Scientists tackle a burning question: When will our quiet sun turn violent?

BOULDER, COLORADO—For all of February the sun is nearly spotless, a smooth circle filled in with a goldenrod crayon. It has been more than a decade since it was so lacking in sunspots—dark magnetic knots as big as Earth that are a barometer of the sun's temperament. Below the surface, however, a radical transition is afoot. In 5 years or so, the sun will be awash in sunspots and more prone to violent bursts of magnetic activity. Then, about 11 years from now, the solar cycle will conclude: Sunspots will fade away and the sun will again grow quiet.

In early March, a dozen scientists descend on the National Center for Atmospheric Research (NCAR) here to predict when the sun will reach its peak, and how unruly it will become. As light reflects off snow caught in the trees and streams through the tall windows of a conference room, the Solar Cycle 25 Prediction Panel comes to order. NASA and the National Oceanic and Atmospheric Administration (NOAA) have sponsored these panels since 1989, aiming to understand what drives the sun's 11-year cycles and assess methods for predicting them. But the exercise is not just academic: The military, satellite operators, and electric utilities all want to know what the sun has in store, because the flares and bursts of charged particles that mark solar maximum can damage their technologies.

Sunspots can be seen with the naked eye, but it wasn't until the mid-1800s that astronomers realized they come and go on a rough schedule. They first appear at midlatitudes and then proliferate, migrating toward the equator over about 11 years. In 1848, Swiss astronomer Johann Rudolf Wolf published an account of the sunspot record, identifying 1755–66 as "Cycle 1," the first period when counts were reliable. He then created a formula for counting the number of daily sunspots—a somewhat subjective technique that has evolved into a counting method used today to marry data sets across the centuries.

The cycles are capricious, however. Sometimes, the sun goes quiet for decades, with anemic sunspot counts across several cycles—as occurred during the 19th century's so-called Dalton minimum. Such variations are what the scientists at NCAR have gathered to forecast. The problem is that no one—in this room or elsewhere—really knows how the sun works.

Most models snatch at reality, but none pieces together the whole puzzle. The last time the panel convened, in 2007, its scientists evaluated dozens of models and came up with a prediction that was far from perfect. It missed the timing of the maximum, April 2014, by almost a year, and also the overall weakness of the past cycle. This panel, a who's who list of solar scientists, doesn't know whether it will do better.

As the NCAR clock ticks toward the start time, the panelists sit in awkward silence, clutching their compostable coffee cups. They know what the next 4 days hold: fights over physics and intuition, belief and data, correlation and causation. Tensions shadow the gathering: Scott McIntosh, director of NCAR's High Altitude Observatory (HAO) here, has an office above the meeting room and his own unorthodox view of what drives the solar cycle and how to predict it. But McIntosh, outspoken and provocative, has not been invited to be on the panel, although a collaborator will present the HAO's research.

At 8:30 a.m., the panel's earnest leader, Doug Biesecker—who works at NOAA's Space Weather Prediction Center here and commutes by bike regardless of the weather—welcomes everyone to the task: sorting through the many models and coming to a consensus about the next cycle. "The mess that you get from the community needs to be synthesized into something that is ideally correct," Biesecker says. "But you know, how can we know what's going to be correct?"

They can't.

As if to prove the point, 14 surprise sunspots appear, seething on the surface that had been so featureless for so long.

