The latest solar minimum came in December 2019 and the Solar Cycle 25 has started
Solar Science Observatory, NAOJ
Sunspot data obtained by the NAOJ show that the sunspot number reached the minimum, which corresponds to the boundary of the Solar Cycles 24 and 25, most probably in December 2019. The Solar Cycle 25 has started. The solar activity, which varies cyclically, has been low for the past few years. Hereafter the Sun will become active year by year toward the solar maximum. Large solar flares and mass ejections, which are harmful to astronauts, satellite operations, and radio communications, will get more frequent with rising solar activity.
Figure 1 shows the variation of the monthly average (black solid line) and the 13-month moving average (red solid line) of the sunspot number (the relative sunspot number) from January 2000 to August 2020. While the monthly average is not smooth, the 13-month moving average shows the long-term variation of the solar activity distinctly. The latest solar minimum in December 2019 which found in the 13-month moving average is indicated at the right-hand side of the figure with a dotted line. Our result is consistent with that derived by SILSO, the world data center for the international sunspot number at the Royal Observatory of Belgium [1].
The transition from the Solar Cycle 24 to 25 was also confirmed with the magnetic properties of the sunspot groups. Figure 2 shows the monthly numbers of sunspot groups belonging to the Cycles 24 and 25 appeared between January 2018 and August 2020. The sunspot groups belonging to the Solar Cycle 25 have become the majority after November 2019, which is close to the minimum.
How will the solar activity develop in the future? It is known that the period of solar activity tend to become longer during the grand solar minimum; the duration of the Solar Cycle 23 was longer than 12 years, and the activity of the Solar Cycle 24 was lower than that of the Solar Cycle 23. However, the Solar Cycle 24 lasted just 11 years. The possibility that the solar activity influences the climate has been discussed, and therefore, the solar activity in the new Solar Cycle 25 draws a particular attention.
Figure 1. Temporal variations in a monthly mean (black solid line) of and a 13-month moving average (red solid line) of relative sunspot numbers from January 2000 to August 2020. The vertical dotted lines indicate two minima of the 13-month moving average.
Figure 2. Monthly numbers of sunspot groups and active regions identified between January 2018 and August 2020. In addition to the numbered NOAA active regions, unnumbered sunspot groups found by the NAOJ are included.
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