Will the Next Solar Cycle Surprise Us?

Now that we’ve passed the longest night of the year, many are eager to see the Sun — and astronomers are among them. Every clear day, solar astronomers image the face of the Sun, counting up the dark spots they see there. And a new prediction says they’ll see far more in the coming months and years than they expected. But whether that prediction holds remains to be seen.

Space Weather Forecast

Sunspots are part of an 11-year up-and-down cycle observed since 1610, “one of the oldest data sets that humans have created,” says Dean Pesnell (NASA Goddard), a solar cycle expert. The sunspot cycle is itself just half of the complete magnetic cycle, called the Hale cycle, in which the Sun’s magnetic-field changes come full circle over a period of 22 years.

Just a few months ago, an expert panel of scientists at NASA and the National Oceanic and Atmospheric Administration announced that the beginning of Cycle 25 had occurred in December 2019. That means we’ve hit bottom on sunspot number and are on our way back up, and with sunspots will come all sorts of other magnetically driven activity, like solar eruptions and flares.

But while the timing of the sunspot cycle is relatively predictable, its magnitude isn’t. “The regular up and down in count of sunspot number rarely repeats itself,” Pesnell explains. The inherent turbulence and chaotic nature of the Sun's dynamo makes predicting its behavior difficult at best. In the end, the panel supported physics-based models, which predict Cycle 25's activity will be similar to the surprisingly weak Cycle 24, peaking around July 2025 with some 115 sunspots per month.

A New Prediction: Termination Events

A consensus is useful for planning purposes, particularly for those who run power grids or launch satellites to low-Earth orbit. For scientists, though, the consensus serves primarily as a point of reference. Because it doesn’t mean that theorists are all done making predictions — far from it!

Indeed, Scott McIntosh (National Center for Atmospheric Research) and colleagues know that they’re going against the consensus when they write, “Sunspot Cycle 25 could have a magnitude that rivals the top few since records began.”

They arrive at this conclusion by analyzing 270 years of sunspot numbers. Using a mathematical technique, they realized that every 22-year Hale cycle terminates markedly and quite quickly, “in a quite astonishing time for a star,” McIntosh says. They call these endings termination events. 

What’s more, the timing of the cycle’s termination event seems to correlate with the magnitude of the next sunspot cycle. If that correlation holds, McIntosh and colleagues say that the previous cycle’s brevity means a strong Cycle 25, surpassing 180 sunspots per month at peak. This prediction differs so much from the consensus that we won’t have to wait years, just months to see which one is holding true.

Not everybody’s laying bets on this one, though. “The fact that their prediction gives a result outside the bounds of any other cycle says that they are obviously pushing the method into areas it has never covered — extrapolating results into unknown territory,” says David Hathaway (Stanford University), who was not involved in the study. Testing the method on earlier cycles to see how well it performed then would have provided more convincing evidence, he explains.

What We'll Learn

The study is emblematic of the ongoing debate in solar cycle prediction, which has become “a high-stakes event,” McIntosh and colleagues write. There are dozens of models that predict solar activity, some of them based on simulations of the physics involved and some of them based on the Sun's own past performances. Some even use neural networks to forecast future sunspot counts.

Upon examining the last cycle, panel co-chair Lisa Upton (Space Systems Research Corporation), says that she and the others on the panel recognized the advantage of physics-based models, particularly those that use measurements of the magnetic field at the Sun’s poles as an input to their models. These polar fields appear to be the “seeds” of the magnetic activity over the next cycle. “We weighted those forecasts more highly,” she says. “All of us really felt like these physics-based models had the highest accuracy.”

The suspicion that these polar fields are a strong factor in determining the Sun's overall activity is one of the driving forces behind Solaris, a NASA mission recently selected for concept study. Solaris would provide never-before-seen views of the Sun's poles (though the European Space Agency's Solar Orbiter will provide oblique views of the poles in 2025).

---

---

So, what if Cycle 25 ends up being strong? “If Cycle 25 does indeed grow significantly beyond that of Cycle 24, then we'll have to reconsider how polar fields contribute to the global evolution of the Sun's magnetism and the dynamo process,” McIntosh says. “It is a real crossroads.”

Regardless what the Sun does over the next few months, we’ll learn something about the inner workings of this fierce dynamo.