Wings of the butterfly: Sunspot groups for 1826–2015R. Leussu, I. G. Usoskin, V. Senthamizh Pavai, A. Diercke, R. Arlt, C. Denker and K. MursulaA&A, 599 (2017) A131DOI: https://doi.org/10.1051/0004-6361/201629533
