://orcid.org/0000-0002-3243-1230",
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"name": "Sowmya, Krishnamurthy"
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"affiliation": "Max Planck Institute for Solar System Research",
"@id": "https://orcid.org/0000-0002-8842-5403",
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"name": "Shapiro, Alexander I."
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"affiliation": "Max Planck Institute for Solar System Research",
"@id": "https://orcid.org/0000-0002-0929-1612",
"@type": "Person",
"name": "Witzke, Veronika"
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"affiliation": "Max Planck Institute for Solar System Research",
"@id": "https://orcid.org/0000-0001-6090-1247",
"@type": "Person",
"name": "N\u00e8mec, Nina-E."
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"affiliation": "Max Planck Institute for Solar System Research",
"@id": "https://orcid.org/0000-0002-0335-9831",
"@type": "Person",
"name": "Chatzistergos, Theodosis"
},
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"affiliation": "Max Planck Institute for Solar System Research",
"@id": "https://orcid.org/0000-0003-3851-7323",
"@type": "Person",
"name": "Yeo, Kok Leng"
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"affiliation": "Max Planck Institute for Solar System Research",
"@id": "https://orcid.org/0000-0002-1377-3067",
"@type": "Person",
"name": "Krivova, Natalie A."
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"affiliation": "Max Planck Institute for Solar System Research",
"@id": "https://orcid.org/0000-0002-3418-8449",
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"name": "Solanki, Sami K."
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],
"url": "https://zenodo.org/record/4564796",
"datePublished": "2021-02-26",
"keywords": [
"The Sun and the Heliosphere",
"Solar activity",
"Solar chromosphere",
"Stellar activity"
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"name": "Wolk, Scott"
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"identifier": "https://doi.org/10.5281/zenodo.4564796",
"@id": "https://doi.org/10.5281/zenodo.4564796",
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"name": "The 20.5th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun"
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"name": "Modelling Solar Ca II H&K Emission Variations"
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