Simulated R′_HKindex distributions of solar mass stars in open clusters with ages of 5, 13, 85, 112, 240, 625, 1000, 2400, and 4000 Myr. The results are compared with measured R′_HK histograms of0.9–1.1 M_⊙ stars in the Pleiades, the Hyades, and M67 scaled to the 979 stars used in the simulation. The short-term variability of the magnetic activity is described by a normal distribution of the short-term CaII chromospheric emission with a variance of 0.327 × ⟨ R′_HK ⟩^1.15 around the average chromospheric emission ⟨ R′_HK⟩ derived from a parametric model of rotation evolution for solar mass stars.