by Natalie Wolchover
to come from cosmic rays interacting with the sun's magnetic field
and then colliding with gas molecules near its surface.
But this long-standing theory doesn't account
for the observed strength and other features
solar gamma-ray signal.
far more high-frequency light than expected,
raising questions about unknown features
of the sun's magnetic field and
the possibility of even more exotic physics...
Stranger still, despite
this extreme excess of gamma rays overall, a narrow bandwidth of
frequencies is curiously absent.
The unexpected signal has emerged in data from the Fermi Gamma-ray Space Telescope (FGST), 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.
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.
Brian Fields, who wasn't involved in the work, said,
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,
...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.
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, David 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.
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:
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,
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,
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.
Visualizations of the sun's magnetic field
on Jan. 1, 1997, June 1, 2003, and Nov. 15, 2013,
based on measurements by the Solar and Heliospheric Observatory.
indicates positive polarity and purple is negative.
Flight Center Scientific Visualization Studio
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.
Cosmic rays, he said, and the pattern of gamma rays they produce,
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.
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 (Unexpected Dip in the Solar Gamma-Ray Spectrum).
However, Elena Orlando emphasized that the sun's motion through the sky makes the data analysis very challenging.
She should know; she and a collaborator discovered (Gamma-ray emission from the solar halo and disk - A study with EGRET data) 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.
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,