SOLARNET Conference: The Many Scales of the Magnetic Sun

Emission in the Ca II H & K spectral lines is sensitive to the changes in the stellar magnetic activity and is a good proxy for studying stellar magnetic fields. This emission is most commonly characterized via the S-index, representing the flux in the Ca II H & K line cores relative to the flux in the nearby continuum. The sources of stellar Ca II emissions are yet to be investigated in detail. Empirical and physics-based models have shown faculae/plage to be the main source of Ca II emissions and thus neglect the contribution from spots. This is because the solar surface coverage by spots is much smaller than those by faculae. Further, spots are assumed to be equally dark in the Ca II line cores and the continuum so that they do not affect the S-index. While this is a good approximation for old less-active stars such as the Sun, for young active stars with large surface coverages by spots, the role played by spots might become significant.

Here we use high quality sunspot observations in the Ca II H line from the Swedish Solar Telescope to determine the Ca II flux from chromospheric counterparts of sunspots. We find that spots are brighter than the surrounding quiet regions in the chromosphere. Using the sunspot flux computed from observations in our physics-based model which also accounts for the effect of faculae, we compute S-index for four solar activity cycles. We find that sunspots lead to an increase in the S-index. However, this increase is quite small to have a significant impact on the observed solar Ca II emissions. Further, we compute S-indices considering cases of stars which are more active than the Sun. Previous studies have indicated that surface coverages of spots increase much faster with increasing activity than the coverages by faculae. We show that with increasing activity and hence spot area coverages, S-index increases steeply suggesting that Ca II emissions of active stars have a significant contribution from spots.