The Sun
Is Stranger Than Astrophysicists Imagined
The sun radiates 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.
Gamma radiation from the sun was
thought 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 of the
solar gamma-ray signal.
May 1, 2019
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.
It’s probably telling us
something very fundamental about the magnetic structure of the sun.
Joe Giacalone
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.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. Green indicates
positive polarity and purple is negative.
NASA’s Goddard Space 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.
“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.”