Time series of the He triplet properties and calcium activity index ($R_{HK}^{\mathrm{\prime }}$) over all days of observation. Left column: from top to bottom are shown the width, depth, position, and EW of the He triplet line, along with the $R_{HK}^{\mathrm{\prime }}$ activity index. Data binned over each day of observation are displayed in various colors, while unbinned data are shown in gray. Sinusoidal variations in the $R_{HK}^{\mathrm{\prime }}$ index highlight the presence of active regions during our observations. This is further corroborated by the Solar Dynamics Observatory (SDO) images shown in Figs. D.1 and D.2. Right column: He triplet properties plotted against the $R_{HK}^{\mathrm{\prime }}$ index, considering only the binned data. Each subplot’s legend displays the corresponding Spearman correlation coefficient, r_S. The black lines denote the linear fit to the correlation studies with the highest coefficients, while the shaded regions show the corresponding 95% confidence intervals. We discuss these proportionality relationships in Sect. 4.2. We provide axes for the width in km/s and depth in % to facilitate the understanding of the analogy between temperature and width, and column density and depth.