In their upcoming study, so far published on the preprint server arXiv and submitted to Physical Review Letters, Linden and his colleagues examined a decade’s worth of data from NASA’s Fermi Gamma-ray Space Telescope to better analyze the sun’s emission of gamma rays—the universe’s most energetic form of electromagnetic radiation. To their surprise, the researchers found the most intense gamma rays appear strangely synced with the quietest part of the solar cycle. During the last solar minimum, from 2008 to 2009, Fermi detected eight high-energy gamma rays (each with energies greater than 100 giga–electron volts, or GeV) emitted by the sun. But over the next eight years, as solar activity built to a peak and then regressed back toward quiescence, the sun emitted no high-energy gamma rays at all. The chances of that occurring at random, Linden says, are extremely low. Most likely the gamma rays are triggered by some aspect of the sun’s activity cycle, but the details remain unclear.
The team speculates these gamma rays are likely emitted when powerful cosmic rays—produced throughout the universe by violent astrophysical events like supernovae and colliding neutron stars—slam into the sun’s surface. If a single cosmic ray collides with a particle in the solar atmosphere, it creates a shower of secondary particles and radiation, including gamma rays. Such showers would usually be wholly absorbed by the sun, however. But according to a hypothesis dating back to the 1990s, some of these secondary showers can be bounced out and away from our star by strong fluctuations in its magnetic field. If this is happening, the gamma rays Fermi has been detecting are likely some of those high-energy escapees.
Read the rest at Scientific American.
Image: SDO and NASA
