When the Hunga Tonga–Hunga Haʻapai volcano erupted on January 15, 2022, it wasn’t just a local impact. The explosion punched through the stratosphere and sent ripples into the edge of space, disturbing satellite orbits and even affecting GPS signals. But one question lingered: how? To be precise, what kind of atmospheric waves were responsible for shaking the thermosphere, the upper layer of our atmosphere?

Now, in a groundbreaking study published in AGU Advances, researchers led by Ruoxi Li from the University of Science and Technology of China have delivered a clear answer. Using cutting-edge satellite data and sophisticated climate models, they found that secondary gravity waves were the main culprits behind the global-scale atmospheric upheaval.

A blast from Earth to space

You’ve probably read some headlines about the 2022 Tonga eruption. It was one of the largest ever recorded, sending enough water into the atmosphere to temporarily warm the planet. Towering ash clouds, tsunami warnings, and shockwaves circled the globe. But what happened above that volcanic plume was even more extraordinary.

“The extraordinary eruption of the Tonga volcano on 15 January 2022 lofted material to heights exceeding 50 km, marking the highest observed since the satellite era,” write the authors of the new study. “This eruption caused significant disturbances spanning from the hydrosphere up to the thermosphere.”

That’s where satellites began picking up strange signals. In particular, the GRACE-FO satellite — designed to measure shifts in Earth’s gravity field — registered large-scale changes in the density of some parts of the atmosphere. But what type of wave caused them?

Two candidates emerged: Lamb waves and gravity waves.

Lamb waves are a type of ultrasonic wave. They propagate along the surface of a plate or shell and move horizontally through the atmosphere like a ringing bell. They seemed to be the most likely culprit. But it was the gravity waves — especially a rarer breed called secondary gravity waves — that this study says caused the ripples.

Chasing waves through the atmosphere

Secondary gravity waves are ripples in the atmosphere created when primary gravity waves, like those triggered by a volcanic eruption, break and release their energy, generating new waves that can travel farther and faster. They act as a second burst of atmospheric motion, spreading disturbances across the globe.

To assess what caused the disturbances observed in satellites, the team turned to two powerful atmospheric models. The first, called WACCM-X, simulated the behavior of Lamb waves. The second, HIAMCM, focused on the behavior of gravity waves, especially the secondary ones created after the primary waves break in the upper atmosphere.

When researchers matched simulated data against actual measurements from GRACE-FO, a clear pattern emerged. The density changes — up to 100% deviations from normal — lined up precisely with the gravity wave simulations. In other words, the secondary gravity waves were responsible for the anomalies observed by GRACE-FO. Lamb waves, by contrast, showed much weaker effects at the satellite’s altitude of around 510 km. According to the model, they accounted for only about 25% of the disturbances observed.

Curiously, the GRACE-FO satellite wasn’t meant for this type of study. The satellite measures changes in Earth’s gravity field by tracking variations in mass, such as water movement, ice loss, and atmospheric density. But as it’s a near-polar satellites, it circles the globe every 90 minutes and provides valuable data for extreme events like this one.

So, what exactly are secondary gravity waves?

The eruption’s vertical plume created primary gravity waves — like ripples from a pebble in a pond. These waves traveled upward and eventually broke, much like ocean waves crashing on a beach.

But the energy didn’t just stop there. As the primary waves dissipated, they created local turbulence and heating, which acted like new mini-explosions. These, in turn, generated secondary gravity waves. Unlike their forebears, these waves had wider reach and higher speeds — up to 600 meters per second — and they spread across the globe.

The HIAMCM model describes the behavior of these secondary waves as they go outward from the eruption site. Their timing and shape closely mirrored the GRACE-FO measurements.

This isn’t just a case of atmospheric forensics. Understanding how energy from Earth’s surface can ripple through the atmosphere and touch the edge of space is crucial. It has real-world implications for satellite operations, GPS accuracy, and even climate models.

The 2022 Tonga eruption was the most powerful atmospheric explosion in decades — possibly since Krakatoa in 1883. It created a rare opportunity to study how the Earth’s systems connect, from the crust to the cosmos.

The study was published in AGU Advances.