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THE CALM BEFORE THE STORMS Downloaded from http://science.sciencemag.org/
A spotless sun, as seen in May by NASA’s Solar Dynamics Observatory.
The sun is nearing on September 13, 2019 solar minimum.
Scientists studying the 11-year solar cycle are trying to predict when a quiet sun will next turn violent
or all of February the sun is nearly By Sarah Scoles, lar maximum can damage their technologies. spotless, a smooth circle filled in in Boulder, Colorado Sunspots can be seen with the naked eye, with a goldenrod crayon. It has been but it wasn’t until the mid-1800s that astrono- more than a decade since it was so will reach its peak, and how unruly it will be- mers realized they come and go on a rough lacking in sunspots—dark magnetic come. As light reflects off snow caught in the schedule. They first appear at midlatitudes knots as big as Earth that are a baro- trees and streams through the tall windows and then proliferate, migrating toward the meter of the sun’s temperament. Be- of a conference room, the Solar Cycle 25 Pre- equator over about 11 years. In 1848, Swiss low the surface, however, a radical diction Panel comes to order. NASA and the astronomer Johann Rudolf Wolf published transition is afoot. In 5 years or so, National Oceanic and Atmospheric Adminis- an account of the sunspot record, identi- the sun will be awash in sunspots and more tration (NOAA) have sponsored these panels fying 1755–66 as “Cycle 1,” the first period Fprone to violent bursts of magnetic activity. since 1989, aiming to understand what drives when counts were reliable. He then created Then, about 11 years from now, the solar cycle the sun’s 11-year cycles and assess methods a formula for counting the number of daily will conclude: Sunspots will fade away and for predicting them. But the exercise is not sunspots—a somewhat subjective technique the sun will again grow quiet. just academic: The military, satellite opera- that has evolved into a counting method used In early March, a dozen scientists descend tors, and electric utilities all want to know today to marry data sets across the centuries. on the National Center for Atmospheric Re- what the sun has in store, because the flares The cycles are capricious, however.
search (NCAR) here to predict when the sun and bursts of charged particles that mark so- Sometimes, the sun goes quiet for decades, TEAMS AND HMI SCIENCE EVE, AIA, NASA/SDO; PHOTO:
818 31 MAY 2019 • VOL 364 ISSUE 6443 sciencemag.org SCIENCE
Published by AAAS Clarification: 20 June 2019. See full text. NEWS with anemic sunspot counts across several jut out from the surface, forming sunspots. effect of such an event on computers and cycles—as occurred during the 19th century’s The sun’s ebb and flow affects Earth. Its communications would be dire. Financial so-called Dalton minimum. Such variations upper atmosphere absorbs the sun’s ultra- transaction systems could collapse. Power are what the scientists at NCAR have gath- violet rays, which dim slightly at solar mini- and water could easily go out. “It probably ered to forecast. The problem is that no one— mum. That causes the atmosphere to cool would be The Hunger Games pretty soon,” in this room or elsewhere—really knows how and shrink, reducing friction for low-flying McIntosh says. the sun works. satellites. In calm solar cycles, operators as- McIntosh doesn’t question the need to Most models snatch at reality, but none sume their satellites will remain in orbit for prepare, but he is skeptical of the panel’s ap- pieces together the whole puzzle. The last longer—and because the same goes for space proach. In fact, he believes its very premise— time the panel convened, in 2007, its scien- junk, the risk of a collision goes up. The predicting the rise and fall of sunspots—is tists evaluated dozens of models and came up sun’s magnetic field also weakens at solar off-base. Sunspots, and the cycle itself, are with a prediction that was far from perfect. minimum, which poses another threat to sat- just symptoms of a still-mysterious story It missed the timing of the maximum, April ellites. The weakened field rebuffs fewer ga- playing out inside the sun. 2014, by almost a year, and also the over- lactic cosmic rays, high energy particles that Lika Guhathakurta, a panel observer from all weakness of the past cycle. This panel, can flip bits in satellite electronics. NASA’s Ames Research Center in California, a who’s who list of solar scientists, doesn’t agrees. “Sunspot is not a physical index of know whether it will do better. anything,” she says, after the morning’s in- As the NCAR clock ticks toward the start “If you design a satellite for troductory talks. “So the fact that we have time, the panelists sit in awkward silence, used it as a proxy in itself kind of presents a clutching their compostable coffee cups. problem.” Using sunspots—a side effect, not
