New results from the High Altitude Water Cherenkov (HAWC) observatory extend the bright, hard gamma-ray emission from the solar disk observed with NASA’s Fermi Gamma-ray Space Telescope, seemingly due to Galactic cosmic rays showering on nuclei in the solar atmosphere. However, current theoretical models are unable to explain the details of how solar magnetic fields shape these interactions. The new detection thus deepens the mysteries of the solar-disk emission.
“The Sun is more surprising than we knew,” said Dr. Mehr Un Nisa, a postdoctoral researcher at Michigan State University.
“We thought we had this star figured out, but that’s not the case.”
The Sun gives off a lot of light spanning a range of energies, but some energies are more abundant than others.
For example, through its nuclear reactions, the Sun provides a ton of visible light — that is, the light we see. This form of light carries an energy of about 1 electron volt, which is a handy unit of measure in physics.
The gamma rays that Dr. Nisa and her colleagues observed had about 1 trillion electron volts, or 1 tera electron volt, abbreviated 1 TeV.
Not only was this energy level surprising, but so was the fact that they were seeing so much of it.
In the 1990s, physicists predicted that the Sun could produce gamma rays when high-energy cosmic rays — particles accelerated by a cosmic powerhouse like a black hole or supernova — smash into protons in the Sun.
But, based on what was known about cosmic rays and the Sun, they also hypothesized it would be rare to see these gamma rays reach Earth.
At the time, though, there wasn’t an instrument capable of detecting such high-energy gamma rays and there wouldn’t be for a while.
The first observation of gamma rays with energies of more than a billion electron volts came from NASA’s Fermi Gamma-ray Space Telescope in 2011.
Over the next several years, the Fermi mission showed that not only could these rays be very energetic, but…
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