Download Our Sun Produces Bizarre Radiation Bursts—Now NASA Knows Why

Our Sun Produces Bizarre
Radiation Bursts—Now NASA
Knows Why
EARTH-ORBITING TELESCOPE SEES LIGHT FROM FAR SIDE OF SUN
NASA’s Fermi Gamma-Ray Space Telescope recorded gamma rays,
the highest-energy form of light, originating from solar
flares on the far side of the sun on three separate occasions.
Astronomers recently got a look at
both sides of the sun during
powerful flares, helping solve a
riddle about our star’s gamma ray
activity.
By Alexandra E. Petri
PUBLISHED FEBRUARY 14, 2017
For the first time, NASA telescopes have worked in tandem to
watch blasts of high-energy radiation triggered by activity
on the far side of the sun. The achievement will help
scientists better understand eruptions of solar material
that, when aimed at Earth, can damage satellites, disrupt
the power grid, and set off beautiful aurora displays.
Launched in 2008, the Fermi Gamma-Ray Space Telescope was
designed to study gamma rays, the most energetic form of
light. These rays are emitted by all kinds of powerful and
exotic phenomena, such as feeding black holes and dying
stars.
Closer to home, solar flares can also produce gamma rays.
Flares happen when active regions on the sun suddenly
release explosions of magnetic energy. That accelerates
particles to incredibly high speeds and creates intense
bursts of light that can briefly outshine the sun itself.
(Also see “How Sun-Watchers Stopped World War III in 1967.”)
Scientists previously thought that the particles accelerated
during a flare would interact locally with the sun’s surface
and produce gamma rays that are close to the active region.
But Fermi has also seen gamma rays coming from the sun even
when no flares are erupting on the side it can see.
Now, scientists think they know what’s happening. Working
with NASA’s Solar Terrestrial Relations Observatory
(STEREO), Fermi has found that solar flares happening on the
far side of the sun can trigger additional bursts of gamma
rays on the side facing Earth.
The trick is that Fermi orbits Earth and shares our planet’s
view of the sun. By contrast, the twin STEREO satellites
orbit the sun, providing clear views of the whole solar
surface. Working together, the satellites watched solar
activity on both sides of the sun on three separate
occasions: October 11, 2013; January 6, 2014; and September
1, 2014.
“We saw high-energy radiation, or gamma rays, on the side of
the sun that faces Earth, but the flare happened on the
other side of the sun,” says Fermi team member Nicola
Omodei, a researcher at Stanford University in California.
Omodei and a large team of international colleagues reported
the results last month in The Astrophysical Journal.
GAMMA RAY ECHO
Thanks to Fermi and STEREO, scientists have now learned that
particles emitted during a flare on the sun’s far side can
travel along magnetic field lines and impact the solar
surface on the side facing Earth, generating a spike in
gamma rays.
“[The Fermi discovery] implies that high-energy particles
need to travel several hundred thousand miles to reach the
visible solar disc,” Omodei says. You can liken the effect
to fireworks bursting from behind a mountain, he adds.
“You cannot see the burst itself, but you can still hear the
sound of it,” he says. “Similarly, we cannot see the flare’s
active region … but we can see a brightening of the sun in
gamma rays as an echo of the flare.”
That’s important for understanding the effects of these
active regions, which can ultimately rotate onto the side of
the sun facing Earth. For instance, the flares Omodei and
his team observed also produced fast coronal mass ejections,
or CMEs.
When these clouds of high-speed charged particles come
racing off the sun, they can bathe spacecraft, astronauts,
and planetary surfaces in damaging radiation. Knowing when a
CME may hit Earth can help people on the planet take
precautions—and it can alert sky-watchers when to expect
especially vivid displays of auroras. (See stunning aurora
pictures from an August 2016 solar storm.)
Now Fermi’s gamma-ray eyes can serve as an additional tool
in our efforts to monitor CMEs and the resulting space
weather.
“It is remarkable that Fermi, even if it is not a mission
dedicated to solar observations, was able to provide such a
big contribution to the field,” Omodei says. “It is a great
experience for me, but also shows the value of this mission
and the importance of having the Fermi telescope on orbit
for a long time ahead.”