Context. The chromospheric Hα 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 II K, are typically used to monitor solar activity. Nonetheless, the Sun is observed constantly in Hα with globally distributed ground-based full-disk imagers.
Aims: The aim of this study is to introduce the imaging Hα 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 Hα excess in the active regions dominates temporal variability in solar Hα observations.
Methods: We used observations of full-disk Hα filtergrams of the Chromospheric Telescope and morphological image processing techniques to extract the imaging Hα 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 ImeanE/D for Hα excess and deficit regions. We describe the evolution of the Hα 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 II index. In particular, we tried to determine how constant the Hα excess and number density of Hα 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 Hα excess and deficit regions on the Sun, which was compared to the imaging Hα excess and deficit and the respective mean intensities averaged for the length of one Carrington rotation. In addition, we present the Hα excess and mean intensity variation of selected active regions during their disk passage in comparison to the number of pixels of Hα excess regions.
Results: The Hα 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 Hα deficit is shortly after the solar maximum. Nonetheless, the correlation of the monthly averages of the Hα excess and deficit is high with a Spearman correlation of ρ = 0.91. The Hα excess is closely correlated to the chromospheric Mg II index with a correlation of 0.95. The highest correlation of the Hα 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 Hα deficit reflects the cyclic behavior of polar crown filaments and their disappearance shortly before the solar maximum. We investigated the mean intensity distribution for Hα 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 Hα excess regions and the Hα excess are strongly correlated with an overall Spearman correlation of 0.92. The correlation between the Hα excess and the mean intensity of Hα excess regions is 0.75. The correlation of the area coverage fraction and the mean intensity of Hα excess regions is in general relatively low (ρ = 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.