Our closest star remains an enigma. Every 11 years or so its activity crescendos, creating flares and coronal mass ejections—the plasma-spewing eruptions that shower Earth with charged particles and beautiful auroral displays—but then it decrescendos. The so-called solar maximum fades toward solar minimum, and the sun’s surface grows eerily quiet.
Scientists have studied this ebb and flow for centuries, but only began understanding its effects on our planet at the dawn of the space age in the mid-20th century. Now it is clear that around solar maximum the sun is more likely to bombard Earth with charged particles that damage satellites and power grids. The solar cycle also plays a minor role in climate, as variations in irradiance can cause slight changes in average sea-surface temperatures and precipitation patterns. Thus, a better understanding of the cycle’s physical drivers is important for sustainable living on Earth.
Yet scientists still lack a model that perfectly predicts the cycle’s key details, such as the exact duration and strength of each phase. “I think the solar cycle is so stable and clear that there is something fundamental that we are missing,” says Ofer Cohen, a solar physicist at the University of Massachusetts Lowell. One obstacle to figuring it out, he says, is that crucial details of the apparent mechanisms behind the cycle—such as the sun’s magnetic field—are largely hidden from our view. But that might be about to change.
Tim Linden, an astronomer at The Ohio State University, and his colleagues recently mapped how the sun’s high-energy glow dances across its face over time. They found a potential link between these high-energy emissions, the sun’s fluctuating magnetic field and the timing of the solar cycle. This, many experts argue, could open a new window into the inner workings of our nearest, most familiar star.
In their upcoming study, so far published on the preprint server arXiv and submitted to Physical Review Letters, Linden and his colleagues examined a decade’s worth of data from NASA’s Fermi Gamma-ray Space Telescope to better analyze the sun’s emission of gamma rays—the universe’s most energetic form of electromagnetic radiation. To their surprise, the researchers found the most intense gamma rays appear strangely synced with the quietest part of the solar cycle. During the last solar minimum, from 2008 to 2009, Fermi detected eight high-energy gamma rays (each with energies greater than 100 giga–electron volts, or GeV) emitted by the sun. But over the next eight years, as solar activity built to a peak and then regressed back toward quiescence, the sun emitted no high-energy gamma rays at all. The chances of that occurring at random, Linden says, are extremely low. Most likely the gamma rays are triggered by some aspect of the sun’s activity cycle, but the details remain unclear.
The team speculates these gamma rays are likely emitted when powerful cosmic rays—produced throughout the universe by violent astrophysical events like supernovae and colliding neutron stars—slam into the sun’s surface. If a single cosmic ray collides with a particle in the solar atmosphere, it creates a shower of secondary particles and radiation, including gamma rays. Such showers would usually be wholly absorbed by the sun, however. But according to a hypothesis dating back to the 1990s, some of these secondary showers can be bounced out and away from our star by strong fluctuations in its magnetic field. If this is happening, the gamma rays Fermi has been detecting are likely some of those high-energy escapees.
If this interpretation is correct, says Randy Jokipii, a retired astronomer from the University of Arizona who was not involved in the study, it is no surprise high-energy gamma rays are more likely to be emitted during solar minimum. When the solar cycle is at low ebb, he says, there is a reduction in its outgoing “winds” of charged particles—which act as a shield to deflect incoming cosmic rays. This reduction allows more cosmic rays to enter our solar system, and our star itself. So an uptick in cosmic rays should lead to an uptick in gamma rays.
But Linden and his colleagues also discovered another curiosity entirely unpredicted by earlier ideas: During solar minimum, most gamma rays above 50 GeV are emitted near the sun’s equator, but throughout the rest of the cycle they tend to come from the polar regions. That means the sun’s total gamma-ray emission is most intense along its equator at solar minimum and at its poles during maximum. To visualize this, imagine looking at a swarm of fireflies in a frosted glass jar. If the brightest fireflies converge near the center of the container, its glow will be most vivid there, even if a higher number of dimmer fireflies populate the container’s perimeter. This situation is somewhat analogous to the sun’s gamma-ray emission during solar minimum. But if the brightest fireflies instead converge at the jar’s bottom and top, its glow will peak at those points instead. That would be analogous to solar max.
The cause of this pole/equator shift in gamma-ray emission remains unknown. “I tried to find an interpretation and I came up—one of the few times in my life—totally without any explanation,” Jokipii says. But Cohen, who was also not involved in the study, points out the shift does correspond with observed motions of sunspots. These dark splotches on the sun’s surface mark intense inner magnetic activity, and much like the newly observed gamma-ray emissions they move from the equator toward the poles as the sun progresses toward solar max. Cohen notes these trends might match, but says he cannot currently explain why. Igor Moskalenko, an astronomer at Stanford University who was not involved in the study, agrees that there is no obvious explanation.
One clue might come from an odd correlation: Although eight high-energy gamma rays greater than 100 GeV were observed in just over a year, two of them were detected within just hours of each other, and at the same time as a coronal mass ejection—“a startling coincidence,” Linden says. When he realized this, he decided to prioritize looking at the last few months of available Fermi data, only to find yet another high-energy gamma ray that had been emitted at the same time as another coronal mass ejection. “It was definitely a ‘wow’ moment,” he says, even though it does not prove the two phenomena are related. Six other high-energy gamma rays did not coincide with ejections, which mostly occur at solar max. So the fact they might only emit high-energy gamma rays when they do occur during a solar minimum would present quite a puzzle. Linden says, however, this potential link could only be part of a solution—much more work remains to be done to clarify such speculations.
Nevertheless, Linden was thrilled to see the first high-energy gamma ray emitted since the eight-year lull created throughout the latest solar max, which occurred in 2013 and 2014. He notes, “That probably indicates that the next few years are going to be really exciting for this sort of science.” He plans to scour the Fermi data every day to search for further bursts, and to work with collaborators to obtain new data from the HAWC Observatory in Mexico and NASA’s upcoming Parker Solar Probe (which will fly closer the sun than ever before). “There’s really a lot of hope that we’re going to get more data and see new physics here,” he says.
So as the sun meanders back toward solar minimum, astronomers are gearing up to study its gamma rays with the hope they might shed light on its mysterious interior. Although Linden and his colleagues cannot yet explain exactly why or how gamma-ray emission shifts in step with the sun’s magnetic field, it is increasingly clear the two are somehow linked. Moskalenko argues gamma-ray emissions could be used to trace the sun’s deep magnetic fields—and potentially to at last solve the lingering mysteries of the solar cycle.
“This is a puzzle that we’ve known about for centuries, but we do not know how to solve it,” Moskalenko says. “Maybe this paper and future studies will provide a hint as to how it can be explained.”