a 10- or 12-year life, Downloaded from They know what the next 4 days hold: fights a cause—to predict the sun’s future behav- over physics and intuition, belief and data, you need to consider the cycle.” ior is like trying to divine the germ theory of correlation and causation. Tensions shadow disease by looking at a runny nose, she and Michael Martinez, Maxar the gathering: Scott McIntosh, director of McIntosh think. NCAR’s High Altitude Observatory (HAO) But because the panelists have convened here, has an office above the meeting room At solar maximum, in contrast, the sun specifically to predict sunspot numbers, they and his own unorthodox view of what drives heats and inflates Earth’s upper atmo- soldier on, reviewing about 60 models over http://science.sciencemag.org/ the solar cycle and how to predict it. But Mc- sphere, and it often flares up and unleashes the next 4 days. Each predicts the number Intosh, outspoken and provocative, has not its own particles. They are not as energetic of sunspots at solar maximum, as well as the been invited to be on the panel, although a as the galactic cosmic rays, but they come in timing of minimum and maximum. collaborator will present the HAO’s research. a flash flood. At solar max, Biesecker says, Many of the models rely on “precursors”— At 8:30 a.m., the panel’s earnest leader, these “coronal mass ejections” of charged observable proxies, not unlike sunspots Doug Biesecker—who works at NOAA’s Space particles are 10 times as frequent as at themselves, that have proved to be empiri- Weather Prediction Center here and com- minimum. Hours or days after the sun spits cally useful in predicting the timing or mag- mutes by bike regardless of the weather— them out, particles rush into Earth’s mag- nitude of solar maximum. A popular one is welcomes everyone to the task: sorting netic field, provoking geomagnetic storms the magnetic field strength at the sun’s poles
through the many models and coming to a that can last for days. The storms can dis- at solar minimum. Telescopes can measure on September 13, 2019 consensus about the next cycle. “The mess rupt communications, interrupt spacecraft this field strength by gauging how atoms that you get from the community needs to and missile tracking, and skew GPS mea- above the sun’s surface absorb certain wave- be synthesized into something that is ideally surements. They can also induce powerful lengths of light. A weak field usually heralds a correct,” Biesecker says. “But you know, how currents in electric grids, which can destroy quiet cycle, because the polar fields represent can we know what’s going to be correct?” transformers and other equipment. Air the seeds that will punch through as sunspots They can’t. crews at high altitudes, particularly near and grow into the activity of the coming solar As if to prove the point, 14 surprise sun- the poles, can be showered with the sun’s cycle. Robert Cameron, a panelist and solar spots appear, seething on the surface that energetic particles—a cancer risk. physicist at the Max Planck Institute for Solar had been so featureless for so long. All of which adds to the practical impor- System Research in Göttingen, Germany, says tance of the panel’s forecasts. “If you de- that over about four cycles of direct observa- EVEN ON ITS CALMEST DAYS, the sun is roiling. sign a satellite for a 10- or 12-year life, you tion and more than a century of indirect data, Fueled by fusion in its core, the sun is a ball of need to consider the cycle,” says Michael the correlation “is good and highly statisti- hot, charged particles, or plasma, that churns Martinez, vice president of mission opera- cally significant.” constantly, generating electric currents that tions at Maxar in Westminster, Colorado, Other precursor models rely on effects of in turn induce magnetic fields. Deep inside which makes high-resolution imaging orbit- the solar cycle on Earth. For 170 years, for ex- the sun is a dense radiative zone, where pho- ers. Designers need to be sure a satellite has ample, observatories around the world have tons slowly fight their way outward. At a cer- enough propellant to combat the friction tracked disturbances in Earth’s magnetic tain point—in the outer third of the sun—the of an expanding atmosphere as the sun ap- field, which tend to be more frequent at solar plasma cools enough to allow convection, a proaches maximum, and they need to shield maximum. But by measuring something on boiling motion that carries energy toward the its electronics from solar particles. Earth rather than the sun, the methods are surface. In this zone, the sun rotates differ- Most worrisome is the prospect of a ma- a step removed, says Dean Pesnell, project entially: faster at the equator than the poles. jor solar storm, such as the Carrington Event scientist for NASA’s Solar Dynamics Obser- The shearing motions that result stretch of 1859. During that storm, the sun ejected vatory and a researcher at NASA’s Goddard and twist the magnetic fields, strengthening billions of tons of charged particles, causing Space Flight Center in Greenbelt, Maryland. them—a process that somehow affects the aurorae as far south as the Caribbean and “They’ve had a mixed record.” 11-year cycle. The tangled field lines some- generating currents in telegraph lines pow- Another approach resembles climate pre- times burst through the convective zone and erful enough to shock operators. Today, the diction: using physics-based simulations of
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A churning, burning star Scientists are trying to understand what drives the sun’s 11-year cycle so they can Whipped into shape predict solar maximum. During this period of heightened magnetic activity, sunspots Dynamo models—3D simulations of the sun—cannot are more common, as are the dangerous solar storms, or coronal mass ejections, make sunspots, but they indicate how motions inside that hurl charged particles at Earth. Differential rotation in the sun’s convective zone the sun transform the shape of its magnetic field is thought to be responsible for winding up and strengthening magnetic fields. over an 11-year solar cycle.
