At 57km high (35 miles), the ash cloud generated by the eruption is the first to have been observed in the mesosphere, a layer of the atmosphere more commonly associated with shooting stars.
The previous record-holder, the 1991 eruption of Mount Pinatubo in the Philippines, caused a plume that was recorded as 40km high, although accurate satellite images such as those taken over Tonga weren't available at the time.
The Tonga eruption took place under the sea around 65km from the country's main island, causing tsunamis that were felt as far away as Russia, the United States, and Chile. The waves claimed six lives, including two people in Peru, 10,000km away.
The research was published in the journal Science.
“It's the first time we've ever recorded a volcanic plume reaching the mesosphere," said Dr Simon Proud, who works with RAL Space's Remote Sensing Group, the National Centre for Earth Observation and University of Oxford. “Krakatau in 1800s might have done as well, but we didn't see that in enough detail to confirm."
A full earth view of the eruption from Japan's Himawari-8 satellite. This shows a day/night ‘overview’ with the volcanic plume appearing blue at night | Credit: Simon Proud / STFC RAL Space / NCEO / JMA
Normally, the height of a volcanic plume can be estimated by measuring the temperature at its top and comparing it to the standard air temperatures found at various altitudes. This is because in the troposphere, the lowest layer of the Earth's atmosphere, temperature decreases with height.
But if the eruption is so large that the plume penetrates the higher layers of the atmosphere, this method becomes unreliable as air temperatures begin to increase again the higher you go.
To overcome this problem, the researchers developed a technique based on a phenomenon called 'the parallax effect'.
You can see this effect for yourself by closing your right eye, and holding out one hand with the thumb raised upwards. If you then switch eyes, so that your left is closed and your right is open, your thumb will appear to shift slightly against the background. By measuring this apparent change in position and combining this with the known distance between your eyes, you can calculate the distance between your eyes and your thumb.
The location of the Tonga volcano is covered by three geostationary weather satellites, 36,000km up in space, so the researchers were able to apply the parallax effect to the aerial images these captured. Crucially, during the eruption itself, the satellites recorded images every 10 minutes, enabling the rapid changes in the plume's trajectory to be documented.
“Thirty years ago, when Pinatubo erupted, our satellites were nowhere near as good as they are now. They could only scan the earth every 30 minutes. Or maybe even every hour," said Dr Proud.
“We think for Pinatubo we actually missed the peak of the activity and the points where it went the highest – it fell between two of the satellite images and we missed it. In reality it probably went quite a bit higher than the estimates that we have for its height."
The atmospheric waves produced by the HTHH eruption, seen by Japan’s Himawari-8 satellite | Credit: Simon Proud / STFC RAL Space / NCEO / JMA
The mesosphere reaches between approximately 48km and 80km high and is the third layer of the atmosphere, above the troposphere and the stratosphere. Meteors falling to earth often burn up in the mesosphere, causing shooting stars in the night sky. It is the coldest part of Earth's atmosphere, with temperatures near the top reaching as low as -143°C.
“The interesting thing is that this eruption put a lot of water into the mesosphere, which is usually a very dry part of the atmosphere," said Dr Proud. “This makes the eruption a useful test case for how well our climate and weather models can cope with unexpected and extreme conditions."
The researchers now intend to construct an automated system to compute the heights of volcano plumes using the parallax method. Co-author Dr Andrew Prata from Oxford's department of Atmospheric, Oceanic & Planetary Physics, said: “We'd also like to apply this technique to other eruptions and develop a dataset of plume heights that can be used by volcanologists and atmospheric scientists to model the dispersion of volcanic ash in the atmosphere."
“Further science questions that we would like to understand are: Why did the Tonga plume go so high? What will be the climate impact of this eruption? And what exactly was the plume composed of?"