From Earth, the Sun appears comfortingly constant, always just about the same brightness in our blue skies during the day. Up close, however, it’s a lot more tumultuous, dotted with sunspots and roiling with solar flares.
Solar activity varies in fairly predictable 11 year cycles, meaning there are more sunspots and flares at certain intervals. But, there’s also fluctuation between cycles—some periods are more intense, and others, like the three decade Maunder minimum in the late 1600s, are almost entirely devoid of sunspots. Astronomers have been trying to pin down the exact physics behind these phenomena for years, although they have a clue as to the culprit: the Sun’s magnetic field.
New research, published in the journal Monthly Notices of the Royal Astronomical Society last year and recently presented at a symposium of the International Astronomical Union, simulates how the stars’ spin can lead to very different kinds of solar cycles. They explain that spin affects a star’s magnetic field, which is born from hot plasma flowing inside the star’s core, creating the differences seen in nearby stars other than our sun. It turns out that the so-called “grand minima” that our sun experiences—like the long-ago Maunder minimum—may not be uniform across all stars.
“Young stars, rapidly rotating and full of energy, are like children: lively, unpredictable, and active… Old stars, with their slow rotation, are reminiscent of the elderly: moving at a more measured pace, and embodying a calmer presence.”
Observations from the past 50 years all point to one trend: that more active stars tend to rotate faster. The new simulations provide a physical explanation for this trend and “confirm the suspected link between a faster rotation and more active stellar cycle,” according to Ryan French, an astronomer at the National Solar Observatory not affiliated with the publication.
The newly published simulations use the physics of fluid dynamics to imitate the rotation and flow of hot plasma within a star. This moving plasma is what generates a star’s magnetic field, known as a magnetic dynamo. The researchers tried these simulations with sun-sized stars that rotate at different speeds, from slow (30 days to complete a revolution), to something similar to our sun (about 25 days), to very fast (1 day). They found that the faster a star rotates, the stronger and more chaotic its magnetic field is. This leads to less predictable solar cycles, and fewer periods of inactivity like the Maunder minimum.
“Young stars, rapidly rotating and full of energy, are like children: lively, unpredictable, and active. These stars have magnetic fields that are strong and chaotic, mirroring the boundless energy and sometimes erratic behaviors of kids,” write lead author Vindya Vashishth and her colleague Anu Sreedevi from the Indian Institute of Technology. “Old stars, with their slow rotation, are reminiscent of the elderly: moving at a more measured pace, and embodying a calmer presence. Their magnetic fields are weaker, and their activity cycles are smooth and predictable, with occasional grand minima. These grand minima become more frequent as the star ages, much like how elderly individuals may have more frequent periods of rest or quietude,” they add.
In order to have grand minima—which they call the “hibernation phase of a star”—like the sun, a star must spin slower than once every ten days according to their models. They have good reason to be confident in their results, too. We have observational evidence that the sun has experienced around 27 grand minima in the past 11,000 years, and their study found a similar frequency of those events in a simulation that spans 11,000 years of the sun’s history.
What is happening with our sun right now?
Some astronomers even think our sun might be in a grand minimum right now, or at least just coming out of one. “Whether the last solar cycle was a Maunder Minimum or not is still debatable,” says Jia Huang, a solar scientist at Berkeley not involved with the new work. “This paper provides a new aspect to understanding the relationship between solar rotations and the occurrence of grand minima, thus it is timely and insightful to understand why and how the grand minima happen.”
The previous solar cycle—Solar Cycle 24—spanned December 2008 to December 2019, and was particularly weak, meaning there weren’t as many sunspots, flares, or other activity on the sun’s surface. A lack of solar activity can actually be good for humanity, as large solar storms or Coronal Mass Ejections (CMEs) can have deleterious effects on our power grids and satellites. On the other hand, extended periods of quiet from the Sun might make Earth a less pleasant place to live; the Maunder minimum seems to coincide with the Little Ice Age, but a causal link has yet to be confirmed.
What’s next for our sun’s solar activity?
We’re now in Solar Cycle 25, which began in December 2019 and will continue until about 2030. Solar scientists are starting to get excited, as the maximum of this particular cycle is set to occur within the next two years. “As we approach the solar maximum of the Sun’s solar cycle, more attention than ever turns to the activity of our local star,” adds French. “Years ago, predictions were made of how this Solar Cycle 25 would play out, and we’re finally close to revealing the truth.”
Scientists at the National Oceanic and Atmospheric Administration claim Cycle 25 will be fairly mild, but will also finally break the trend of weakening solar activity, avoiding a situation that would truly mimic the Maunder minimum. They’ve also predicted that Cycle 25’s maximum might bring some excitement, including “impactful space weather events” in 2024 and possibly extra-radiant aurora. That’s not all for the sun’s time in the spotlight, either — solar activity will be on full display in April 2024, when a total solar eclipse will allow viewers in North America a gorgeous glimpse at the sun’s corona. It’s the last such event expected for this continent until 2045, so don’t miss out!
