The Hunga Tonga-Hunga Haʻapai volcanic column is one of the tallest on record

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The Hunga Tonga-Hunga Haʻapai eruption in mid-January this year was particularly spectacular. It is one of the largest volcanic eruptions ever recorded. Thanks to satellite images, a team from the University of Oxford affirms that the volcanic cloud generated by this eruption reached a height of 57 kilometers, that is, beyond the stratosphere. This is the first time a plume has been seen penetrating the stratopause.

The January 2022 eruption was so powerful that it was heard nearly 10,000 kilometers from the Tonga islands. The phenomenon caused tsunamis that were felt as far away as Russia, the United States and Chile. According to some experts, the event was comparable to the eruption of Krakatoa in 1883. However, specialists have estimated that this eruption would have little effect on the climate, due to the location of the volcano (relatively isolated) and the amount of material ejected. (about 400 kilotons).

Large explosive volcanic eruptions can spew materials such as ash, gases, and water into the stratosphere, with measurable impacts on atmospheric composition and climate. The 1991 Mount Pinatubo eruption, for example, ejected almost 10 km3 of volcanic material, lowering temperatures by about half a degree for a year or two. The height of its plume was estimated at the time to be 40 kilometers. This time, and for the first time, the ash cloud was observed in the lower mesosphere. This suggests that the height reached by the Pinatubo eruption plume may have been underestimated.

An altitude measured thanks to the parallax effect

Measuring the height of a volcanic column requires precise observations and careful calculations. It is usually calculated by measuring the temperature at its top and comparing it to standard air temperatures recorded at different altitudes, knowing that in the troposphere, the lowest layer of the Earth’s atmosphere, temperature decreases with altitude. But in the case of large eruptions, this method becomes unreliable: when the volcanic column reaches the stratosphere, at an average altitude of about 12 kilometers, the air temperature increases with altitude (because the air heats up by absorbing the rays UV from the sun). ).

Therefore, the researchers used another method to estimate altitude, based on the parallax effect. This effect, the basis of binocular vision, results in the difference in the apparent position of an object seen along different lines of sight. Parallax can be experienced as follows: close one eye, then extend your arm out in front of you and hold up your thumb; switch eyes, you will see your thumb move slightly relative to the background. By measuring this apparent change in position and knowing the distance between your eyes, it is possible to calculate the distance between your eyes and your thumb.

The Hunga Tonga-Hunga Haʻapai eruption as seen by the Japanese Himawari-8 satellite on January 15, 2022. Left: eruption at 4:20 UTC (approximately 15 min after eruption onset); medium: at 4:50 UTC; right: at 5:40 UTC. © Simon Proud/STFC Space RAL/NCEO/JMA.

It is the same principle that was used to measure the plume of the Tonga volcano. This was observed by three geostationary meteorological satellites: GOES-17 (USA), Himawari-8 (Japan) and GeoKompSat-2A (Korea). During the eruption, images were captured every 10 minutes, allowing the rapid changes in the trajectory of the plume to be observed; therefore, the researchers applied the parallax effect to these satellite images. They determined an altitude of 57 kilometers, which corresponds to the mesosphere.

An advance in the mesosphere that undoubtedly has precedents

The mesosphere refers to the part of the atmosphere located between 48 and 80 kilometers above sea level. It is at this altitude that meteors falling to Earth burn up and cause shooting stars. It is also the coldest part of the atmosphere; at the highest point, temperatures can reach -143°C.

The team notes that the Tonga volcano eruption sent a lot of water into the mesosphere, which is usually a very dry part of the atmosphere. ” This makes the eruption a useful test of how well our weather and climate models can cope with unexpected and extreme conditions. said Dr. Simon Proud, Senior Scientist at the National Center for Earth Observation at the University of Oxford.

Not only does this finding suggest that the January 2022 eruption could have a climate impact, the nature and extent of which have yet to be determined, but it implies that other past eruptions may have generated a similar plume, which the instrumentation of the time did not. . it is possible to estimate in its true measure. ” Thirty years ago, when Pinatubo erupted, our satellites were not as good as they are today. They could only scan the Earth every 30 minutes. Or maybe even every hour says Dr. Proud.

So it could be that in 1991, the researchers simply missed the peak of the activity and the point of the highest column if they occurred between two satellite images. Even Krakatoa’s 1883 eruption could have produced a similar plume, but there was obviously no way to verify this at the time.

The team now plans to develop an automated system to calculate the heights of volcanic columns using the parallax method. Such a tool would help scientists model the dispersion of volcanic ash in the atmosphere. It also remains to be determined what exactly this plume was made of and why it rose so high.

Source: S. Proud et al., Science

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