A correlation between sunspot observations and solar Ca <scp>ii</scp> H&amp;K activity proxies

ABSTRACT The magnetic phenomena on the solar surface have been the subject of several investigations over the last 400 yr. An early indicator of solar magnetic activity was sunspot counting. Currently, the main sunspot indicators are the international sunspot number, the sunspot group number, the total sunspot area, and the photometric sunspot index. Several improvements in observational techniques have allowed magnetic activity to be measured using solar/stellar spectra. Standard spectroscopic activity indicators are the $S_{\rm MW}$ index, based on the Ca ii H&amp;K emission lines, and the chromospheric component $R^{\prime }_{\rm HK}$ index. In this context, we present a correlation between sunspot observations and solar Ca ii H&amp;K activity proxies. We present our comparisons between the spectroscopic chromospheric activity proxies ($S_{\rm MW}$ and $R^{\prime }_{\rm HK}$) and the sunspot indicators over the last decades, using solar measurements (spectroscopic and spot proxy) performed on the same day. In general, our results indicate a linear fit between the chromospheric proxies and sunspot indicators. In addition, using the long-term sunspot group number records, we estimate an average spectroscopic proxy along the solar Maunder minimum (MM) phase, corresponding to $\mathinner {\langle {S_{\rm MW}}\rangle } = 0.167 \pm 0.013$ and $\mathinner {\log \langle {R^{\prime }_{\rm HK}}\rangle } = -4.913 \pm 0.363$. The estimated variability is $\sigma _{S_{\rm MW}} = 1.137 \times 10^{-7}$ and $\sigma _{\log R^{\prime }_{\rm HK}} = 2.704 \times 10^{-6}$. Our linear regression analysis, applied annually, suggests that the variability level of the chromospheric activity in the MM phase is significantly lower than in the normal period of activity and that this could be the result of linear regression on annually averaged data, combined with minimal sunspot activity during the solar MM phase. Further observations of MM analogues will be needed to test this hypothesis.