0 1750 Sunspot number 1800 1850 1900 Modern maximum Dalton minimum Cycle 1 Cycle 24 1950 2000 50 100 150 200 250 Meridional flow Poloidal field Radiative zone Toroidal field Convective zone Sunspot Sunspots are dark because they are cooler than the surrounding hot gas. These intensely magnetic knots burst through the surface, unleashing energy for days or weeks before fading. Telltale spots Since accurate sunspot counts began in the mid-1700s, the sun has been through 24 cycles, each lasting about 11 years. Forecasts suggest the 25th cycle will be moderately weak. Pulse of the sun Dynamo models—3D simulations of the sun—cannot make sunspots, but they indicate how motions inside the sun transform the shape of its magnetic field over an 11-year solar cycle. Whipped into shape A churning, burning star Scientists are trying to understand what drives the sun’s 11-year cycle so they can predict solar maximum. During this period of heightened magnetic activity, sunspots are more common, as are the dangerous solar storms, or coronal mass ejections, that hurl charged particles at Earth. Differential rotation in the sun’s convective zone is thought to be responsible for winding up and strengthening magnetic fields. At solar minimum , the sun’s magnetic field is poloidal, like a bar magnet’s. 1 4 5 6 7 2 3 At solar maximum , the sunspots are more common and closer to the equator. Turbulent motions tear apart sunspots. The meridional flow nudges remnants to the poles. The remnants cancel the original poloidal field and build up a new one. The sun returns to solar minimum , but the polarity of its poloidal field is reversed. Motions in the sun wind up the fields until they become toroidal. Kinks in the fields emerge as sunspots , first at midlatitudes. Jupiter Earth Sun Core Drawn to scale Coronal mass ejection Meridional flow Slower spin at poles Faster spin at equator Radiative zone Convective zone Corona Looping magnetic fields Sunspot Hot rising gas

Even on its calmest days, the sun is roiling. Fueled by fusion in its core, the sun is a ball of hot, charged particles, or plasma, that churns constantly, generating electric currents that in turn induce magnetic fields. Deep inside the sun is a dense radiative zone, where photons slowly fight their way outward. At a certain point—in the outer third of the sun—the plasma cools enough to allow convection, a boiling motion that carries energy toward the surface. In this zone, the sun rotates differentially: faster at the equator than the poles. The shearing motions that result stretch and twist the magnetic fields, strengthening them—a process that somehow affects the 11-year cycle. The tangled field lines sometimes burst through the convective zone and jut out from the surface, forming sunspots.

The sun's ebb and flow affects Earth. Its upper atmosphere absorbs the sun's ultraviolet rays, which dim slightly at solar minimum. That causes the atmosphere to cool and shrink, reducing friction for low-flying satellites. In calm solar cycles, operators assume their satellites will remain in orbit for longer—and because the same goes for space junk, the risk of a collision goes up. The sun's magnetic field also weakens at solar minimum, which poses another threat to satellites. The weakened field rebuffs fewer galactic cosmic rays, high energy particles that can flip bits in satellite electronics.

At solar maximum, in contrast, the sun heats and inflates Earth's upper atmosphere, and it often flares up and unleashes its own particles. They are not as energetic as the galactic cosmic rays, but they come in a flash flood. At solar max, Biesecker says, these "coronal mass ejections" of charged particles are 10 times as frequent as at minimum. Hours or days after the sun spits them out, particles rush into Earth's magnetic field, provoking geomagnetic storms that can last for days. The storms can disrupt communications, interrupt spacecraft and missile tracking, and skew GPS measurements. They can also induce powerful currents in electric grids, which can destroy transformers and other equipment. Air crews at high altitudes, particularly near the poles, can be showered with the sun's energetic particles—a cancer risk.

All of which adds to the practical importance of the panel's forecasts. "If you design a satellite for a 10- or 12-year life, you need to consider the cycle," says Michael Martinez, vice president of mission operations at Maxar in Westminster, Colorado, which makes high-resolution imaging orbiters. Designers need to be sure a satellite has enough propellant to combat the friction of an expanding atmosphere as the sun approaches maximum, and they need to shield its electronics from solar particles.

Most worrisome is the prospect of a major solar storm, such as the Carrington Event of 1859. During that storm, the sun ejected billions of tons of charged particles, causing aurorae as far south as the Caribbean and generating currents in telegraph lines powerful enough to shock operators. Today, the effect of such an event on computers and communications would be dire. Financial transaction systems could collapse. Power and water could easily go out. "It probably would be The Hunger Games pretty soon," McIntosh says.

If you design a satellite for a 10- or 12-year life, you need to consider the cycle. Michael Martinez, Maxar

McIntosh doesn't question the need to prepare, but he is skeptical of the panel's approach. In fact, he believes its very premise—predicting the rise and fall of sunspots—is off-base. Sunspots, and the cycle itself, are just symptoms of a still-mysterious story playing out inside the sun.

