Solar cycle around the onset of the Spoerer Minimum

Indirect observations of long-term solar activity using cosmogenic nuclides such as carbon-14 in tree rings and beryllium-10 in ice cores have suggested that the Sun has experienced five grand solar minima, among which the Spoerer Minimum occurred in the 14^th to the 15^th century is the longest. Detailed analyses of solar cycles based on carbon-14 in absolutely dated annual tree rings around the onset of such event may give clues on how the periods of extremely weak solar activity occurs; however, it's been a challenging work as the decadal-scale variation of carbon-14 is significantly attenuated in the carbon cycle. By improving the precision of measurement using the Accelerator Mass Spectrometer (AMS) at Yamagata Univ., we have recently achieved the precision of about 0.1% and succeeded to reconstruct solar cycles around the onset of the Spoerer Minimum. For this study, we used Thujopsis dolabrata (Asunaro tree) excavated at Shimokita Peninsula in Japan. Although the precise timing of the onset of sunspot disappearance is not clarified for the Spoerer Minimum, we find that solar cycles had started to be lengthened at least two cycles ahead of the grand minimum.