Peak solar activity is arriving sooner than expected, reaching levels not seen in 20 years | Science | AAAS

In 2019, as the Sun approached a minimum in its 11-year cycle of magnetic activity, a dozen scientists assembled for a traditional exercise: forecasting the next peak. Now, a few years into the Sun’s resurgence, it’s becoming clear that the official prediction from the panel, convened by NASA, the National Oceanic and Atmospheric Administration (NOAA), and the International Space Environment Service (ISES), missed the mark. The Sun’s activity has already surpassed the forecast, reaching levels not seen in 20 years, and solar maximum may arrive within the next year, months ahead of its presumed schedule. “Obviously the panel underestimated it,” says Ilya Usoskin, a physicist at the University of Oulu.

The discrepancy highlights a need for better observations of the Sun. It may also point to unknown factors influencing the churning dynamo of ionized gas that gives rise to the Sun’s magnetic field. “I’d like to think we’re making progress in terms of understanding the dynamo, but there’s work to do,” says Mark Miesch, a solar physicist at the University of Colorado Boulder.

The stakes are high. At peak activity, the Sun more often unleashes particle storms that crash into Earth, threatening satellites, jamming radio transmissions, and overloading power grids. Because the previous cycle was unusually mild, “We’ve been lulled into a false sense of complacency,” says Tamitha Skov, a heliophysicist at Millersville University.

Scientists typically track solar cycles by counting sunspots—flares of activity spurred by knots of magnetic field loops. The sunspot number climbs over the course of a solar cycle, then drops near zero as magnetic activity subsides. When the NASA-NOAA-ISES prediction panel met in 2019, it analyzed about 60 different forecast models, each offering an estimate for the peak number of sunspots and when it would arrive.

Some of the models are purely statistical, making forecasts by extrapolating centuries of sunspot observations. Others rely on observable “precursors” thought to be correlated with the solar cycle, such as the strength of the magnetic field at the Sun’s poles at solar minimum. As the cycle progresses, that “seed field” gets more powerful as its field lines are wound up into a doughnut shape by the way the Sun rotates—faster at the equator than at the poles. A third category relies on advanced computer models that work like climate models, ingesting as much observable data as possible and then using the laws of physics to simulate the Sun’s dynamo and shifting magnetic fields.

After a week of discussing the merits of different approaches, the panel voted and hashed out a consensus: The monthly sunspot count would peak at about 115, sometime around July 2025—making it a relatively weak cycle, much like the preceding one. But the Sun has already woken up faster and is feistier than expected. It sported 159 sunspots in July and 115 in August.

“Did we get it absolutely right? No,” says Lisa Upton, a physicist at the Southwest Research Institute who co-chaired the panel. “But considering the level of uncertainty that’s associated with what we’re trying to do here, it’s actually a quite good prediction.”

Upton believes one reason the panel’s prediction fell short is the quality and longevity of the observations that feed and drive the precursor and dynamo models—most importantly, the strength of the polar magnetic field. Those values come primarily from the Wilcox Solar Observatory, which can see the imprint of the polar field on the spectrum of sunlight. But the telescope has relatively poor resolution and a limited view. NASA mission concepts such as Firefly and Solaris would send spacecraft closer to the Sun to probe its polar fields directly, but they’re still in the development phase.

Other researchers suspect a deeper snag. The relationship between polar magnetic fields and subsequent solar activity is drawn from measurements spanning only a few decades, and other factors may be at work. Clues are coming from observations led by Scott McIntosh, a solar physicist and deputy director of the National Center for Atmospheric Research.

For 2 decades, he and colleagues have tracked millions of “bright points” in extreme-ultraviolet images of the Sun that they think trace bands of magnetic field traveling under the Sun’s skin. The bright points seem to follow a pattern across two solar cycles: Clusters routinely emerge at midlatitudes at the start of the first solar cycle. They then migrate toward the equator as the solar activity peaks, falls, and peaks again. At the end of the second cycle, the points suddenly disappear in what the researchers call a “terminator event.” Just after this event, the bright points reappear at midlatitudes and start the cycle afresh.

McIntosh believes the double-cycle pattern means the underlying field bands from subsequent cycles must be interacting—sometimes constructively, leading to increased solar activity. And he thinks the timing of consecutive terminator events can be used to forecast this interference—and the height and timing of the next solar maximum. After spotting the most recent terminator event in December 2021, he and colleagues predicted this cycle’s sunspots would peak at about 184 sometime near early 2024.

“It’s a fascinating pattern and something that will challenge dynamo theory,” Miesch says. Dibyendu Nandi, an astrophysicist at the Indian Institute of Science Education and Research Kolkata who worked on dynamo models used in the 2019 panel prediction, doesn’t buy the predictive power of the terminator events. He does, however, still believe that bright points may be an important signal. “I think they’re onto something,” he says.

The dynamo simulations have come a long way in the past decade and now predict the polar seed fields pretty well, Nandi says. If overall solar activity continues to ramp up far beyond predictions, scientists might have to reconsider whether polar fields are really the only thing driving the solar cycle, he reasons. Perhaps, as McIntosh’s observations suggest, the interaction of lingering magnetic fields in the Sun’s interior are leaving a footprint on the next cycle. It’s a possibility that Nandi is investigating in his models now.

“If there’s one certainty in this field of prediction,” Nandi says, “it’s that we should be always ready to be proved wrong and go back to our drawing boards.”

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