The Sun Is Stranger Than Astrophysicists Imagined

A decade’s worth of telescope observations of the sun have revealed a startling mystery: Gamma rays, the highest frequency waves of light, radiate from our nearest star seven times more abundantly than expected. Stranger still, despite this extreme excess of gamma rays overall, a narrow bandwidth of frequencies is curiously absent.

The surplus light, the gap in the spectrum, and other surprises about the solar gamma-ray signal potentially point to unknown features of the sun’s magnetic field, or more exotic physics.

“It’s amazing that we were so spectacularly wrong about something we should understand really well: the sun,” said Brian Fields, a particle astrophysicist at the University of Illinois, Urbana-Champaign.

The unexpected signal has emerged in data from the Fermi Gamma-ray Space Telescope, a NASA observatory that scans the sky from its outpost in low-Earth orbit. As more Fermi data have accrued, revealing the spectrum of gamma rays coming from the sun in ever-greater detail, the puzzles have only proliferated.

“We just kept finding surprising things,” said Annika Peter of Ohio State University, a co-author of a recent white paper summarizing several years of findings about the solar gamma-ray signal. “It’s definitely the most surprising thing I’ve ever worked on.”

Not only is the gamma-ray signal far stronger than a decades-old theory predicts; it also extends to much higher frequencies than predicted, and it inexplicably varies across the face of the sun and throughout the 11-year solar cycle. Then there’s the gap, which researchers call a “dip” — a lack of gamma rays with frequencies around 10 trillion trillion hertz. “The dip just defies all logic,” said Tim Linden, a particle astrophysicist at Ohio State who helped analyze the signal.

Fields, who wasn’t involved in the work, said, “They’ve done a great job with the data, and the story it tells is really kind of amazing.”

The likely protagonists of the story are particles called cosmic rays — typically protons that have been slingshotted into the solar system by the shock waves of distant supernovas or other explosions.

Physicists do not think the sun emits any gamma rays from within. (Nuclear fusions in its core do produce them, but they scatter and downgrade to lower-energy light before leaving the sun.) However, in 1991, the physicists David Seckel, Todor Stanev and Thomas Gaisser of the University of Delaware hypothesized that the sun would nonetheless glow in gamma rays, because of cosmic rays that zip in from outer space and plunge toward it.

Occasionally, the Delaware trio argued, a sunward-plunging cosmic ray will get “mirrored,” or turned around at the last second by the sun’s loopy, twisty magnetic field. “Remember the Road Runner cartoon?” said John Beacom, a professor at Ohio State and one of the leaders of the analysis of the signal. “Imagine the proton runs straight toward that sphere, and at the last second it changes its direction and comes back at you.” But on its way out, the cosmic ray collides with gas in the solar atmosphere and fizzles in a flurry of gamma radiation.

Based on the rate at which cosmic rays enter the solar system, the estimated strength of the sun’s magnetic field, the density of its atmosphere, and other factors, Seckel and colleagues calculated the mirroring process to be roughly 1 percent efficient. They predicted a faint glow of gamma rays.

Yet the Fermi Telescope detects, on average, seven times more gamma rays coming from the solar disk than this cosmic-ray theory predicts. And the signal becomes up to 20 times stronger than predicted for gamma rays with the highest frequencies. “We found that the process was consistent with 100 percent efficiency at high energies,” Linden said. “Every cosmic ray that comes in has to be turned around.” This is puzzling, since the most energetic cosmic rays should be the hardest to mirror.

And Seckel, Stanev and Gaisser’s model said nothing about any dip. According to Seckel, it’s difficult to imagine how you would end up with a deep, narrow dip in the gamma-ray spectrum by starting with cosmic rays, which have a smooth spectrum of energies. It’s hard to get dips in general, he said: “It’s much easier to get bumps than dips. If I have something that comes out of the sun, OK, that’s an extra channel. How do I make a negative channel out of that?”

Perhaps the strong glow of gamma rays reflects a source other than doomed cosmic rays. But physicists have struggled to imagine what. They’ve long suspected that the sun’s core might harbor dark matter — and that the dark matter particles, after being drawn in and trapped by gravity, might be dense enough there to annihilate each other. But how could gamma rays produced by annihilating dark matter in the core avoid scattering before escaping the sun? Attempts to link the gamma-ray signal to dark matter “seem like a Rube Goldberg-type thing,” Seckel said.

Some aspects of the signal do point to cosmic rays and to the broad strokes of the 1991 theory.

For instance, the Fermi Telescope detects many more gamma rays during solar minimum, the phase of the sun’s 11-year cycle when its magnetic field is calmest and most orderly. This makes sense, experts say, if cosmic rays are the source. During solar minimum, more cosmic rays can reach the strong magnetic field near the sun’s surface and get mirrored, instead of being deflected prematurely by the turbulent tangle of field lines that pervades the inner solar system at other times.

On the other hand, the detected gamma rays drop off as a function of frequency at a different rate than cosmic rays. If cosmic rays are the source, the two rates would be expected to match.

Whether or not cosmic rays account for the entire gamma-ray signal, Joe Giacalone, a heliospheric physicist at the University of Arizona, says the signal “is probably telling us something very fundamental about the magnetic structure of the sun.” The sun is the most extensively studied star, yet its magnetic field — generated by the churning maelstrom of charged particles inside it — remains poorly understood, leaving us with a blurry picture of how stars operate.