Magnetograms and averaged sunspot number. Panel a: photospheric solar disk magnetic field observation by HMI on 2 December 2014 12:00:00. The color represents the strength of magnetic field. The region marked by the red lines shows −50° to ∼50° in latitude and −10° to ∼10° in longitude. Panel b: day-averaged sunspot numbers in each year during the time of data sets. The colors represent the phases: Yellow for ascending phase, deep red for maximum phase, blue for declining phase, and dark blue for minimum phase.