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Today's Top Science Headline: Enigma of our Nearest Star --"Sun is Bombarding Earth with Strange Patterns of Gamma Rays—and No One Knows Why"  

 

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Scientists have recently mapped how the sun’s high-energy glow dances across its face over time. They found a potential link between these high-energy emissions, the sun’s fluctuating magnetic field and the timing of the 11-year solar cycle. This, many experts argue, could open a new window into the inner workings of our nearest, most familiar star.

In their upcoming study, Tim Linden, an astronomer at The Ohio State University, and his colleagues 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, writes Shannon Hall in Scientific American, 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.

 

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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.

 Linden and his colleagues also discovered another curiosity entirely unpredicted by earlier ideas: During solar minimum, most gamma rays above 50 GeV are emitted near the sun’s equator, but throughout the rest of the cycle they tend to come from the polar regions. That means the sun’s total gamma-ray emission is most intense along its equator at solar minimum and at its poles during maximum.

To visualize this, imagine looking at a swarm of fireflies in a frosted glass jar. If the brightest fireflies converge near the center of the container, its glow will be most vivid there, even if a higher number of dimmer fireflies populate the container’s perimeter. This situation is somewhat analogous to the sun’s gamma-ray emission during solar minimum. But if the brightest fireflies instead converge at the jar’s bottom and top, its glow will peak at those points instead. That would be analogous to solar max.

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