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.
Giacalone points to the corona, the wispy plasma envelope that surrounds the sun. To efficiently mirror cosmic rays, the magnetic field in the corona is probably stronger and oriented differently than scientists thought, he said. However, he noted that the coronal magnetic field must be strong only very close to the sun’s surface so as not to mirror cosmic rays too soon, before they’ve entered the zone where the atmosphere is dense enough for collisions to occur. And the magnetic field seems to become particularly strong near the equator during solar minimum.
These fresh clues about the structure of the magnetic field could help unravel the long-standing mystery of the solar cycle.
“Every 11 years, the whole magnetic field of the sun reverses,” said Igor Moskalenko, a senior scientist at Stanford University who is part of the Fermi scientific collaboration. “We have south in the place of north and north in the place of south. This is a dramatic change. The sun is huge, and why we observe this change of polarity and why it is so periodic nobody actually knows.” Cosmic rays, he said, and the pattern of gamma rays they produce “may answer this very important question: Why is the sun changing polarity every 11 years?”
But there are no good guesses about how the sun’s magnetic field might create the dip in the gamma-ray spectrum at 10 trillion trillion hertz. It’s such an unusual feature that some experts doubt that it’s real. But if the absence of gamma rays around that frequency is a miscalculation or a problem with Fermi’s instruments, no one has figured out the cause. “It does not seem to be any instrumental effect,” said Elena Orlando, an astrophysicist at Stanford and a member of the Fermi team.
When Peter, Linden, Beacom and their collaborators found the dip in Fermi’s data last year, they tried hard to get rid of it before publishing their discovery. “I think there are 15 pages in the appendix of different tests we ran to see whether we were miscalculating,” Linden said. “Statistically, the dip appears very prominent.”
However, Orlando emphasized that the sun’s motion through the sky makes the data analysis very challenging. She should know; she and a collaborator discovered the stream of gamma rays coming from the sun for the first time in 2008 using the EGRET satellite, Fermi’s predecessor. Orlando has also been centrally involved in processing Fermi’s solar gamma-ray data. In her view, more data and independent analyses will be needed to confirm that the dip in the spectrum is real.
A solar panel malfunction kept the Fermi Telescope mostly pointed away from the sun for the last year, but workarounds have been found — just in time for solar minimum. The sun’s magnetic field lines are currently curving tidily from pole to pole; if this solar minimum is like the last, the gamma-ray signal is now at its most robust. “That’s what makes this so exciting,” Linden said. “Right now we’re just hitting the peak of solar minimum, so hopefully we’ll see higher-energy [gamma-ray] emission with a number of telescopes.”
This time, along with Fermi, a mountaintop observatory called HAWC (for High-Altitude Water Cherenkov experiment) will be taking data. HAWC detects gamma rays at higher frequencies than Fermi, which will reveal more of the signal. Scientists are also eager to see whether the spatial pattern of gamma rays changes relative to 11 years ago, since cosmic rays remain positively charged but the sun’s north and south poles have reversed.
These clues could help solve the solar mystery. HAWC scientists hope to report their first findings within a year, and scientists both within the Fermi collaboration and outside it have started to pore over its accruing data already. Since NASA is publicly funded, “anybody can download it if they want to glance through,” said Linden, who downloads Fermi’s new data almost every day.
“The worst that can happen here is that we find out that the sun is stranger and more beautiful than we ever imagined,” Beacom said. “And the best that could happen is we discover some kind of new physics.”