Cosmic blast seared Earth’s atmosphere from 2 billion light-years away

On 9 October 2022, for 7 minutes, high energy photons from a gigantic explosion 1.9 billion light-years away toasted one side of Earth as never before observed. The event, called a gamma ray burst (GRB), was 70 times brighter than the previous record holder. But what astronomers dub the “BOAT”—the brightest of all time—did more than provide a light show spanning the electromagnetic spectrum. It also ionized atoms across the ionosphere, which spans from 50 to 1000 kilometers in altitude, researchers say. The findings highlight the faint but real risk of a closer burst destroying Earth’s protective ozone layer.

“It was such a massive event, it affected all levels of the atmosphere,” says solar physicist Laura Hayes of the European Space Agency (ESA).

Astronomers aren’t yet sure what causes a GRB, which we see shining as intensely as a bright star in the Milky Way, despite being billions of times more distant. To hurl so much energy across billions of light-years, GRBs must be among the biggest explosions since the big bang. At least some of them may be born in a particular type of supernova that occurs when a dying, massive star collapses into a neutron star or black hole. A typical GRB releases as much energy in a few seconds as the Sun will in its entire 10-billion-year lifetime.

Even by those standards, the October 2022 event, known as GRB 221009A, was exceptional, saturating detectors on NASA’s Fermi Gamma-ray Space Telescope and leaving an afterglow at longer visible wavelengths that even amateur astronomers could see for hours.

When Hayes heard that detectors on ESA’s Solar Orbiter spacecraft had picked up x-rays from GRB 221009A, she wondered whether the event might have been witnessed by another instrument she had been using to detect the impact of solar flares on the ionosphere. That instrument, called SuperSID, is a simple antenna at Ireland’s Dunsink Observatory tuned to pick up very low frequency (VLF) radio signals. Because VLF waves can penetrate saltwater, militaries use them to communicate with submarines. The waves can also travel thousands of kilometers around the globe by bouncing between the sea surface and charged particles in the lower ionosphere. SuperSID can’t decrypt naval messages, but it can analyze changes in the shape and strength of passing VLF signals to gather information on the state of the lower ionosphere.

When Hayes looked back in the SuperSID data to 9 October 2022, she saw a jump in the strength of a VLF signal at the time GRB 221009A occurred, one that was roughly equivalent in size to the effect of a small to medium solar flare, she and a colleague reported in Research Notes of the AAS in October 2022. The observation showed that the burst’s photons were reaching down to just 60 kilometers above the surface and ionizing more molecules in the air, creating a better reflector. Past VLF detections of GRBs had occurred at night, when they weren’t drowned out by the Sun’s activity. This time, the researchers caught one in the middle of the day. “We were surprised it was so big,” Hayes says. “It overcame even the Sun.

But what of the rest of the ionosphere? It also was stirred up, a second fortuitous observation shows. Mirko Piersanti, a space weather scientist at the University of L’Aquila, and colleagues were analyzing data from a small satellite called the China Seismo-Electromagnetic Satellite (CSES), which aims to sense earthquake precursors from their impact on the upper ionosphere. The satellite orbits at 507 kilometers altitude and measures the characteristics of particles in the rarified atmosphere around it.

CSES happened to be in the right place to witness the strike of GRB 221009A. It recorded a spike in the electric field of the region of ionosphere it was traveling through, which could have been caused by a sudden increase in ionization from the GRB, the team reports today in Nature Communications . To rule out other causes, such as solar flares, the team modeled the effect of a sudden gamma ray pulse on the upper atmosphere. It chimed with the size of the electric field spike and the time delay between GRB 221009A and the spike. “It’s the first time we’ve been able to observe such a variation in the upper part of the ionosphere,” Piersanti says.

Hayes concurs. “Effects were expected down below but had not been seen at high altitude,” she says. “It changed the dynamics of the whole system.”

Scientists have long mused about what might happen if a GRB were to go off in a nearby galaxy or even inside the Milky Way . If a GRB 1.9 billion light-years away can affect the whole of our ionosphere in this way, “one closer could be very serious for us,” Piersanti says. “It would destroy the ozone layer,” which shields living things from the Sun’s harmful ultraviolet light, he says.

Born from interactions between air and sunlight, ozone, molecular O 3 , forms naturally in the stratosphere at altitudes of 15 to 35 kilometers. But an intense blast of gamma rays from a nearby GRB would create nitrogen oxides that destroy ozone faster than it can regenerate. It could take up to a decade to recover, Piersanti says. In that time, plants and animals could suffer much more DNA damage from increased ultraviolet levels, according to models.

Astronomers estimate, however, that GRB 221009A was a one-in-10,000-year event and so a closer one would be even rarer. “I’m an optimist,” Piersanti says. “But from a physical point of view, it could happen.”