For all we’ve learned about the sun, our home star remains shrouded in mystery. Now, after seven years of relative calm, the sun is set to become more temperamental—and a fleet of sun-gazing spacecraft are ready to watch as it awakens. Those spacecraft are offering scientists an unprecedented chance to study our stormy star and the ways it can affect our cosmic neighborhood.
Like broods of cicadas that periodically emerge and vanish, the sun cycles between periods of storm and slumber over the course of 11 years or so. These cycles are linked to the sun’s internal magnetic activity, and are betrayed by telltale phenomena such as sunspots and solar flares.
Scientists keep a close eye on the sun’s temperament because solar outbursts can wreak havoc on our power grids and communication systems—technologies that are vital for modern civilization—as well as any human and robotic explorers in orbit and beyond. But understanding the oft-invisible threads that tie the sun to its planetary system has been tricky.
“When I step back, as an innocent bystander, I think, How the heck can we possibly know as much as we do?” says NASA solar physicist James Klimchuk. “But there’s a tremendous amount more that we don’t understand.”
Now, the next solar cycle has ignited, with peak activity predicted around 2025. And this time, the sun will shake off its slumber while NASA’s Parker Solar Probe is continuously dive-bombing the sun, swooping closer to the star than any craft has yet dared.
“It still gives me goosebumps when I think about it,” says NASA’s Madhulika Guhathakurta, a solar physicist. “I think I stayed on at NASA headquarters for one—and only one—mission, and that is Parker Solar Probe.”
The European Space Agency’s Solar Orbiter is also looping around the sun, and it will ultimately deliver our first good observations of the sun’s poles. Until this mission, scientists mostly have been limited to scrutinizing the faces of the sun that we can see from Earth, and observing its poles is crucial for understanding its magnetic activity and the intensity of activity during each 11-year cycle.
Perhaps even more thrilling for space fans, the peak of this cycle is expected to happen very close to the time a total solar eclipse will be visible from North America, in April 2024. When the moon blots out the sun, people in the path of totality will be able to see the delicate, diaphanous halo of the sun’s upper atmosphere, or corona, and it should be a spectacular sight so close to solar maximum.
“It will have the appearance of stuff coming out from everywhere, very dynamic,” Guhathakurta says.
Plotting the terminators
While solar experts agree the next cycle has begun, debate swirls over how strong it’s likely to be. In September 2020, the Solar Cycle 25 Prediction Panel announced that cycle 25 had kicked off—and they predicted it would be mild. Traditionally, these predictions are based on counting dark, transient patches on the sun’s surface known as sunspots. Appearing in regions where magnetic fields are strong, sunspots bloom and shrivel as the sun’s activity waxes and wanes.
In December 2019, scientists recorded a minimum number of sunspots. That observation marked the end of cycle 24, the panel later said, and based on how fast the spots had started reappearing, it seemed that cycle 25 would be similar in intensity to the relative calm of cycle 24.
However, other solar cycle experts reached a dramatically different conclusion: Cycle 25 could be one of the strongest since record-keeping began in 1755. Instead of counting sunspots, Robert Leamon of the University of Maryland, Baltimore County, and his collaborators based their prediction on something they call the terminator, or the point when all magnetic activity from a previous solar cycle vanishes. Sunspots generally track that transition, but the true terminator tends to lag behind the sunspot minimum by somewhere between 12 and 18 months.
“If there’s one take-home I can get to everybody, it’s that there’s more to activity than sunspots,” says Leamon, whose team published their contradictory prediction in the journal Solar Physics.
By plotting terminator events over 270 years, Leamon and his colleagues found that the timing between terminators is tightly linked to the strength of the next cycle, with shorter gaps portending stronger activity. And that’s the situation we’re in now, he says, where the gap between terminators is short, and magnetic activity from cycle 25 is likely to take over in the next couple of months.
“We are close,” Leamon says. “That’s when you’ll see a big jump in activity.”
Forecasting disasters
A strong solar cycle might spell trouble for Earth. Sunspots can unleash massive explosions called solar flares, and those flares sometimes sling volleys of radiation and charged particles into space called coronal mass ejections, or CMEs. If a sufficiently strong CME collides with Earth, it could cause a damaging geomagnetic storm.
Perhaps the best-known of these storms occurred in 1859, during solar cycle 10. Known as the Carrington Event, it disrupted telegraphs and shocked operators at the controls, and it lit the skies with auroras that were visible as far south as the Caribbean. Today, a storm of this magnitude would be devastating. It could crash power grids, knock out satellites, endanger astronauts in orbit, change planned flight routes, and render Earth’s upper atmosphere impenetrable to ground-based communication systems.
Weaker eruptions are also dangerous. On March 12, 1989, the entire province of Quebec lost power when a CME a fraction the strength of the Carrington event smashed into Earth and fried the power grid, trapping people in elevators and tunnels. In orbit, multiple satellites temporarily went dark or had trouble maintaining altitude, and sensors tripped aboard the space shuttle Discovery—which had launched earlier that day.
The influence of solar activity on Earth is called space weather, and for years, scientists have been fine-tuning how to anticipate when the sun might turn hazardous. Better forecasts for solar storms mean better opportunities for protecting vulnerable infrastructure, similar to how weather forecasts allow people in a storm’s path to prepare for its onslaught.
“We’re not going to be able to stop the sun from misbehaving, but we will hopefully be able to mitigate it by switching to alternative power grids if we have sufficient warning,” says Western Kentucky University’s Gordon Emslie, chair of the American Astronomical Society’s Solar Physics Division public policy committee.
Recognizing the destructive power of solar storms, the U.S. government adopted the PROSWIFT Act in October 2020, which directs various government agencies to work together to crack the mysteries of space weather and improve predictions. That means prioritizing basic research into how solar eruptions occur and how coronal mass ejections travel through space, as well as setting up various monitoring programs and satellites.
