Solar H<i>α</i> excess during Solar Cycle 24 from full-disk filtergrams of the Chromospheric Telescope

<jats:p><jats:italic>Context.</jats:italic> The chromospheric H<jats:italic>α</jats:italic> spectral line is a strong line in the spectrum of the Sun and other stars. In the stellar regime, this spectral line is already used as a powerful tracer of stellar activity. For the Sun, other tracers, such as Ca <jats:sc>II</jats:sc> K, are typically used to monitor solar activity. Nonetheless, the Sun is observed constantly in H<jats:italic>α</jats:italic> with globally distributed ground-based full-disk imagers.</jats:p> <jats:p><jats:italic>Aims.</jats:italic> The aim of this study is to introduce the imaging H<jats:italic>α</jats:italic> excess and deficit as tracers of solar activity and compare them to other established indicators. Furthermore, we investigate whether the active region coverage fraction or the changing H<jats:italic>α</jats:italic> excess in the active regions dominates temporal variability in solar H<jats:italic>α</jats:italic> observations.</jats:p> <jats:p><jats:italic>Methods.</jats:italic> We used observations of full-disk H<jats:italic>α</jats:italic> filtergrams of the Chromospheric Telescope and morphological image processing techniques to extract the imaging H<jats:italic>α</jats:italic> excess and deficit, which were derived from the intensities above or below 10% of the median intensity in the filtergrams, respectively. These thresholds allowed us to filter for bright features (plage regions) and dark absorption features (filaments and sunspots). In addition, the thresholds were used to calculate the mean intensity <jats:italic>I</jats:italic><jats:sub>mean</jats:sub><jats:sup>E/D</jats:sup> for H<jats:italic>α</jats:italic> excess and deficit regions. We describe the evolution of the H<jats:italic>α</jats:italic> excess and deficit during Solar Cycle 24 and compare it to the mean intensity and other well established tracers: the relative sunspot number, the F10.7 cm radio flux, and the Mg <jats:sc>II</jats:sc> index. In particular, we tried to determine how constant the H<jats:italic>α</jats:italic> excess and number density of H<jats:italic>α</jats:italic> excess regions are between solar maximum and minimum. The number of pixels above or below the intensity thresholds were used to calculate the area coverage fraction of H<jats:italic>α</jats:italic> excess and deficit regions on the Sun, which was compared to the imaging H<jats:italic>α</jats:italic> excess and deficit and the respective mean intensities averaged for the length of one Carrington rotation. In addition, we present the H<jats:italic>α</jats:italic> excess and mean intensity variation of selected active regions during their disk passage in comparison to the number of pixels of H<jats:italic>α</jats:italic> excess regions.</jats:p> <jats:p><jats:italic>Results.</jats:italic> The H<jats:italic>α</jats:italic> excess and deficit follow the behavior of the solar activity over the course of the cycle. They both peak around solar maximum, whereby the peak of the H<jats:italic>α</jats:italic> deficit is shortly after the solar maximum. Nonetheless, the correlation of the monthly averages of the H<jats:italic>α</jats:italic> excess and deficit is high with a Spearman correlation of <jats:italic>ρ</jats:italic> = 0.91. The H<jats:italic>α</jats:italic> excess is closely correlated to the chromospheric Mg <jats:sc>II</jats:sc> index with a correlation of 0.95. The highest correlation of the H<jats:italic>α</jats:italic> deficit is found with the F10.7 cm radio flux, with a correlation of 0.89, due to their peaks after the solar activity maximum. Furthermore, the H<jats:italic>α</jats:italic> deficit reflects the cyclic behavior of polar crown filaments and their disappearance shortly before the solar maximum. We investigated the mean intensity distribution for H<jats:italic>α</jats:italic> excess regions for solar minimum and maximum. The shape of the distributions for solar minimum and maximum is very similar, but with different amplitudes. Furthermore, we found that the area coverage fraction of H<jats:italic>α</jats:italic> excess regions and the H<jats:italic>α</jats:italic> excess are strongly correlated with an overall Spearman correlation of 0.92. The correlation between the H<jats:italic>α</jats:italic> excess and the mean intensity of H<jats:italic>α</jats:italic> excess regions is 0.75. The correlation of the area coverage fraction and the mean intensity of H<jats:italic>α</jats:italic> excess regions is in general relatively low (<jats:italic>ρ</jats:italic> = 0.45) and only for few active regions is this correlation above 0.7. The weak correlation between the area coverage fraction and mean intensity leaves us pessimistic that the degeneracy between these two quantities can be broken for the modeling of unresolved stellar surfaces.</jats:p>