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These solar flares were imaged in extreme ultraviolet light by NASA's STEREO satellites, which at the time were viewing the side of the sun facing away from Earth. All three events launched fast coronal mass ejections (CMEs). Although NASA's Fermi Gamma-ray Space Telescope couldn't see the eruptions directly, it detected high-energy gamma rays from all of them. Scientists think particles accelerated by the CMEs rained onto the Earth-facing side of the sun and produced the gamma rays. The central image was returned by the STEREO A spacecraft, all others are from STEREO B. Credit: NASA/STEREO

An international science team has observed a high-energy light from solar eruptions located on the far side of the sun.

The team used NASA’s Fermi Gamma-ray Space Telescope to observe the hidden solar flare with direct imaging of beyond-the-limb solar flares associated with high-energy gamma rays.

The apparent paradox is providing solar scientists with a unique tool for exploring how charged particles are accelerated to nearly the speed of light and move across the sun during solar flares.

Nicola Omodei, a researcher at Stanford University, described the observation.

“Fermi is seeing gamma rays from the side of the sun we're facing, but the emission is produced by streams of particles blasted out of solar flares on the far side of the sun,” Omodei said in a statement. “These particles must travel some 300,000 miles within about five minutes of the eruption to produce this light.”

The Fermi telescope has doubled the number of these rare events—called behind-the-limb flares—since it began scanning the sky in 2008.

The Large Area Telescope has captured gamma rays with energies reaching 3 billion electron volts, 30 times greater than the most energetic light previously associated with hidden flares.

Melissa Pesce-Rollins, a researcher at the National Institute of Nuclear Physics in Pisa, Italy, and a co-author of the paper, said the observation will lead to bigger discoveries in space.

“Observations by Fermi's LAT continue to have a significant impact on the solar physics community in their own right, but the addition of [NASA’s] STEREO [Solar Terrestrial Relations Observatory] observations provides extremely valuable information of how they mesh with the big picture of solar activity,” Pesce-Rollins said in a statement.

The hidden flares occurred Oct. 11, 2013, Jan. 6, 2014 and Sept. 1, 2014, with all three events associated with fast coronal mass ejections (CME), where billion-ton clouds of solar plasma were launched into space. 

The CME from the latest event was moving at nearly 5 million miles an hour as it left the sun.

The researchers believe that particles accelerated at the leading edge of the CME’s were responsible for the gamma-ray emission.

Large magnetic field structures can connect the acceleration site with distant part of the solar surface but because charged particles must remain attach to magnetic field lines, the research team theorize that particles accelerated at the CME traveled to the sun’s visible side along magnetic field lines connecting both locations.

As the particles impacted the surface, they generated gamma-ray emission through a variety of processes with one prominent mechanism is thought to be proton collisions that result in a particle called a pion, which quickly decays into gamma rays.

Fermi has detected high-energy emission from more than 40 solar flares in its first eight years.

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

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