The most energetic light ever seen emitted from the Sun has been detected, posing a new puzzle for solar physicists to solve.

A six-year observing campaign conducted by more than 30 institutions in North America, Europe and Asia has resulted in the first detection of solar gamma radiation in the teraelectronvolt (TeV) range.

However, contrary to expectations, this was not associated with increased solar activity, or any solar activity at all. On the contrary, the Sun was quite calm during the detection, and our current solar models cannot fully explain this.

The Sun is the most intensively studied star in the universe. We have observations covering the entire spectrum, from low-frequency radio waves to gamma radiation, the most energetic light in the universe. Despite all this, there is still much we don't know about our home star. Details about its processes still elude us.

We know that magnetic fields are important and play a crucial role in almost all the solar activity we observe, but their mechanisms are not fully understood. To gain a deeper understanding, we are studying the light emitted by the Sun.

The High Altitude Water Cherenkov (HAWC) observatory in Mexico is one of the instruments we use to study gamma and cosmic radiation coming from space and colliding with Earth's atmosphere.

The atmosphere prevents this radiation from reaching us, but doing so can produce high-energy particles that can be detected in the dark, and the main gamma-ray energy and direction can be calculated from this detection.

"HAWC is one of the few detectors capable of observing the Sun in the TeV range," writes the international HAWC Collaboration, which conducted the research. "The large field of view and high live-time ratio allow for continuous exposure as the Sun crosses the sky."

In data collected using HAWC between 2014 and 2021, the researchers detected emissions ranging from 0.5 to 2.6 TeV (one TeV is one trillion electronvolts) from the direction of the Sun. The new analysis pipeline determined that this emission comes from the Sun with a probability of 6.3 sigma.

This is not the highest-energy light ever seen in space. That record belongs to the 450 TeV gamma-ray observation from the direction of the Crab Nebula a few years ago. But this is a new record for our own star.

"After looking at six years of data, this excess of gamma rays appeared," says Mehr Un Nisa, an astroparticle physicist at Michigan State University.

"When we first saw this, we said, 'We've definitely messed this up.' The Sun can't be this bright at these energies."

We know that the Sun sometimes gets violent. It attacks with solar flares and coronal mass ejections. But the Sun doesn't just emit light. It's also hit by light. Cosmic rays from the wider galaxy flow continuously through space. On Earth, these cosmic rays interact with the atmosphere to produce a gamma-ray glow.

A similar process is thought to occur on the Sun. Galactic cosmic rays collide with nuclei in the solar atmosphere in the gigaelectronvolt (1 billion electronvolts) range.

The team thinks that the TeV emission is probably the result of this interaction. They also looked at GeV data collected by HAWC and NASA's Fermi LAT gamma-ray space telescope and found that GeV emission is brighter and more abundant than expected during the Sun's quiet periods.

What we don't know is how the emission got so bright. It doesn't match the theoretical models. Given how bad the Sun's diffuse magnetic fields are, these are likely to play a role. Indeed, a paper published earlier this year suggested that the Sun's magnetic fields act as accelerators for cosmic ray electrons to produce synchrotron gamma radiation.

But more modelling will be needed to determine exactly how this happens.

"Models of cosmic ray interactions on the Sun ... already underestimate the observed gamma-ray flux from the Sun in the GeV range," the HAWC Collaboration writes. "Our observations emphasise the need for a revised framework that can explain the anomalous gamma-ray excess from the Sun also in the TeV range."

 

Source: https://www.sciencealert.com/