The period from 1640-1710 when almost no sunspots were observed. It coincided with a period of colder-than-average temperatures in northern Europe called the Little Ice Age.
Little Ice Age, Sunspot
Schröder, W. J. Geom. Geoelctr. 1992.
Schröder, W. Das Phänomen des Polarlichts. Darmstadt, 1984.