“Just as you understand microbursts and tornadoes and hurricanes better if you observe them and try to understand them, so it is with space weather,” Emslie says.
Space agencies already have a number of sun-observing telescopes on the ground and in orbit that collectively offer views of our home star in multiple wavelengths. Recently, two more spacecraft joined the fleet, and they are giving scientists ringside views of the soon-to-be stormy star—with data from the probes already starting to untangle the sun’s mysteries.
Gather ‘round the campfire
Launched in February 2020, ESA’s Solar Orbiter is currently looping around the sun, using gravitational encounters with Venus to help boost it into an orbit where it can see the sun’s poles. For now, the spacecraft is busy studying our home star with a variety of onboard instruments that will help illuminate our understanding of its influence on Earth.
Last May, those cameras caught sight of some 1,500 miniature flares in the low solar atmosphere—or rather, flares that are miniature by solar standards, since some of them would span entire continents. The small eruptions last for tens of seconds, and the team named them “campfires.”
“It’s an observational term that was coined, describing really, really small things that flare up for a really small amount of time—but everywhere,” says ESA’s Daniel Müller, Solar Orbiter’s project scientist.
Occurring in quiet regions of the sun, campfires are lit when tension builds up in twisted, tangled magnetic field lines, causing them to snap like overloaded rubber bands. As the lines rupture and reconnect, they emit heat and produce the tiny flares. In late April, scientists announced at a meeting of the European Geosciences Union that the energy released by these campfires could be sufficient to explain an abiding solar mystery.
While the surface of the sun roasts at around 9,900 degrees Fahrenheit, the corona is a searing million degrees or more. It’s an enigma that solar physicists have been trying to solve for nearly a century.
“You can’t move away from your fireplace, and suddenly when you’re 100 yards away, it’s really hot,” Müller says. “How could that be?” By simulating the smoldering solar surface, a team led by Yajie Chen of Peking University calculated that the heat produced by these campfires appears to be enough to cook the corona.
“The models are now at the degree of sophistication that you can start to believe what they show,” Müller says. “And they show that we are on the right track.”
That is, perhaps, not exactly surprising, says NASA’s Klimchuk. The campfires appear to be a larger version of nanoflares, he says, a type of small solar explosion that physicist Eugene Parker suggested could be responsible for the hot corona back in 1988.
The new work comes with caveats that mean the case is not yet closed, including the fact that campfires occur in quiet solar regions and tend to dump heat low in the solar atmosphere, whereas nanoflares occur higher up and are especially powerful in active regions. Still, the observations from Solar Orbiter are helping scientists to better understand the phenomenon.
But perhaps the most pressing unanswered question about our home star is how it generates and controls its magnetic field.
“That is the six-billion-dollar question,” Leamon says—and it’s one that these sun-orbiting spacecraft may be poised to answer.
The sun’s magnetic field is generated as hot gases churn deep within the sun, producing what’s called a dynamo. In this system, magnetic field lines erupt through the sun’s surface and spread out. Understanding that process requires looking at the sun’s poles, which have been tough to study because of the energy required to sling a spacecraft into a polar-observing orbit. By swinging high over the star’s poles in a few more years, Solar Orbiter will collect data from that region, providing us with a crucial missing piece of the puzzle.
Simultaneously, NASA’s Parker Solar Probe is snuggling up extremely close to the sun—even swooping in and out of its upper atmosphere—which means it will have a front-row seat as solar cycle 25 unfolds. For the first time, scientists will be able to study the sun from up close as it shakes off its slumber, connecting the information from Parker Solar Probe with the tendrils of solar activity that stretch all the way back to Earth.
Spectacular totality
For this solar cycle, though, people won’t need to be on a spacecraft team to get a good look at the sun in action. On April 8, 2024, the moon will slide between Earth and the sun and completely blot out our star’s face. For nearly four and a half minutes, this total solar eclipse will be visible across a narrow swath of the United States from Texas to Maine—and it will occur as the sun is reaching peak activity.
Guhathakurta, an eclipse chaser, has been following the moon’s shadow as it washes over our planet since she saw her first total solar eclipse off the coast of Mexico in 1991—an experience that for her, was surprisingly transcendental.
“It changes you,” she says. “It just provokes you to think deeply about the cosmos and our relationship to it, about the very basic questions we have as human beings.”
This next eclipse will look different than the one that took eclipse chasers by storm in 2017. Although that event was visible across unusually large portions of the U.S., it occurred as solar activity was waning, meaning one of the main draws of an eclipse—a view of the corona—was rather modest.
Normally obscured by sunlight, the solar corona extends millions of miles from the sun’s surface. But while the sun’s face is obscured during an eclipse, viewers can make out the wispy sheath that looks like a shimmering halo adorning the blackened lunar disc.
During eclipses at solar minimum, features such as streamers are visible mainly near the solar equator, and perhaps a handful of small flares twinkle near the moon’s edge. At solar maximum, however, the corona will be spectacular.
“You’re going to see many such streamers, not just from the equatorial latitudes, but from the polar region,” Guhathakurta says. “We might catch some prominences or post loops.”
Those streamers and other solar snazziness won’t be visible until the moon completely blots out the sun’s face, which is the only time it’s safe to stare at our home star with uncovered eyes. Watching the spectacle while the moon creeps across the solar disk, while crescent shadows dapple sidewalks and an eerie twilight deepens, requires protective eyewear. Even our intrepid, sun-gazing spacecraft are heavily shielded.
But when the corona blazes behind the moon, its delicate loops and streamers will trace the sun’s otherwise invisible magnetic field lines, illuminating an enigma that science may be on the cusp of cracking.
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