Sun blasts out highest-energy radiation ever recorded, raising questions for solar physics

"We thought we had this star figured out, but that's not the case."

By Monisha Ravisetti


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An image of the sun. (Image credit: NASA/SDO)
In a record-breaking discovery, scientists detected our very own sun emitting an extraordinary amount of gamma rays — wavelengths of light known to carry the most energy of any other wavelength in the electromagnetic spectrum. This is quite a big deal as it marks the highest-energy radiation to ever be documented coming from our planet's host star.

Something like 1 trillion electron volts, to be exact.

"After looking at six years' worth of data, out popped this excess of gamma rays," Meher Un Nisa, a postdoctoral research associate at Michigan State University and co-author of a new paper about the findings released Wednesday (Aug. 3), said in a statement. "When we first saw it, we were like, 'We definitely messed this up. The sun cannot be this bright at these energies.'"

Upon deliberation, however, the team realized that such brightness definitely existed — and it was simply due to the sheer amount of gamma rays the sun seemed to be spitting out.

"The sun is more surprising than we knew," Nisa said.

Before you start worrying, no, these rays can't harm us. But what they can do is have a pretty important ripple effect for the future of solar physics. In fact, they have already raised some important questions about the sun, such as what role its magnetic field might play in the newly observed gamma-ray phenomenon.

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What an excess of solar gamma rays looks like to the High-Altitude Water Cherenkov Observatory Collaboration. (Image credit: Courtesy of the HAWC Collaboration)
It's all thanks to a unique lens on the cosmos called the High-Altitude Water Cherenkov Observatory, or HAWC. In short, this observatory, completed in the spring of 2015, is a facility specifically designed to observe particles associated with very high-energy gamma rays and cosmic rays, the latter of which are equally energetic but also mysterious in that they often travel across the universe without exhibiting a clear starting point.

"In this particular energy regime, other ground-based telescopes couldn't look at the sun because they only work at night," Nisa said. "Ours operates 24/7."

HAWC basically uses a network of 300 large water tanks, a press release on the new study explains. Each of these tanks is filled with about 200 metric tons of purified water, and they all sit nestled between two dormant volcano peaks in Mexico more than 13,000 feet (3,962 meters) above sea level. All of this purified water is important because, as high-energy particles from space strike the liquid, the collision results in a phenomenon known as Cherenkov radiation (which you may have heard of if you've watched the TV show "Chernobyl").

Named after 1958 Physics Nobel Prize laureate Pavel Cherenkov, Cherenkov radiation essentially refers to a bluish glow that happens when electrically charged particles move at a certain speed through a certain medium, in this case water.

Tapping into this concept, HAWC's overall field of view covers 15% of the sky, allowing it to survey a total two-thirds every 24 hour period and figure out the roots of various high-energy particles headed to Earth.

An illustration depicting charged particles hitting the water tanks of the HAWC.

A composite image shows a photograph of the High-Altitude Water Cherenkov Observatory in Mexico observing particles, whose paths are shown as red lines, generated by high-energy gamma rays from the sun. (Image credit: Mehr Un Nisa)

What's normal solar radiation like?

Even though scientists have observed the sun sending out gamma ray emissions before, such observations are connected to incredibly extreme solar events such as super powerful solar flares. The recent gamma-ray discovery doesn't seem to be associated with that kind of scenario.

Within the sun, nuclear fusion processes are also expected to produce these strong wavelengths, however, gamma rays created that way don't exactly make it out of the star — let alone far enough to be detected by Earth-based instruments.

Instead, most of the time, what we see radiating out from our host star are infrared wavelengths, ultraviolet wavelengths and, of course, visible wavelengths that we can see with the unaided eye.

For context, one of those visible wavelengths carries an energy of about 1 electron volt. The gamma rays Nisa and fellow researchers witnessed, by contrast, exuded about 1 trillion electron volts. And, there were a lot of them.

The first time scientists observed gamma rays with energies of more than a billion electron volts, according to the release, was in 2011 with NASA's Fermi Gamma-ray Space Telescope. But Fermi had a limit. It maxed out at finding gamma rays with about 200 billion electron volts. So in 2015, the new study's research team started collecting gamma ray data with HAWC as this observatory didn't seem to have the same restriction.

"They nudged us and said, 'We're not seeing a cutoff. You might be able to see something," Nisa said.
Which brings us to the present — the first time we've seen sun rays with energies extending into a trillion electron volts. And, according to Nisa, that does not appear to be the maximum.

"We thought we had this star figured out, but that's not the case."

