[Reading Science] Why Is the Sun Hotter These Days? ... A 1 Trillion Volt-Class Explosion Occurred

Michigan State University Research Team
Observed 1 PeV-Class Gamma Rays
"Largest Scale Ever, Brightest Shine"

The sun is acting unusually. Recently, an ultra-powerful gamma-ray burst with an energy scale of a staggering 10 trillion electron volts (eV) was observed pouring down on Earth.

A research team from Michigan State University in the United States published a paper containing this information on the 3rd in the physics journal Physical Review Letters. The team used a unique instrument called the High-Altitude Water Cherenkov Observatory (HAWC), installed at an altitude of 3,962 meters in the Mexican highlands. It can observe solar activity and cosmic rays. The facility consists of 300 large tanks, each storing about 200 tons of water. It detects and analyzes the light generated when high-energy gamma rays or cosmic ray particles from space pass through the water tanks. This facility is based on the principle discovered by Pavel Cherenkov, who won the Nobel Prize in Physics in 1957. It applies the fact that charged particles emit light when passing through certain media such as water.

This observatory has been operating 24/7 since its completion in 2015, and recently detected the largest gamma-ray burst since its operation began, with an energy scale reaching as high as 10 trillion eV. This is the strongest intensity of gamma rays ever measured coming from the sun.

The mystery lies in the unclear cause of this ultra-powerful gamma-ray burst of 10 trillion eV observed this time. Although strong gamma-ray emissions have often been observed from the sun before, they were associated with powerful solar flare explosions, which was not the case this time. Strong gamma rays are also produced during the sun’s internal nuclear fusion processes, but even those are not strong enough to be detected on Earth. Instead, the sun emits strong infrared and ultraviolet wavelengths, as well as visible light wavelengths that we can see with the naked eye. Visible light has an energy of about 1 eV, but the solar gamma rays observed by the research team exceeded 10 trillion eV and were detected multiple times.

Astrophysicists began detecting gamma rays with energies above 1 billion eV through NASA’s Fermi Gamma-ray Space Telescope launched in 2011. However, due to equipment limitations, it could only detect up to 200 billion eV. Since 2015, the research team has been observing gamma rays with even higher energies using HAWC. Even if such high-energy gamma-ray bursts as this one occur, Earth is not in danger. However, it opens a new chapter in the study of solar activity.

Space.com noted, "At first, the researchers thought the observation was incorrect due to the unimaginable intensity, but eventually realized that the sun could indeed produce explosions of such powerful brightness," adding, "Understanding the role of the sun’s magnetic field in this ultra-powerful gamma-ray burst phenomenon is an important issue that can broaden our understanding of the sun."