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Progress in Earth and Planetary Science
Fig. 2 | Progress in Earth and Planetary Science

Fig. 2

From: Solar evolution and extrema: current state of understanding of long-term solar variability and its planetary impacts

Fig. 2

The yearly mean total sunspot number time series. Observations spanning four centuries show that the number of sunspots observed on the solar surface increases and decreases in a quasi-periodic fashion with a mean period of 11 years. The plot shows the yearly mean ISN (v 2.0) from 1700 until 2019.5. The plot 1700 (the vertical dashed line) show Group Number series (v 1.0). This is scaled by a multiplicative factor of 10 to amplify the plot until the dashed line. The amplitude of sunspot cycles varies from one cycle to another and displays secular trends that are not well-understood. A phase of very low solar activity with very few sunspot observations—known as the Maunder minimum occurred between 1645 and 1715. Solar activity reconstructions over thousands of years indicate the presence of many more extremely low or strong activity phases in the solar cycle (Usoskin 2017). Credit: SILSO, Royal Observatory of Belgium, Brussels

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