Solar cycle 25 kicked off last year. Forecasters thought it would be a mild one, but it's turning out to be quite the opposite. From its onset, this solar cycle has been steadily outpacing predictions, producing more sunspots and spewing way more solar wind, flares and eruptions than the world's leading experts predicted.
But while most space weather scientists are scratching their heads, saying "We still know very little about our star," one heliophysicist has become the dark horse in space weather forecasting. His model of the sun's behavior seems to have gotten it right.
A brief glance at the sun's radiant disk reveals little about the star's dynamic life. As ordinary Earthlings, we feel more affected by pesky clouds obstructing its rays. But astronomers equipped with telescopes have known since the 17th century that the sun's surface changes from day to day, breaking out with dark spots that grow and shrink, change shape and move across the sun's surface, and disappear over time to be replaced with new ones. Since 1749, astronomers have been carefully recording those sunspots. By the middle of the 19th century, they realized that the number and size of these spots ebbs and flows following a roughly 11-year solar cycle. Since records began, the star has completed 24 cycles, with cycle 25 currently underway.
But there is more to this ebb and flow. Just like the number of sunspots grows and recedes within a single cycle, the productivity of these cycles varies from one cycle to the next, following patterns that are still little explored.
The two forecasts
Cycle 24, which officially ended in December 2019 but overlapped with the ensuing cycle 25 for quite a while, was one of the weakest on record. When a group of experts from NASA and the National Oceanic and Atmospheric Administration (NOAA) came together to estimate the sun's activity for cycle 25, they predicted that the upcoming cycle would be just as weak.
But at about the same time, another forecast came out. Put together by a team led by Scott McIntosh, a solar physicist and deputy director of the U.S. National Center for Atmospheric Research, this forecast predicted the exact opposite: that cycle 25 might be among the stronger ones in recorded history.
"We looked back over 140-plus years of data about the sun's magnetic activity and its relation to the number of sunspots," McIntosh told Space.com. "And there was a pattern that shaped how large or small the upcoming sunspot cycle was going to be. We predicted the same pattern to take place before solar cycle 25. Based on that, we made a wild scientific guess that cycle 25 could possibly be as high as double the amplitude of cycle 24."
The team published their predictions in the journal Solar Physics in November 2020. Since then, while experts at NOAA and NASA were puzzled, McIntosh and his colleagues watched the sun do exactly what they expected.
For example, whereas NOAA and NASA predicted a meager 27 sunspots for December 2021, the sun produced 67 — more than double the estimate. And in May 2022, instead of the 37 sunspots predicted by NOAA and NASA, there were 97, producing solar flares and coronal mass ejections. The intense solar activity also caused geomagnetic storms on Earth, wreaking havoc on satellites in orbit and triggering magnificent auroral displays.
The official forecast for the peak of the solar maximum, which is expected in April 2025, is 115 sunspots that month. But if the curve follows the current trend, which reflects McIntosh and his team's forecast, the sun will easily reach 115 monthly sunspots by the end of this year and peak two years later at over 210 monthly sunspots.
"It's really wild that the sun continues to do this," McIntosh said. "Month after month, it continues to follow the track. But we'll see. The sun sometimes does weird things, and the cycle could completely fall over tomorrow."
Tzu-Wei Fang, a space scientist at NOAA's Space Weather Prediction Center, agrees that the official solar cycle forecast is not working out and admits that scientists' current understanding of the factors driving the sun's behavior is rather limited.
"We don't know what is driving this strong solar activity," Fang told Space.com. "The sun's behavior changes based on different cycles, from short cycles of 11 days to long cycles of 80 years. There are still a lot of unknowns, and we just don't have enough data points or knowledge to [accurately predict] solar activity.
The Hale cycle
So on what, exactly, do McIntosh and his colleague base their "wild scientific guess"? The team studied the relationship between the roughly 11-year cycle of sunspot generation and the so-called Hale cycle, a 22-year cycle of magnetic activity during which magnetic waves of opposite polarity move from the sun's poles toward the equator, where they eventually meet and cancel each other out.
The Hale cycle, discovered in the early 20th century by American astronomer George Ellery Hale, is likely driven by the so-called solar dynamo, the motion of molten material in the sun's core that generates the sun's magnetic field. The Hale cycle encompasses two sunspot cycles, with the magnetic poles of the star swapping when each sunspot cycle ends. The Hale cycle concludes after 22 years, when the magnetic field returns to the original state.
McIntosh and his colleagues think the interference of the magnetic waves during the Hale cycle drives the creation of spots on the sun's surface.
"Since about a decade ago, we've been tracking the evolution of the Hale cycle," McIntosh said. "Once you know what you're looking for, you can go back all the way to 1860 and extract those features from observations right at the dawn of photography."
The scientists modeled the progression of the magnetic cycle and compared it with historical observations of sunspots. They found a corresponding pattern: At the beginning of every new 11-year cycle, sunspots appear at solar midlatitudes and only later start springing up nearer and nearer to the equator, which matches the motion of those magnetic waves.
"It takes [the magnetic waves] about 19 years to reach the equator, but when they meet, then literally over the course of a few months, the sun goes from having [sunspot] activity at the equator to having nothing," McIntosh said. "And when it goes to having nothing, then new activity springs into life at midlatitudes and a new sunspot cycle starts."
This "meeting in the middle" is when the opposing magnetic fields neutralize each other. The event seems to terminate the previous solar cycle and kick-start the new one, leading scientists to nickname the event "the terminator." Exactly how this terminator event occurs seems to have particular importance for the strength of the next solar cycle, McIntosh said.
"We first spotted [the terminator] about nine years ago," McIntosh said. "And when we looked through the 140 years of data, we were able to make crude measurements of these terminator events at the equator and measure the overlap of the magnetic systems. When you plot that against the upcoming cycle strength, you get a very strong linear relationship."
The data seemed to suggest that the more time between the terminators, the weaker the next solar cycle.
The terminator between solar cycles 24 and 25 was a bit delayed, McIntosh said; solar cycle 24 didn't seem to want to go away. The event finally occurred in December 2021, which enabled McIntosh's team to finalize their solar cycle forecast.
"It will be just above the historical average with a monthly smoothed sunspot number of 190 ± 20," McIntosh told Space Weather Archive in February.
Fang, however, cautions that it's too early to make conclusions about the current solar cycle.
"The fact that we have a strong solar activity now doesn't mean that we are certainly looking at a much higher solar cycle," she said. "It still requires some work for scientists before we can make that conclusion."