Solar Gamma Rays Behaving Strangely

Physics 11, s110
Nearly 10 years of Fermi telescope images show unexpected changes in the numbers and energies of gamma-ray photons coming from the Sun.

When speedy interstellar protons known as cosmic rays slam into the Sun, they produce gamma rays that can be observed from Earth. By tracking how this gamma ray flux changes with time, researchers hope to better understand how the Sun interacts with its surroundings and how those interactions relate to its ever-changing magnetic engine. Now, astronomers have unveiled the first gamma-ray maps of the Sun, which detail the ebb and flow of these high-energy photons over nearly a decade of observations. The data raise more questions then they answer.

Tim Linden, of The Ohio State University in Columbus, and colleagues analyzed gamma-ray data of the Sun taken by NASA’s Fermi space telescope from 2008 to 2017. This period covered most of the last solar cycle, the latest round of the 11-year oscillation in the Sun’s magnetic activity. They found that the Sun’s poles steadily emitted gamma rays throughout the cycle. But surprisingly, the flux from the solar equator varied dramatically, outshining the poles during the solar minimum in 2009 before going largely quiet in gamma rays for the rest of the cycle.

The team also detected nine photons—all from the equator—with energies exceeding 100 GeV, the first detections from the Sun at such high energy. Eight arrived over the course of one year, during the period of minimum activity in the last cycle. The ninth showed up this past February, just as a new round of minimum activity was beginning.

To explain this behavior, the researchers imagined different scenarios, such as magnetic focusing and cosmic-ray trapping, but none worked. As the Sun starts its next cycle, the team will continue monitoring it to see how (and if) gamma rays correlate with other phenomena such as explosive solar eruptions.

This research is published in Physical Review Letters.

–Christopher Crockett

Christopher Crockett is a freelance writer based in Arlington, Virginia.

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