THE SUN IS ABOUT TO LOSE ITS MEMORY: That’s the surprising conclusion of a new study by researchers at the University of Oulu in Finland. The team examined 180 years of the aa index (a record of geomagnetic activity stretching back to 1844) and found that after odd-numbered solar cycles, the sun forgets what happened in the past.
We’re approaching such a moment now, as Solar Cycle 25 descends from its peak.
Their research begins with the little-known “Gnevyshev-Ohl rule.” Discovered in 1948, this rule says that solar cycles come in pairs, with the odd-numbered half stronger than the even. Take a look at this historical plot of sunspot numbers:
Once you notice the Gnevyshev-Ohl rule, you can’t unsee it. With only one exception in the past 180 years (Solar Cycles 22-23), odd cycles are always stronger than evens. The current pair, Cycles 24-25, definitely follow the rule.