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.")

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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.