The paper was published Thursday (Aug. 3) in the journal Physical Review Letters
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Scientists detect 'highest-energy light' coming from the Sun

The new study showcases that the gamma-ray energy could extend maximum into the TeV range – up to roughly 10 TeV.

By Mrigakshi Dixit

Created: Aug 04, 2023

In 2011, the arrival of NASA's Fermi Gamma-ray Space Telescope resulted in the first observation of gamma rays with enormous energy of more than a billion electron volts.

Fermi’s measurement of the Sun’s gamma rays estimated the maximum energy to be around 200 billion electron volts.

The new study showcases that the gamma-ray energy could far exceed this figure. In fact, it could extend maximum into the TeV range – up to roughly 10 TeV.

However, scientists are still uncertain how these gamma rays acquire such extraordinarily high energy.

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High-Altitude Water Cherenkov Observatory in Mexico. Jordan A. Goodman/Wikimedia Commons

According to the authors, gamma rays leave behind "telltale signatures" that may be seen in the atmosphere.

A specialist High-Altitude Water Cherenkov Observatory, or HAWC, identified this signal.

HAWC is unlike the conventional telescope. It employs a network of 300 colossal water tanks, each holding around 200 metric tons of water. The network is located in Mexico, more than 13,000 feet above sea level, between two dormant volcanic summits.

“From this vantage point, it can observe the aftermath of gamma rays striking air in the atmosphere. Such collisions create what are called air showers, which are a bit like particle explosions that are imperceptible to the naked eye,” explained the release.

When shower particles reach the water in HAWC's tanks, they produce what is known as Cherenkov radiation, which may be detected using the observatory's equipment.

“Currently, the discovery creates more questions than answers. Solar scientists will now scratch their heads over how exactly these gamma rays achieve such high energies and what role the sun’s magnetic fields play in this phenomenon,” they concluded.

The study was published in the journal Physical Review Letters.

Study abstract:

We report the first detection of a TeV γ-ray flux from the solar disk (6.3σ), based on 6.1 years of data from the High Altitude Water Cherenkov (HAWC) observatory. The 0.5–2.6 TeV spectrum is well fit by a power law, dN/dE=A(E/1  TeV)−γ, with A=(1.6±0.3)×10−12  TeV−1 cm−2 s−1 and γ=3.62±0.14. The flux shows a strong indication of anticorrelation with solar activity. These results extend the bright, hard GeV emission from the disk observed with Fermi-LAT, seemingly due to hadronic Galactic cosmic rays showering on nuclei in the solar atmosphere. However, current theoretical models are unable to explain the details of how solar magnetic fields shape these interactions. HAWC’s TeV detection thus deepens the mysteries of the solar-disk emission.


The Sun Is Stranger Than Astrophysicists Imagined

Quanta Magazine

The Fermi Telescope detects many more gamma rays during solar, or Maunder, minimum, the phase of the sun’s 11-year cycle when its magnetic field is calmest and most orderly. This makes sense if cosmic rays are the source. During solar minimum, more cosmic rays can reach the strong magnetic field near the sun’s surface and get mirrored, instead of being deflected prematurely by the turbulent tangle of field lines that pervades the inner solar system at other times. On the other hand, the detected gamma rays drop off as a function of frequency at a different rate than cosmic rays. If cosmic rays are the source, the two rates would be expected to match.

Visualizations of the sun’s magnetic field on Jan. 1, 1997, June 1, 2003, and Nov. 15, 2013, based on measurements by the Solar and Heliospheric Observatory. Green indicates positive polarity and purple is negative. NASA’s Goddard Space Flight Center Scientific Visualization Studio

Along with Fermi, a mountaintop observatory called HAWC (for High-Altitude Water Cherenkov experiment) will be taking data. HAWC detects gamma rays at higher frequencies than Fermi, which will reveal more of the signal. Scientists are also eager to see whether the spatial pattern of gamma rays changes relative to 11 years ago, since cosmic rays remain positively charged but the sun’s north and south poles have reversed as they do in their 11 year solar cycle.

These clues could help solve the solar mystery. HAWC scientists hope to report their first findings within a year, and scientists both within the Fermi collaboration and outside it have started to pore over its accruing data already. Since NASA is publicly funded, “anybody can download it if they want to glance through,” said Linden, who downloads Fermi’s new data almost every day.


Produced throughout the Universe by violent astrophysical events like supernova explosions, quasars and active galactic nuclei, 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. But finding solar emissions of gamma rays with an energy of 10 TeV or more was a shock, indeed.