Our sun's steady heat and light
makes life on Earth possible, but instruments operating above
our atmosphere see a more dynamic star. Magnetic
activity associated with sunspots can fire off torrents of
high-energy radiation and launch billion-ton clouds of plasma--
called coronal mass ejection--into space.
NASA's Fermi Gamma-ray Space Telescope orbits Earth and monitors the entire
sky for gamma rays, the highest-energy light there is.
Most of the time, the sun is merely a faint source of gamma rays,
which are produced when high-energy particles called cosmic rays
interact with either its surface gas or its lower-energy light.
Occasionally, tangled magnetic fields near
sunspots suddenly release their pent-up energy.
This produces an explosion that rapidly accelerates charged particles to near
the speed of light. Confined by magnetic fields,
some of these particles toward the sun and excite gamma-ray emission.
Suddenly, the sun may become the brightest object
in Fermi's sky.
Now, scientists say Fermi has caught gamma rays from solar storms located on the
opposite side of the sun, where the spacecraft shouldn't be able to see them at all.
Here's one of them.
NASA's STEREO B spacecraft has a clear view of the solar flare, but
the eruption cannot be seen by NASA's Solar Dynamics Observatory,
which views the sun from the same direction as Earth and Fermi. Yet Fermi's
Large Area Telescope detected gamma rays associated with this flare
for nearly two hours. Here's what's going on.
Magnetic field lines extend high above each active region on the sun
and charged particles must travel along them.
Some particles accelerated at the leading edge of a coronal mass ejection can
follow these lines and strike the opposite side of the sun, traveling
about 300,000 miles in less than five minutes.
So far, Fermi has observed two additional
farside flares, doubling the number of these rare detections.
These observations will help scientists better understand how particles
accelerate, travel, and interact to produce gamma rays
during solar storms.
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