A violent, active sun, as seen in ultraviolet light in October 2014—near solar maximum in its 11-year solar cycle. As the sun approaches solar minimum, scientists are trying to predict the timing and strength of the next solar maximum.
By Sarah ScolesMay. 30, 2019 , 2:00 PM
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?”
As if to prove the point, 14 surprise sunspots appear, seething on the surface that had been so featureless for so long.
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 DigitalGlobe 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, DigitalGlobe
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.