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.

### Citation Style Language JSON Export

{
"publisher": "Zenodo",
"DOI": "10.5281/zenodo.4564796",
"language": "eng",
"title": "Modelling Solar Ca II H&K Emission Variations",
"issued": {
"date-parts": [
[
2021,
2,
26
]
]
},
"abstract": "<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>",
"author": [
{
"family": "Sowmya, Krishnamurthy"
},
{
"family": "Shapiro, Alexander I."
},
{
"family": "Witzke, Veronika"
},
{
"family": "N\u00e8mec, Nina-E."
},
{
"family": "Chatzistergos, Theodosis"
},
{
"family": "Yeo, Kok Leng"
},
{
"family": "Krivova, Natalie A."
},
{
"family": "Solanki, Sami K."
}
],
"id": "4564796",
"event-place": "virtually anywhere",
"type": "graphic",
"event": "The 20.5th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun (CS20.5)"
}
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