The Sun emitted its largest flare of Solar Cycle 25 on October 3, 2024. NASA classified it as X9.0, where X is the strongest class, and the higher the number, the more powerful the flare. Credit: NASA/SDO
In an October 15 teleconference, representatives from NASA and the National Oceanic and Atmospheric Administration (NOAA) announced that the Sun has reached the peak level in its current activity cycle, the 25th for which humans have held detailed records. The 25th cycle was much more active than the previous one, especially this year, when auroral displays delighted skywatchers around the world.
Solar cycles last about 11 years, slowly moving between periods of high activity – which can include sunspots, solar flares, coronal mass ejections and more – and calmer, less active periods. However, the cycles are irregular and difficult to predict. Previous cycles have lasted as little as nine or as long as 14 years, and on a day-to-day basis, solar time is just as difficult to predict as Earth’s time.
The metric by which solar cycles are measured is based on counting sunspots, which are dark areas on the Sun that are cooler than their surroundings. They usually last days or weeks and often appear in clusters. They are also related to many other types of solar weather, all of which arise from tangles and knots in the Sun’s powerful magnetic field.
In fact, the solar cycle indicates when the Sun, which is a giant magnet, reverses its polarity. In its quieter phases, the Sun is a well-behaved dipole, that is, a magnet with a positive and a negative pole, like a bar magnet. But over the course of a solar cycle, the Sun’s outer layers are churned by a phenomenon called differential rotation, the fact that the Sun’s plasma spins faster at the equator than at the poles. As a result, the magnetic field becomes more convoluted and complex, causing the chaos and activity we see now. Eventually, the Sun’s polarity reverses completely and our star returns to its more placid phase.
RELATED: Is artificial intelligence the key to predicting solar storms?
Researchers say it will be impossible to identify the exact peak of the cycle until at least a year later. But what they can do now, said Lisa Upton, co-chair of the Solar Cycle 25 Prediction Panel, is compare the shape of the gradual increase in sunspot numbers over the past few years, from about 2020 to 2022, with the shape of previous cycles solar. This comparison, and current data, show that sunspot levels have begun to level off, rather than continuing to rise.
From here on it’s all downhill, perhaps.
Don’t trust your eyes
On October 10, skywatchers around the world had their second chance of the year to watch a massive G4 geomagnetic storm light up the skies with auroras. The previous storm, in May 2024, had reached G5 levels, the strongest type. Both storms followed a series of X-class flares from the Sun earlier in the week.
Yet these storms are not in themselves evidence of the Sun’s peak state. Just as July can bring cool, rainy days and January might deliver a day of T-shirt weather, the Sun’s climate varies enormously even within a single cycle . A day near solar maximum might not show any sunspots, and a day near solar minimum might shock with a powerful flare.
Bill Murtagh, program coordinator for NOAA’s Space Weather Prediction Center (SWPC), points out that the famous 2003 Halloween solar storm – one of the most powerful ever recorded – occurred about two years after solar maximum. And the Sun remained at extremely similar levels to today for much of 2023, without the spectacular auroral displays.
So while it’s certainly more likely to see strong solar weather near solar maximum periods, it’s far from guaranteed. But as night falls earlier in the Northern Hemisphere in the coming months, it will remain a good time to see the Northern Lights, even if you’re a little further south.
