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

The magnetic phenomena on the solar surface have been the subject of several investigations over the last 400 years. The early indicator of the solar magnetic activity was the 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 measuring the magnetic activity using solar/stellar spectra. Standard spectroscopic activity indicators are the S(MW )index, based on the Ca II H&K emission lines, and the chromospheric component R-HK ' index. In this context, we present a correlation between sunspot observations and solar Ca II H&K activity proxies. We present our comparisons between the spectroscopic chromospheric activity proxies (SMW and RHK ') and the sunspot indicators along 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 sunspots 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 < S-MW > = 0.167 +/- 0.013 and log < R-HK '>=-4.913 +/- 0.363. The estimated variability is sigma S-MW=1.137x10(-7) and sigma log R-HK '=2.704x10(-6). Our linear regression analysis, applied annually, suggests that the variability level of the chromospheric activity in MM phase is significantly lower than in the normal period of activity or could be due to linear regression on annually averaged data, combined with the minimal sunspot activity during solar Maunder Minimum. Further observations of MM analogues will be needed to test this hypothesis.