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A New Monitor for Cosmic Rays in the Solar System: Inverse-Compton Emission from Cosmic-Ray Electrons Scattering with Sunlight

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Why our Sun fires out bizarre gamma rays

Our closest star remains enigmatic. During periods of decreased activity, elusive phenomena trigger enhanced emission of energetic gamma rays. This unexpected explosive mood of our Sun hints at elusive magnetic field mechanisms or the presence of exotic matter.


Produced throughout the Universe by violent astrophysical events like supernova explosions and quasars, cosmic rays interacting with the solar atmosphere produce a cascade of protons, electrons, neutrons, muons and electromagnetic radiation. This slew of secondary ‘messengers’ and radiation deluge the Sun’s atmosphere with gamma rays. This high-energy radiation is certainly entirely different from gamma rays generated through fusion processes in the Sun’s core that never make it to the outer layers before being converted into lower-energy radiation. “A decade’s worth of detailed observations of our Sun using NASA’s Fermi Gamma-Ray Space Telescope have shown that the solar magnetic fields should strongly affect how cosmic rays interact with the solar atmosphere. The detailed mechanisms of how high fluxes of gamma-ray beams are produced at the high-energy end remain a mystery,” notes Kenny Chun Yu Ng, project coordinator of the SolarIC project that received funding under the Marie Skłodowska-Curie Actions programme.

Deciphering the role of magnetic fields

“Understanding the production mechanisms of gamma rays could provide a new way to study magnetic fields on the Sun,” adds Ng. Powerful eruptions close to the Sun’s surface known as coronal mass ejections driven by kinks in the magnetic field largely account for space weather phenomena on Earth, ranging from beautiful auroral lights to satellite damage. Solar gamma rays can certainly serve as a tool to monitor space weather. Within the context of SolarIC, researchers simulated cosmic-ray propagation close to the solar atmosphere, considering the magnetic fields outside the atmosphere. “In a first for solar astronomy, we showed that the magnetic fields above the solar surface led to the production of the lower-energy gamma rays (with energy measuring about 1 billion times that of visible light). Intriguingly, magnetic fields bent the trajectories of cosmic rays, creating favourable circumstances for the production of observable gamma rays,” explains Ng. Although the simulation results provide strong hints for some of the observed solar gamma rays, they can still not solve the full solar gamma-ray puzzle. “To the scientists’ surprise, the Fermi telescope recorded gamma rays 1 trillion times more energetic than visible light during the solar minimum, namely in the quietest part of the solar cycle,” notes Ng. “Based on what we learnt from this work, a new kind of solar magnetic field is needed, much stronger than that we used. We are currently actively investigating how to incorporate this in our simulations.” While SolarIC demystified how magnetic fields drive the production of gamma rays, it also highlighted the need for new ideas and quantitative studies to explain the intense radiation that appeared in the last solar minimum. “Despite its proximity and extreme importance for life on Earth, the Sun is still a burning mystery. The solar magnetic fields are just so complicated, making the prediction of cosmic-ray propagation in the solar atmosphere, and thus gamma-ray production, very challenging,” Ng points out. “To make things even more interesting, gamma-ray evidence could signal the annihilation of dark matter, making the Sun a possible source of exotic states of matter.”


SolarIC, Sun, magnetic field, gamma rays, cosmic rays, solar minimum, exotic matter, dark matter

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