Scientists puzzled by super-bright light from the sun

A stream of energetic particles, called cosmic rays, constantly bombards earth from space. It is kept from reaching the ground by earth’s atmosphere and magnetic field. Scientists know that cosmic rays come from the centres of large galaxies and from the explosive deaths of massive stars, or supernovae, based on detectors fit on satellites.

Scientists have also detected cosmic rays coming from the direction of the sun, which is because the star’s magnetic field has deflected them towards earth. Sometimes, particles in the cosmic rays interact with the sun’s atmosphere to produce gamma rays, which scientists study as solar gamma rays.

Theory v. observation mismatch

As products of the solar magnetic field, the composition of the solar atmosphere, and the composition of cosmic rays, scientists can deduce much about these three things by studying the solar gamma rays themselves. Now, a new measurement has indicated to scientists that they may be missing something in this relationship.

Researchers at the High-Altitude Water Cherenkov (HAWC) Observatory, in Puebla, Mexico, reported on August 3 in the journal Physical Review Letters that HAWC had detected TeV-energy gamma rays from the sun. TeV stands for tera-electron-volt, or 1 trillion eV, a very high amount of energy for particles. This, the paper’s authors write, is the first time such energetic gamma rays have been detected from the sun.

HAWC also found more such high-energy gamma rays than expected.

Existing models of the sun’s magnetic field and atmosphere can’t account for this ‘excess’ energy and brightness, and scientists will need to figure out why.

Checking for Cherenkov radiation

“Most ongoing research is focused on trying to correctly model how the cosmic rays interact in the sun’s atmosphere,” Michigan State University postdoctoral research associate and a corresponding author of the new study Mehr Un Nisa told this writer in an email. “The cosmic-ray composition is relatively well-understood. However, the solar magnetic fields at various distances from the sun’s surface and their evolution with time is a complicated problem.”

When gamma rays from space smash into atoms in earth’s upper atmosphere, they produce a shower of particles that stream downwards, leading to a cascade of more particles. HAWC is located at 4.1 km above sea level, between two dormant volcanoes, to minimise the distance between the shower and its detectors. It consists of 300 tanks, each holding 200 tonnes (or 2 lakh litres) of pure water.

When an energetic charged particle, like an electron, streams into this water, it may move faster than the speed of light in water. This creates radiation known as Cherenkov radiation. The phenomenon is similar to when a jet flies faster through the air than the speed of sound in air: it creates a shockwave that can be heard as a sonic boom. Cherenkov radiation is the “shockwave” created by a charged particle moving through a medium faster than light can in that medium.

Photomultiplier tubes – devices that excel at detecting light – at the bottom of each tank record this radiation and relay it to a computer for analysis.

The sun shadow

The researchers recorded such data for six years, from November 2014 to January 2021. In their analysis, in order to find high-energy gamma rays, they had to subtract the gamma rays that would have reached earth if not for the sun blocking them. These rays are called the “sun shadow”.

The team had gone into the study looking specifically for high-energy gamma rays. The Fermi Gamma-ray Space Telescope can detect gamma rays of up to 200 billion eV, or 200 GeV, and the scientists operating it found that it was recording energies all the way up to the limit. “They nudged us and said, ‘We’re not seeing a cutoff. You might be able to see something,” Nisa said in a statement.

When they subtracted the sun shadow from the recorded data, the researchers found a “gamma ray excess” in the direction of the sun, with a statistical significance exceeding that required to claim a discovery. One particular reading went up to 2,600 GeV.

Because of the duration over which they collected data, the researchers found another quirk. The amount of activity on the sun changes in a 22-year cycle: for 11 years, the amount of activity increases, and for the next 11 years it decreases. The researchers found that HAWC detected more high-energy gamma rays when the solar activity was at a minimum, and vice versa – i.e. an inverse relationship.

Physics Magazine corresponding editor Ryan Wilkinson, who trained as a physicist, wrote that this suggests “that the sun’s magnetic field affects the flux – a result that will be useful for modelling”.

Data, data, data

The researchers also found that beyond around 400 GeV, the number of gamma rays at even higher energies drops rapidly. “We have Fermi measurements up to 200 GeV and HAWC measurements begin above 500 GeV,” Nisa explained. “Somewhere in between, the emission spectrum changed its slope”.

The task before researchers now is to update existing models or build new ones such that they can explain what we already know about the sun as well as accommodate the new findings.

“The observations at this point hint at multiple energy-dependent components instead of a single mechanism” causing the mismatch, according to Nisa. “Then there is also a small possibility of new physics out there” – in the form of a hitherto undetected particle, for example – “that may manifest itself through solar gamma rays”.

“Decisive new probes are needed to solve these puzzles, and observations at high energies are especially important,” the team wrote in its paper. To this end, Nisa said the LHAASO facility in China – “a bigger and more sensitive counterpart of HAWC” – could record more TeV-energy gamma rays. The Southern Wide-field Gamma-ray Observatory, to be built in South America, could bridge the energy gap between HAWC and Fermi data.

To complete the picture, she said, “future NASA probes should be able to add to our MeV-GeV knowledge of the sun”.