Every so often, our star fires off a plasma bomb in a random direction. Our best hope the next time Earth is in the crosshairs? Capacitors.

TO A PHOTON, the sun is like a crowded nightclub. It’s 27 million degrees inside and packed with excited bodies—helium atoms fusing, nuclei colliding, positrons sneaking off with neutrinos. When the photon heads for the exit, the journey there will take, on average, 100,000 years. (There’s no quick way to jostle past 10 septillion dancers, even if you do move at the speed of light.) Once at the surface, the photon might set off solo into the night. Or, if it emerges in the wrong place at the wrong time, it might find itself stuck inside a coronal mass ejection, a mob of charged particles with the power to upend civilizations.

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The cause of the ruckus is the sun’s magnetic field. Generated by the churning of particles in the core, it originates as a series of orderly north-to-south lines. But different latitudes on the molten star rotate at different rates—36 days at the poles, and only 25 days at the equator. Very quickly, those lines stretch and tangle, forming magnetic knots that can puncture the surface and trap matter beneath them. From afar, the resulting patches appear dark. They’re known as sunspots. Typically, the trapped matter cools, condenses into plasma clouds, and falls back to the surface in a fiery coronal rain. Sometimes, though, the knots untangle spontaneously, violently. The sunspot turns into the muzzle of a gun: Photons flare in every direction, and a slug of magnetized plasma fires outward like a bullet.

The sun has played this game of Russian roulette with the solar system for billions of years, sometimes shooting off several coronal mass ejections in a day. Most come nowhere near Earth. It would take centuries of human observation before someone could stare down the barrel while it happened. At 11:18 am on September 1, 1859, Richard Carrington, a 33-year-old brewery owner and amateur astronomer, was in his private observatory, sketching sunspots—an important but mundane act of record-keeping. That moment, the spots erupted into a blinding beam of light. Carrington sprinted off in search of a witness. When he returned, a minute later, the image had already gone back to normal. Carrington spent that afternoon trying to make sense of the aberration. Had his lens caught a stray reflection? Had an undiscovered comet or planet passed between his telescope and the star? While he stewed, a plasma bomb silently barreled toward Earth at several million miles per hour.

When a coronal mass ejection comes your way, what matters most is the bullet’s magnetic orientation. If it has the same polarity as Earth’s protective magnetic field, you’ve gotten lucky: The two will repel, like a pair of bar magnets placed north-to-north or south-to-south. But if the polarities oppose, they will smash together. That’s what happened on September 2, the day after Carrington saw the blinding beam.

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Electrical current raced through the sky over the western hemisphere. A typical bolt of lightning registers 30,000 amperes. This geomagnetic storm registered in the millions. As the clock struck midnight in New York City, the sky turned scarlet, shot through with plumes of yellow and orange. Fearful crowds gathered in the streets. Over the continental divide, a bright-white midnight aurora roused a group of Rocky Mountain laborers; they assumed morning had arrived and began to cook breakfast. In Washington, DC, sparks leaped from a telegraph operator’s forehead to his switchboard as his equipment suddenly magnetized. Vast sections of the nascent telegraph system overheated and shut down.

The Carrington Event, as it’s known today, is considered a once-in-a-century geomagnetic storm—but it took just six decades for another comparable blast to reach Earth. In May 1921, train-control arrays in the American Northeast and telephone stations in Sweden caught fire. In 1989, a moderate storm, just one-tenth the strength of the 1921 event, left Quebec in the dark for nine hours after overloading the regional grid. In each of these cases, the damage was directly proportional to humanity’s reliance on advanced technology—more grounded electronics, more risk.

When another big one heads our way, as it could at any time, existing imaging technology will offer one or two days’ notice. But we won’t understand the true threat level until the cloud reaches the Deep Space Climate Observatory, a satellite about a million miles from Earth. It has instruments that analyze the speed and polarity of incoming solar particles. If a cloud’s magnetic orientation is dangerous, this $340 million piece of equipment will buy humanity—with its 7.2 billion cell phones, 1.5 billion automobiles, and 28,000 commercial aircraft—at most one hour of warning before impact.

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ACTIVITY ON THE solar surface follows a cycle of roughly 11 years. At the beginning of each cycle, clusters of sunspots form at the middle latitudes of both solar hemispheres. These clusters grow and migrate toward the equator. Around the time they’re most active, known as solar maximum, the sun’s magnetic field flips polarity. The sunspots wane, and solar minimum comes. Then it happens all over again. “I don’t know why it took 160 years of cataloging data to realize that,” says Scott McIntosh, a blunt-speaking Scottish astrophysicist who serves as deputy director of the US National Center for Atmospheric Research. “It hits you right in the fucking face.”

Today, in the 25th solar cycle since regular record-­keeping began, scientists don’t have much to show beyond that migration pattern. They don’t fully understand why the poles flip. They cannot explain why some sunspot cycles are as short as nine years while others last 14. They cannot reliably predict how many sunspots will form or where coronal mass ejections will occur. What is clear is that a big one can happen in any kind of cycle: In the summer of 2012, during the historically quiet Cycle 24, two mammoth coronal mass ejections narrowly missed Earth. Still, a more active cycle increases the chances of that near miss becoming a direct hit.

When navigation and communication systems fail, the 10,000 or so commercial planes in the sky will attempt a simultaneous grounding. Pilots will eyeball themselves into a flight pattern while air traffic controllers use light signals to guide the planes in. 

Without a guiding theory of solar dynamics, scientists tend to take a statistical approach, relying on strong correlations and after-the-fact rationales to make their predictions. One of the more influential models, which offers respectable predictive power, uses the magnetic strength of the sun’s polar regions as a proxy for the vigor of the following cycle. In 2019, a dozen scientists empaneled by NASA predicted that the current solar cycle will peak with 115 sunspots in July 2025—well below the historical average of 179.

