Upper panel: simulated Hα ST variability (reconstructed from observed Ca II) versus observed variability, for plage hypothesis A (orange), hypothesis B (brown), and hypothesis C (green), and for the three categories of stars: category #1 (stars), category #2 (diamonds), and category #3 (triangle). The black dashed line is the y = x line. Middle panel: same for the LT Hα variability. Lower panel: simulated ST Hα/Ca versus simulated LT Hα/Ca, for the three categories of stars (same symbols and colour code). The black symbols correspond to the observed values for comparison.