Understanding variability of solar Balmer lines

Precise, adequately high-cadence, long-term records of spectral variability at different temporal scales lead to better understanding of a wide variety of phenomena including stellar atmospheres and dynamos, evolution of the magnetic fields on a stellar photosphere, convective motions, and rotational periods. These, in turn, are fundamental for the detectability of exoplanets, the characterization of their atmospheres and habitability, as well as characterization of stellar magnetospheres and winds. The Sun, viewed as a star via spectral irradiance measurements, offers a means of exploring such measurements while also having the imaging capability to help discern the causes of observed spectral variations. In this study, we investigate the variability of solar Balmer lines (H-α, β, γ and δ) observed by space-borne radiometers, combining these precise, long-term observations with abundant, high-resolution data from the ground-based NSO/ISS spectrograph. We relate the detected variability to magnetic features on the solar disk. We find that on solar-rotation timescales (~month), the Balmer line activity indices (defined as line-core to line-wing ratios) closely follow variations in the total solar irradiance (which is predominantly photospheric), thus frequently (specifically, during passages of big sunspot groups) deviates from behavior of the line-activity indices that track chromospheric activity levels. At longer timescales (years), the correlation with chromospheric indices increases, with periods of low- or even anti- correlation found at intermediate timescales. Comparisons with Balmer-line variability patterns obtained from a semi-empirical model indicate the periods of low/anti correlations should be attributed to the increase of the relative abundance of network, which affects the Ca-index while leaving almost un-altered the Hα-index