McIntosh, who was not invited to join the NASA panel, calls this “made-up physics.” He believes the old-school models are concerned with the wrong thing—sunspots, rather than the processes that create them. “The magnetic cycle is what you should be trying to model, not the derivative of it,” he says. “You have to explain why sunspots magically appear at 30 degrees latitude.”

McIntosh’s attempt to do that goes back to 2002, when, at the behest of a postdoctoral mentor, he began plotting tiny ultraviolet concentrations on the solar surface, known as brightpoints. “I think my boss knew what I would find if I let a full cycle pass,” he recalls. “By 2011, I was like, holy fuck.” He found that brightpoints originate at higher latitudes than sunspots do but follow the same path to the equator. To him, this implied that sunspots and brightpoints are twin effects of the same underlying phenomenon, one not found in astrophysics textbooks.

His grand unified theory, developed over a decade, goes something like this: Every 11 years, when the sun’s polarity flips, a magnetic band forms near each pole, wrapped around the circumference of the star. These bands exist for a couple of decades, slowly migrating toward the equator, where they meet in mutual destruction. At any given time, there are usually two oppositely charged bands in each hemisphere. They counteract each other, which promotes relative calm at the surface. But magnetic bands don’t all live to be the same age. Some reach what McIntosh calls “the terminator” with unusual speed. When this happens, the younger bands are left alone for a few years, without the moderating influence of the older bands, and they have a chance to raise hell.

McIntosh and his colleague Mausumi Dikpati believe that terminator timing is the key to forecasting sunspots—and, by extension, coronal mass ejections. The faster one set of bands dies out, the more dramatic the next cycle will be.

The most recent terminator, their data suggests, happened on December 13, 2021. In the days that followed, magnetic activity near the sun’s equator dissipated (signaling the death of one set of bands) while the number of sunspots at midlatitude rapidly doubled (signaling the solo reign of the remaining bands). Because this terminator arrived slightly sooner than expected, McIntosh predicts above-average activity for the current solar cycle, peaking at around 190 sunspots.

A clear victor in the modeling wars could emerge later this year. But McIntosh is already thinking ahead to the next thing—tools that can detect where a sunspot will emerge and how likely it is to burst. He yearns for a set of satellites orbiting the sun—a few at the poles and a few around the equator, like the ones used to forecast terrestrial weather. The price tag for such an early-­warning system would be modest, he argues: eight craft at roughly $30 million each. But will anyone fund it? “I think until Cycle 25 goes bananas,” he says, “nobody’s going to give a shit.”

WHEN THE NEXT solar storm approaches Earth and the deep-space satellite provides its warning—maybe an hour in advance, or maybe 15 minutes, if the storm is fast-moving—alarms will sound on crewed spacecraft. Astronauts will proceed to cramped modules lined with hydrogen-rich materials like polyethylene, which will prevent their DNA from being shredded by protons in the plasma. They may float inside for hours or days, depending on how long the storm endures.

The plasma will begin to flood Earth’s ionosphere, and the electron bombardment will cause high-frequency radio to go dark. GPS signals, which are transmitted via radio waves, will fade with it. Cell phone reception zones will shrink; your location bubble on Google Maps will expand. As the atmosphere heats up, it will swell, and satellites will drag, veer off course, and risk collision with each other and space debris. Some will fall out of orbit entirely. Most new satellites are equipped to endure some solar radiation, but in a strong enough storm, even the fanciest circuit board can fry. When navigation and communication systems fail, the commercial airline fleet—about 10,000 planes in the sky at any given time—will attempt a simultaneous grounding. Pilots will eyeball themselves into a flight pattern while air traffic controllers use light signals to guide the planes in. Those living near military installations may see government aircraft scrambling overhead; when radar systems jam, nuclear defense protocols activate.

Through a weird and nonintuitive property of electromagnetism, the electricity coursing through the atmosphere will begin to induce currents at Earth’s surface. As those currents race through the crust, they will seek the path of least resistance. In regions with resistive rock (in the US, especially the Pacific Northwest, Great Lakes, and Eastern Seaboard), the most convenient route is upward, through the electrical grid.

The weakest points in the grid are its intermediaries—machines called transformers, which take low-voltage current from a power plant, convert it to a higher voltage for cheap and efficient transport, and convert it back down again so that it can be piped safely to your wall outlets. The largest transformers, numbering around 2,000 in the United States, are firmly anchored into the ground, using Earth’s crust as a sink for excess voltage. But during a geomagnetic storm, that sink becomes a source. Most transformers are only built to handle alternating current, so storm-induced direct current can cause them to overheat, melt, and even ignite. As one might expect, old transformers are at higher risk of failure. The average American transformer is 40 years old, pushed beyond its intended lifespan.

If just nine transformers were to blow out in the wrong places, the US could experience coast-to-coast outages for months.

Modeling how the grid would fail during another Carrington-class storm is no easy task. The features of individual transformers—age, configuration, location—are typically considered trade secrets. Metatech, an engineering firm frequently contracted by the US government, offers one of the more dire estimates. It finds that a severe storm, on par with events in 1859 or 1921, could destroy 365 high-voltage transformers across the country—about one-fifth of those in operation. States along the East Coast could see transformer failure rates ranging from 24 percent (Maine) to 97 percent (New Hampshire). Grid failure on this scale would leave at least 130 million people in the dark. But the exact number of fried transformers may matter less than their location. In 2014, The Wall Street Journal reported findings from an unreleased Federal Energy Regulatory Commission report on grid security: If just nine transformers were to blow out in the wrong places, it found, the country could experience coast-to-coast outages for months. Prolonged national grid failure is…

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