The Sun belches out a powerful solar flare and coronal mass ejection (CME) in this image taken by NASA’s Solar Dynamics Observatory on September 6, 2017. New analysis of data from NASA’s Fermi space telescope suggests that the Sun’s emission of gamma rays—the most energetic electromagnetic radiation in the universe—may be linked with CMEs and other outbursts, which follow a roughly 11-year cycle of activity. Credit: NASA, SDO
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.
They 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.
The shift does correspond with observed motions of sunspots. These dark splotches on the Sun’s surface mark intense inner magnetic activity, and much like the newly observed gamma-ray emissions they move from the equator toward the poles as the sun progresses toward solar max.
One clue might come from an odd correlation: Although eight high-energy gamma rays greater than 100 GeV were observed in just over a year, two of them were detected within just hours of each other, and at the same time as a coronal mass ejection - a “startling coincidence.” So, the fact they might only emit high-energy gamma rays when they do occur during a solar minimum would present quite a puzzle. However, this potential link could only be part of a solution - much more work remains to be done to clarify such speculations.