Lika Guhathakurta, a panel observer from NASA's Ames Research Center in California, agrees. "Sunspot is not a physical index of anything," she says, after the morning's introductory talks. "So the fact that we have used it as a proxy in itself kind of presents a problem." Using sunspots—a side effect, not a cause—to predict the sun's future behavior is like trying to divine the germ theory of disease by looking at a runny nose, she and McIntosh think.

But because the panelists have convened specifically to predict sunspot numbers, they soldier on, reviewing about 60 models over the next 4 days. Each predicts the number of sunspots at solar maximum, as well as the timing of minimum and maximum.

Many of the models rely on "precursors"—observable proxies, not unlike sunspots themselves, that have proved to be empirically useful in predicting the timing or magnitude of solar maximum. A popular one is the magnetic field strength at the sun's poles at solar minimum. Telescopes can measure this field strength by gauging how atoms above the sun's surface absorb certain wavelengths of light. A weak field usually heralds a quiet cycle, because the polar fields represent the seeds that will punch through as sunspots and grow into the activity of the coming solar cycle. Robert Cameron, a panelist and solar physicist at the Max Planck Institute for Solar System Research in Göttingen, Germany, says that over about four cycles of direct observation and more than a century of indirect data, the correlation "is good and highly statistically significant."

Other precursor models rely on effects of the solar cycle on Earth. For 170 years, for example, observatories around the world have tracked disturbances in Earth's magnetic field, which tend to be more frequent at solar maximum. But by measuring something on Earth rather than the sun, the methods are a step removed, says Dean Pesnell, project scientist for NASA's Solar Dynamics Observatory and a researcher at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "They've had a mixed record."

Another approach resembles climate prediction: using physics-based simulations of the sun to predict how it will evolve. The models, which combine theories of electromagnetism and fluid dynamics, start with the sun's current conditions and calculate its evolution through the cycle. And they're improving, says Maria Weber, a panelist and fellow at the University of Chicago in Illinois. Increased computing power and better algorithms mean scientists can run simulations in a few hours that a decade ago would have taken weeks. They also have more measurements to calibrate the models: not just sunspot counts and polar field measurements, but also helioseismology data—measurements of vibrations that probe the sun's interior—that can capture the flow of plasma beneath the sun's surface.

These "dynamo" models are providing insights into how the shape of the sun's magnetic field changes over the course of a cycle. At first the field is primarily poloidal—with field lines running from pole to pole like a bar magnet's. But as the sun's differential rotation twists up the magnetic field, its shape becomes toroidal, wrapping around the star like a doughnut. "That's when the magnetism specifically creates sunspots," Weber says.

Over time, the "meridional flow," an equator-to-pole circulation in the convective zone, ushers these superficial magnetic fields back toward the poles, converting the toroidal fields back into poloidal ones. Although the models can re-create this basic 11-year cycle, Weber says they still have one big failing. "No dynamo model has been able to actually create sunspots," Weber says. The modelers use intense toroidal magnetism as a proxy for sunspot-producing bands.

Still other models seek correlations like a conspiracy theorist: anywhere they can find them. One looks at how the decline of sunspots three cycles ago relates to the peak of the current cycle. Another links the prior cycle length to the minimum sunspot number. "There's not very much physics involved," concedes panelist Rachel Howe of the University of Birmingham in the United Kingdom, who has been tasked with reviewing the mishmash of statistical models. "There's not very much statistical sophistication either."

Panelist Andrés Muñoz-Jaramillo of the Southwest Research Institute in Boulder agrees with Howe. "There is no connection whatsoever to solar physics," he says in frustration. McIntosh, who by now has walked downstairs from his office and appears in the doorway, is blunter. "You're trying to get rid of numerology?" he says, smirking.

"That's how some science has occurred," protests Lisa Upton, Biesecker's co-chair and a physicist at Space Systems Research Corporation in Alexandria, Virginia: You find an obscure quantitative relationship you don't understand, and only later do you model what it means physically.

