Contributed Talk - Splinter Solar

Wednesday, 23 September 2020, 15:31   (virtual room L)

Modeling the Ca II H & K emission variations - effect of inclination on the S-index

K. Sowmya, A. I. Shapiro, V. Witzke, N.-E. Nemec, T. Chatzistergos, N. A. Krivova, S. K. Solanki
Max Planck Institute for Solar System Research

The emission in the near ultraviolet Ca II H & K lines, quantified via the S-index, has been serving as a prime proxy of magnetic activity for several decades. The value of the S-index is suspected to be influenced by the inclination angle between the stellar rotation axis and the observer's line-of-sight. Stars are observed at random inclination angles which are normally unknown. It is crucial to understand the inclination effect on the S-index measurements in order to properly use the S-indices to compare the magnetic activity of different stars. However, until now such an inclination effect on the S-index has remained largely unexplored. In order to fill this gap we develop a physics-based model to calculate the S-index, using the Sun which is a perfect gateway to understand other stars. To this end, we make use of the distributions of the solar magnetic features derived from ground and spaced based observations together with the Ca II H & K spectra synthesized using a non local thermodynamic equilibrium radiative transfer code. We validate our model by successfully reconstructing the observed variations of solar S-index across four recent activity cycles and subsequently use the model to obtain a time series of the S-index dating back to 1700. Further, we investigate the intricate relationship between the inclination and the S-index variability, both on the magnetic activity cycle and the rotational timescales. We achieve this by using the distribution of magnetic features over the visible solar disk obtained from the surface flux transport simulations for various inclinations. We find that with decreasing inclination, the amplitude of S-index variations decreases weakly on the activity cycle and strongly on the stellar rotation timescales. Also, the absolute value of the S-index depends only weakly on the inclination angle. We use the model to identify the range in the S-index variations and the mean activity level caused by the change in the inclination. Using this we show that the chromospheric emission variations for the Sun is normal for a star with solar like activity level. All in all, our model facilitates a better understanding of the connection between the inclination and the S-index, thus enhancing the diagnostic potential of the Ca II H & K emission measurements.