Abstract

The most commonly used index of stellar magnetic activity is the instrumental flux scale of singly ionized calcium \(H\) & \(K\) line core emission, \(S\), developed by the Mount Wilson Observatory (MWO) HK Project, or the derivative index \({R}_{\mathrm{HK}}^{\prime }\). Accurately placing the Sun on the \(S\) scale is important for comparing solar activity to that of the Sun-like stars. We present previously unpublished measurements of the reflected sunlight from the Moon using the second-generation MWO HK photometer during solar cycle 23 and determine cycle minimum \({S}_{23,\min }=0.1634\pm 0.0008\), amplitude \({\rm{\Delta }}{S}_{23}=0.0143\pm 0.0012\), and mean \(\langle {S}_{23}\rangle =0.1701\pm 0.0005\). By establishing a proxy relationship with the closely related National Solar Observatory Sacramento Peak calcium \(K\) emission index, itself well correlated with the Kodaikanal Observatory plage index, we extend the MWO \(S\) time series to cover cycles 15–24 and find on average \(\langle {S}_{\min }\rangle =0.1621\pm 0.0008\), \(\langle {\rm{\Delta }}{S}_{\mathrm{cyc}}\rangle =0.0145\pm 0.0012\), \(\langle {S}_{\mathrm{cyc}}\rangle =0.1694\pm 0.0005\). Our measurements represent an improvement over previous estimates that relied on stellar measurements or solar proxies with non-overlapping time series. We find good agreement from these results with measurements by the Solar-Stellar Spectrograph at Lowell Observatory, an independently calibrated instrument, which gives us additional confidence that we have accurately placed the Sun on the \(S\)-index flux scale.