Sunspot Index and Long-Term Solar Observations (SILSO) in Belgium said this month that the minimum between Solar Cycles 24 and 25 “most probably” took place last December. SILSO, a part of the Royal Observatory of Belgium and formerly known as SIDC, cited as evidence the January 2020 increase in the 13-month smoothed sunspot number — the first upswing since the Cycle 24 maximum in April 2014.
“[F]or now, this latest smoothed value in January 2020 is the very first point indicating a rise of the activity. So, the date of the minimum still needs a full confirmation over the coming months,” SILSO said on its website. “For now, preliminary smoothed values, limited to less than 13 months, hint at increasing values over coming months. If the rising trend indeed continues, this [December 2019] date will become fully definitive.”
SILSO said another indication of the transition between the two solar cycles can be drawn from counting individual sunspot groups that belong to either the old or new solar cycle. “While most sunspot groups belonged to the last solar cycle [Cycle 24] until September 2019, the dominance switched to groups of the new cycle in November 2019,” SILSO said.
SILSO said that in terms of the number of active regions, the minimum between Cycle 24 and Cycle 25 falls in October 2019. “This is close to December 2019,” SILSO said. It attributes the difference to three factors:
- The sunspot number also takes into account the total number of spots, and thus the size of the emerging active regions.
- The time of the minimum depends on the respective trends of the declining phase of the past cycle, and of the rising phase of the new cycles, over the 12 months surrounding the minimum.
- The date of the minimum has a significant uncertainty range. Near minimum the activity hardly varies and is close to minimum during a few months. “The date of the minimum is thus always less sharply defined than the date of the maximum of the cycles, which are more sharply peaked,” SILSO explained.
- “This late-breaking upward trend is now expected to accelerate over the coming months,” SILSO predicted. “So be prepared for a more eruptive and interesting sun!”
SILSO is preserving the longest record of solar activity, which spans the last 4 centuries.