Radiative 1 At solar zone minimum, JupiterJupiter Earth Sun the sun’s Meridional magnetic 5ow field is DraDrawnwn to scascalel poloidal, like a bar magnet’s. Poloidal Radiative feld Slower spin zone at poles Coronal mass ejection 2 Motions in the sun wind up the Downloaded from fields until they become Convective toroidal. zone Core Toroidal feld Faster spin http://science.sciencemag.org/ at equator 3 Kinks in the fields emerge as sunspots, first at midlatitudes.
Convective 4 At solar Sunspot zone maximum, the sunspots
are more on September 13, 2019 common Corona and closer to the equator.
Looping 5 Turbulent Telltale spots magnetic motions tear Sunspots are dark because they Eelds apart sunspots. are cooler than the surrounding The meridional hot gas. These intensely Sunspot flow nudges magnetic knots burst through remnants to the surface, unleashing energy Meridional the poles. for days or weeks before fading. Hot rising fow gas
Pulse of the sun 6 The remnants Since accurate sunspot counts began in the mid-1700s, the sun has been through 24 cycles, cancel the each lasting about 11 years. Forecasts suggest the 25th cycle will be moderately weak. original poloidal field and build Modern maximum up a new one. 250 Cycle 1 Cycle 24 200 Dalton 150 minimum 7 The sun returns 100 to solar minimum, but
Sunspot number Sunspot 50 the polarity of its poloidal field 0 is reversed. 1750 1800 1850 1900 1950 2000
820
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the sun to predict how it will evolve. The you don’t understand, and only later do In the midst of their research, they dis- models, which combine theories of electro- you model what it means physically. covered they weren’t alone: In the 1980s, magnetism and fluid dynamics, start with Biesecker concedes the point. “But we other scientists had published a paper in the sun’s current conditions and calculate haven’t really found one that seems to work,” Nature describing basically the same idea. its evolution through the cycle. And they’re he says. “And we’ve been doing it for hun- But that work disappeared into obscurity. improving, says Maria Weber, a panelist dreds of years.” Now, the idea of an extended, 22-year cycle and fellow at the University of Chicago in McIntosh is irritated that the panel is is catching on again with some researchers. Illinois. Increased computing power and weighing models he considers dubious. HAO scientist Mausumi Dikpati recently better algorithms mean scientists can run “This is how churches spring up,” he says. published a Nature paper that builds on simulations in a few hours that a decade “You’re a disciple of a disciple of a disciple.” McIntosh’s ideas. The magnetic bands, ago would have taken weeks. They also have McIntosh, who didn’t study astrophysics in she hypothesizes, also produce “magnetic more measurements to calibrate the models: school and instead focused on math and dams,” which hold back piled-up plasma. not just sunspot counts and polar field mea- physics, has his own idea of how the sun When the bands meet and annihilate each surements, but also helioseismology data— works—and it doesn’t spring from one of the other, the dams break. The plasma rolls up measurements of vibrations that probe the popular models. from the equator toward the midlatitudes sun’s interior—that can capture the flow of at 300 meters per second in what Dikpati plasma beneath the sun’s surface. calls a “solar tsunami.” The waves drive These “dynamo” models are providing “If the predictions hold, magnetic fields to the surface, creating insights into how the shape of the sun’s the first sunspots of the next cycle a few magnetic field changes over the course at some point someone has to weeks later. Downloaded from of a cycle. At first the field is primarily Dikpati, who’s an adviser to the panel, poloidal—with field lines running from pole sit up and take notice.” presents this research to the panelists, who, to pole like a bar magnet’s. But as the sun’s Scott McIntosh, High Altitude Observatory by this point, have a lot to consider before differential rotation twists up the magnetic they cast their votes. field, its shape becomes toroidal, wrapping around the star like a doughnut. “That’s Around 2002, he started to catalog bright BY THE FINAL DAY, the snow has melted off http://science.sciencemag.org/ when the magnetism specifically creates features that, in extreme ultraviolet images of the pines. It is time for the panel to make sunspots,” Weber says. the sun’s outer atmosphere, or corona, look its prediction. Biesecker looks tired as he Over time, the “meridional