The eruption of Hunga Tonga-Hunga Ha’apai

A new analysis led by researchers from the University of Oxford has found that the devastating January 2022 Hunga Tonga-Hunga Ha’apai eruption created the highest volcanic plume on record. At a height of 57 km (35 miles), the ash cloud generated by the eruption is also the first observed in the mesosphere, a layer of the atmosphere more often associated with shooting stars.

Using satellite images, researchers have confirmed that the January 2022 eruption of the Hunga Tonga-Hunga Ha’apai volcano produced the highest plume on record. The colossal eruption is also the first directly observed and has broken through to the mesosphere layer of the atmosphere. The results, by a team of scientists from the[{” attribute=””>University of Oxford’s Department of Physics and RAL Space, were published on November 3 in the journal Science.

On January 15, 2022, Hunga Tonga–Hunga Haʻapai, a submarine volcano in the Tongan archipelago in the southern Pacific Ocean, violently erupted. The explosion was one of the most powerful ever observed, sending shock waves around the world and triggering devastating tsunamis that left thousands homeless. A towering column of ash and water was ejected into the atmosphere – but until now, scientists lacked an accurate way to measure just how tall this was.

Normally, the height of a volcanic plume can be estimated by measuring the temperature recorded at the top by infrared-based satellites and comparing this to a reference vertical temperature profile. This is because in the troposphere (the first and lowest layer of the Earth’s atmosphere), temperature decreases with height. But if the eruption is so large that the plume penetrates into the next layer of the atmosphere (the stratosphere), this method becomes ambiguous because the temperature begins to increase again with height (due to the ozone layer absorbing solar ultraviolet radiation).

A zoomed-in image of the Hunga Tonga-Hunga Ha’apai eruption on January 15, 2022, captured by NOAA’s GOES-17 weather satellite. Credit: Simon Proud and Simeon Schmauß / Uni Oxford, RALSpace NCEO / NOAA

To solve this problem, the researchers used a new method based on a phenomenon called “the parallax effect.” This is the apparent difference in an object’s position when viewed from multiple lines of sight. You can see this for yourself by closing your right eye and extending one hand with the thumb up. Then when you switch eyes so that your left side is closed and your right side is open, your thumb seems to shift slightly against the background. By measuring this apparent change in position and combining it with the known distance between your eyes, you can calculate the distance to your thumb.

The Tonga volcano’s location is covered by three geostationary weather satellites, so the researchers were able to apply the parallax effect to the aerial photos they captured. Crucially, the satellites themselves recorded images every 10 minutes during the eruption, documenting the rapid changes in the plume’s orbit.

The results showed that the plume reached a height of 57 kilometers (35 miles) at its highest point. This is significantly higher than the previous record holders: the 1991 eruption of Mount Pinatubo in the Philippines (40 km / 25 miles at the highest point) and the 1982 eruption of El Chichón in Mexico (31 km / 19 miles). It also makes the plume the first observational evidence of a volcanic eruption that injects material through the stratosphere and directly into the mesosphere, which begins about 50 km (31 miles) above Earth’s surface.

“It’s the first time we’ve ever recorded a volcanic plume reaching the mesosphere. Krakatoa in the 1800s might have done just as well, but we haven’t seen that in enough detail to confirm,” said Dr. Simon Proud, senior scientist at the National Center for Earth Observation at the University of Oxford and the Science and Technology Facilities Council’s RAL Space facility.

“It’s an extraordinary result, because we’ve never seen such a high cloud,” Proud added. “In addition, it is only possible to estimate altitude as we did (using the parallax method) now that we have good satellite coverage. Ten years ago, that would not have been possible.”

The Oxford researchers now plan to build an automated system to calculate the height of volcanic plumes using the parallax method.

Co-author Dr. Andrew Prata of the Atmospheric, Oceanic & Planetary Physics subsection added: “We want to apply this technique to other eruptions as well and develop a plume height dataset that can be used by volcanologists and atmospheric scientists to model the distribution of volcanic ash in the atmosphere. “Further scientific questions we would like to understand are: Why did the Tonga plume go so high? What are the climatic effects of this eruption? And what exactly did the plume consist of?”

Reference: “The January 2022 Hunga Tonga-Hunga Ha’apai Volcano Eruption Reached the Mesosphere” by Simon R. Proud, Andrew T. Prata and Simeon Schmauß, November 3, 2022, Science.
DOI: 10.1126/science.abo4076

In addition to the University of Oxford, the study also involved the Rutherford Appleton Laboratory and the National Center for Earth Observation in Harwell and the University of Applied Sciences in Munich.

The three satellites used to record and evaluate the eruption were GOES-17 (US), Himawari-8 (Japan) and GeoKompSat-2A (Korea). The open access data was processed by Britain’s Jasmin Supercomputer in the Science and Technology Facilities Council’s Rutherford Appleton Lab.