Biesecker concedes the point. "But we haven't really found one that seems to work," he says. "And we've been doing it for hundreds of years."

McIntosh is irritated that the panel is weighing models he considers dubious. "This is how churches spring up," he says. "You're a disciple of a disciple of a disciple." McIntosh, who didn't study astrophysics in school and instead focused on math and physics, has his own idea of how the sun works—and it doesn't spring from one of the popular models.

Around 2002, he started to catalog bright features that, in extreme ultraviolet images of the sun's outer atmosphere, or corona, look like buoys floating in the glowing plasma. These bright points, he found, follow a similar path across the sun as the sunspots, except they start higher, at about 55° latitude, before marching toward the equator. McIntosh hypothesizes that both sunspots and bright spots reflect parallel bands of magnetic flux that, at the beginning of each cycle, crop up at high latitudes and, like clockwork, meet at the equator at the cycle's end. The bright points, however, could be better markers for the bands—more closely linked to what's going on deep inside the sun.

If the predictions hold, at some point someone has to sit up and take notice. Scott McIntosh, High Altitude Observatory

During the last solar minimum, he watched as the bright points—and presumably, the bands—overlapped at the equator. McIntosh calls the encounter "the Terminator," because he thinks it is the moment when the two bands—which have opposite magnetic polarity—cancel each other out, marking the abrupt end of one 11-year cycle and the beginning of the next. But because the sun's north and south magnetic pole are flipped at the end of each cycle, McIntosh prefers to talk about an extended 22-year cycle. He hopes that by understanding the bands, scientists will finally be able to produce reliable and accurate forecasts.

The team is still working out exactly why these supposed bands would form. In a 2014 paper in The Astrophysical Journal, McIntosh and his colleagues set out their best guess: Giant swirling cells near the base of the convective zone form tubes of magnetic flux that appear on the surface as activity bands.

In the midst of their research, they discovered they weren't alone: In the 1980s, other scientists had published a paper in Nature describing basically the same idea. But that work disappeared into obscurity. Now, the idea of an extended, 22-year cycle is catching on again with some researchers. HAO scientist Mausumi Dikpati recently published a Nature paper that builds on McIntosh's ideas. The magnetic bands, she hypothesizes, also produce "magnetic dams," which hold back piled-up plasma. When the bands meet and annihilate each other, the dams break. The plasma rolls up from the equator toward the midlatitudes at 300 meters per second in what Dikpati calls a "solar tsunami." The waves drive magnetic fields to the surface, creating the first sunspots of the next cycle a few weeks later.

Dikpati, who's an adviser to the panel, presents this research to the panelists, who, by this point, have a lot to consider before they cast their votes.

By the final day, the snow has melted off the pines. It is time for the panel to make its prediction. Biesecker looks tired as he stands before the panelists. "A consensus among experts can often be a better prediction of the future state of a system than the set of individual predictions," he says.

McIntosh hovers in the doorway again as the panelists solemnly vote, their predictions and estimates of uncertainty based on an instinctive assessment of the models. Biesecker dutifully tabulates the estimates, and comes up with a peak sunspot range: 95 to 130. This spells a weak cycle, but not notably so, and it's marginally stronger than the past cycle. He does the same with the votes for the timing of minimum. The consensus is that it will come sometime between July 2019 and September 2020. Maximum will follow sometime between 2023 and 2026.

McIntosh has his own private prediction: a peak of 155 sunspots, in mid-2023. He concedes he could be wrong. But a successful prediction, he hopes, will win his model some acceptance. "If the predictions hold," McIntosh says, "at some point someone has to sit up and take notice."

Who, if anyone, is right won't be known for years. Meanwhile the sun, approaching minimum, is proving as surprising as ever. The night before, that active region of sunspots erupted for an hour straight. The particles from the coronal mass ejection will arrive in a matter of days.

As the panel preps its predictions and perfects its messaging, the storm charges toward Earth, ready or not.

*Clarification, 19 June, 2 p.m.: Michael Martinez’s affiliation has been changed to reflect the absorption of DigitalGlobe by new parent company Maxar.