The 25th solar cycle began last year. It's turning out to be quite the opposite of what forecasters expected. In terms of sunspots, solar wind, flares, and eruptions, this solar cycle has surpassed world experts' predictions since its onset, producing more than expected.
However, one heliophysicist is emerging as the dark horse in space weather forecasting, even as most space weather scientists scratch their heads. It appears that his model of the sun's behaviour is accurate.
An in-depth look at the sun's radiant disk provides little insight into the star's dynamic life. When its rays are obscured by clouds, we feel more at odds with our surroundings. Since the 17th century, astronomers have known that the sun's surface changes from day to day, revealing dark spots that grow and shrink, change shape and move across its surface, and disappear over time. A careful record of those sunspots has been kept by astronomers since 1749. In the middle of the 19th century, scientists discovered that the number and size of these spots ebb and flow according to an 11-year solar cycle. With cycle 25 underway, the star has completed 24 cycles since records began.
There is more to this ebb and flow than meets the eye. Sunspot numbers grow and recede within a single cycle, and their productivity varies from cycle to cycle, according to patterns that are not well understood.
Where NASA Went Wrong.
Cycle 24, which ended in December 2019 but overlapped with cycle 25 for a while, was one of the weakest in history. NASA and the National Oceanic and Atmospheric Administration (NOAA) predicted the upcoming cycle would be equally weak as cycle 25, based on their estimates.
A second forecast came out around the same time. Developed by Scott McIntosh, a solar physicist and deputy director of the National Center for Atmospheric Research, this forecast predicted the exact opposite: cycle 25 might be among the strongest ever recorded.
"We looked back over 140-plus years of data about the sun's magnetic activity and its relation to the number of sunspots," McIntosh said. "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."
Solar Physics published the team's predictions in November 2020. McIntosh and his colleagues have watched the sun do exactly what they predicted, while NOAA and NASA experts were puzzled.
NOAA and NASA predicted 27 sunspots for December 2021, but the sun produced 67 - more than twice what they predicted. In May 2022, instead of the 37 sunspots predicted by NOAA and NASA, there were 97, causing solar flares and coronal mass ejections. As a result of the intense solar activity, geomagnetic storms also occurred on Earth, causing havoc on satellites in orbit and triggering magnificent auroral displays.
A solar maximum of 115 sunspots is expected in April 2025, which is the official forecast. But if the curve follows the current trend, which reflects McIntosh and his team's forecast(opens in new tab), 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."
Physicist Tzu-Wei Fang admits that scientists' understanding of the factors that drive the sun's behaviour is limited and that the official NASA solar cycle forecast is not working out.
"We don't know what is driving this strong solar activity," Fang told Space.com. "The sun's behaviour 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
#TerminatorWatch. Why has #SunspotCycle25 taken off.......? Well..... the #Terminator has arrived. In this thread we'll show some of the preliminary observations and discuss the implications for our forecast of #SunspotCycle25 strength. pic.twitter.com/hrgMWyslzz
— Scott McIntosh (@swmcintosh) January 14, 2022
What exactly is the basis for McIntosh's "wild scientific guess"? In their study, the team explored the relationship between sunspot generation and the Hale cycle, a 22-year magnetic activity cycle in which oppositely polarised magnetic waves move from the poles to the equator and cancel each other out.
As George Ellery Hale discovered in the early 20th century, the Hale cycle is likely driven by the solar dynamo, the movement of molten material in the sun's core that creates the sun's magnetic field. Hale cycles consist of two sunspot cycles in which the magnetic poles of the star swap at the end of each cycle. After 22 years, the magnetic field returns to its original state, completing the Hale cycle.
During the Hale cycle, McIntosh and his colleagues believe that magnetic waves interfere with each other and create spots on the surface of the sun.
"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."
Based on historical observations of sunspots, the scientists modelled the magnetic cycle progression. As sunspots spring up nearer and nearer to the equator at the start of every new 11-year cycle, the motion of magnetic waves matches the appearance of sunspots at solar midlatitudes.
"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."
The "terminator"
The opposing magnetic fields meet in the middle and neutralize each other. A terminator event both ends the previous solar cycle and initiates a new one, which led scientists to call the event "the terminator." The exact mechanism by which the terminator event occurs may have an impact on 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."
According to the data, the next solar cycle is weaker when there is more time between terminators.
A bit of a delay occurred between solar cycles 24 and 25, McIntosh said; solar cycle 24 didn't want to disappear. McIntosh's team was able to finalize their solar cycle forecast after the event occurred in December 2021.
"It will be just above the historical average with a monthly smoothed sunspot number of 190 ± 20," McIntosh told Space Weather Archive in February.
As for the current solar cycle, Fang warns that it's too early to make any conclusions.
"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."
#Space_Aus
