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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<oai_dc:dc xmlns:dc="" xmlns:oai_dc="" xmlns:xsi="" xsi:schemaLocation="">
  <dc:contributor>Wolk, Scott</dc:contributor>
  <dc:creator>Sowmya, Krishnamurthy</dc:creator>
  <dc:creator>Shapiro, Alexander I.</dc:creator>
  <dc:creator>Witzke, Veronika</dc:creator>
  <dc:creator>Nèmec, Nina-E.</dc:creator>
  <dc:creator>Chatzistergos, Theodosis</dc:creator>
  <dc:creator>Yeo, Kok Leng</dc:creator>
  <dc:creator>Krivova, Natalie A.</dc:creator>
  <dc:creator>Solanki, Sami K.</dc:creator>
  <dc:description>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 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 H&amp;K spectra synthesized using a non-local thermodynamic equilibrium (non-LTE) radiative transfer code.

We show that the value of the solar S-index is influenced by the inclination angle between the solar rotation axis and the observer’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.</dc:description>
  <dc:subject>The Sun and the Heliosphere</dc:subject>
  <dc:subject>Solar activity</dc:subject>
  <dc:subject>Solar chromosphere</dc:subject>
  <dc:subject>Stellar activity</dc:subject>
  <dc:title>Modelling Solar Ca II H&amp;K Emission Variations</dc:title>
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