flow,” an like buoys floating in the glowing plasma. stands before the panelists. “A consensus equator-to-pole circulation in the convective These bright points, he found, follow a simi- among experts can often be a better predic- zone, ushers these superficial magnetic fields lar path across the sun as the sunspots, except tion of the future state of a system than the back toward the poles, converting the toroi- they start higher, at about 55° latitude, be- set of individual predictions,” he says. dal fields back into poloidal ones. Although fore marching toward the equator. McIntosh McIntosh hovers in the doorway again as the models can re-create this basic 11-year cy- hypothesizes that both sunspots and bright the panelists solemnly vote, their predictions cle, Weber says they still have one big failing. spots reflect parallel bands of magnetic flux and estimates of uncertainty based on an in-
“No dynamo model has been able to actually that, at the beginning of each cycle, crop up stinctive assessment of the models. Biesecker on September 13, 2019 create sunspots,” Weber says. The modelers at high latitudes and, like clockwork, meet dutifully tabulates the estimates, and comes use intense toroidal magnetism as a proxy at the equator at the cycle’s end. The bright up with a peak sunspot range: 95 to 130. This for sunspot-producing bands. points, however, could be better markers for spells a weak cycle, but not notably so, and Still other models seek correlations like a the bands—more closely linked to what’s go- it’s marginally stronger than the past cycle. conspiracy theorist: anywhere they can find ing on deep inside the sun. He does the same with the votes for the tim- them. One looks at how the decline of sun- During the last solar minimum, he watched ing of minimum. The consensus is that it will spots three cycles ago relates to the peak of as the bright points—and presumably, the come sometime between July 2019 and Sep- the current cycle. Another links the prior bands—overlapped at the equator. McIntosh tember 2020. Maximum will follow some- cycle length to the minimum sunspot num- calls the encounter “the Terminator,” be- time between 2023 and 2026. ber. “There’s not very much physics involved,” cause he thinks it is the moment when the McIntosh has his own private prediction: concedes panelist Rachel Howe of the Uni- two bands—which have opposite magnetic a peak of 155 sunspots, in mid-2023. He versity of Birmingham in the United King- polarity—cancel each other out, marking the concedes he could be wrong. But a success- dom, who has been tasked with reviewing the abrupt end of one 11-year cycle and the begin- ful prediction, he hopes, will win his model mishmash of statistical models. “There’s not ning of the next. But because the sun’s north some acceptance. “If the predictions hold,” very much statistical sophistication either.” and south magnetic pole are flipped at the McIntosh says, “at some point someone has Panelist Andrés Muñoz-Jaramillo of the end of each cycle, McIntosh prefers to talk to sit up and take notice.” Southwest Research Institute in Boulder about an extended 22-year cycle. He hopes Who, if anyone, is right won’t be known agrees with Howe. “There is no connection that by understanding the bands, scientists for years. Meanwhile the sun, approaching whatsoever to solar physics,” he says in frus- will finally be able to produce reliable and ac- minimum, is proving as surprising as ever. tration. McIntosh, who by now has walked curate forecasts. The night before, that active region of sun- downstairs from his office and appears in the The team is still working out exactly spots erupted for an hour straight. The par- doorway, is blunter. “You’re trying to get rid why these supposed bands would form. In ticles from the coronal mass ejection will of numerology?” he says, smirking. a 2014 paper in The Astrophysical Journal, arrive in a matter of days. “That’s how some science has occurred,” McIntosh and his colleagues set out their As the panel preps its predictions and protests Lisa Upton, Biesecker’s co-chair best guess: Giant swirling cells near the perfects its messaging, the storm charges and a physicist at Space Systems Research base of the convective zone form tubes of toward Earth, ready or not. j Corporation in Alexandria, Virginia: You magnetic flux that appear on the surface as
GRAPHIC: (OPPOSITE PAGE) C. BICKEL/ SCIENCE C. PAGE) (OPPOSITE GRAPHIC: find an obscure quantitative relationship activity bands. Sarah Scoles is a journalist in Denver.
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