About a month ago, the Honga Tonga-Hun Aha Apay volcano eruption in Tonga reached an explosive climax (see Once in a Thousand Years for details). The energy it rapidly releases in the oceans brings tsunamis, and at the same time, it also brings meteorological tsunamis in the atmosphere, a kind of air pressure disturbance, and the resulting pressure waves quickly spread around the world.
Image credit: NASA Earth Observatory
Atmospheric wave patterns near volcanic eruptions are quite complex, but thousands of kilometers away, the barometric pressure wave, as an isolated wavefront, spreads outward horizontally at a speed of about 1,000 kilometers per hour.
James Garvin, chief scientist at NASA's Goddard Space Flight Center, said in an interview that it is estimated that the power of the explosion is almost equivalent to 4-18 megatons of TNT. Observed from satellites using infrared sensors, the ripples in the atmosphere look like ripples from a stone thrown in a pond. The shock wave of sound waves produced by the explosion is enough to disturb the ionosphere, that is, the outer layer of the atmosphere about 80 kilometers above the Earth's surface.
Image credit: Mathew Barlow/University of Massachusetts Lowell
As the pulse of a wave passes through North America, India, Europe, and many other parts of the globe, it manifests itself as a disturbance that lasts for several minutes at atmospheric pressure. People are constantly paying attention to the movement of the wavefront, and the atmospheric wave spreads to the whole earth and turns back in about 35 hours.
The wavefront expansion produced by the eruption of The Tonga volcano is a particularly spectacular example of the global spread of atmospheric waves, which has occurred in the wake of other historic explosions. The eruption was particularly powerful, and it made the atmosphere "ring" like a bell (though the frequency was too low for us to actually hear). In fact, it is also a re-validation of a wave theory proposed more than 200 years ago.
Special atmospheric wave theory
More than 200 years ago, Pierre-Simon Laplace, a French mathematician, physicist and astronomer, predicted the behavior of a global atmosphere.
Pierre-Simon Laplace. | Image source: Wikimedia
Laplace built his theory on the basis of the physical equations that govern atmospheric motion at the global scale. He predicted that there should be a class of motion in the atmosphere that can spread rapidly but always "hug" the earth's surface tightly.
Specifically, Laplace found that gravity and atmospheric buoyancy favor air movement in the horizontal direction relative to vertical air movement, with the consequent effect that atmospheric waves can move along the curvature of the Earth.
For much of the 19th century, this was still a slightly abstract idea. But the pressure data after the 1883 eruption of Mount Krakatoa suggests in a dramatic way that Laplace was right. These movements along the curvature of the Earth can be excited and propagated over huge distances.
Krakatoa Volcano in 1883
The first such pressure waves, which attracted scientific attention, were generated by the 1883 eruption of Mount Krakatoa in Indonesia.
Krakatoa erupted in 1883. | Image source: Wikimedia Commons
The wave pulses of Krakatoa can be found in barometric pressure observations around the world. Of course, communication was much slower in those days, but over the course of a few years, scientists had been able to combine various independent observations and map the world map of how the pressure fronts spread in the hours and days after the outbreak.
The wavefront propagates outward from Mount Krakatoa and has been observed on at least three "global trips". The Royal Society of London published a series of maps in a famous report on volcanic eruptions in 1888 that illustrate the propagation of wavefronts.
A map drawn from an 1888 report reveals how the position of the pressure waves from the 1883 Krakatoa eruption changed every two hours. | Image credit: Kevin Hamilton, based on Royal Society of London images, CC BY-ND
The waves produced after the eruption of The Krakatoa Volcano are very low-frequency sound waves. During propagation, a change in local pressure exerts a force on neighboring air, which then accelerates, causing expansion or compression, accompanied by a change in pressure, which in turn makes the air travel farther along the path of the wave.
From what we know about high-frequency sound waves, we expect it to propagate in a straight line. But these global pressure pulses have a distinctive feature, and they bend with the curvature of the Earth as they propagate. It is precisely one manifestation of Laplace's theory.
The atmosphere rings like bells
This eruption of the Tonga volcano also shows this. These eruptions cause the atmosphere to "ring" like bells, that is, vibrate at certain frequencies.
In addition to these extreme events, it wasn't until recent years that scientists discovered that similar phenomena exist in the background oscillations of the atmosphere.
These global atmospheric oscillations are very low in frequency, but they are constantly stimulated by all the other movements in the atmosphere, providing a very gentle and continuous "background music" for more intense weather fluctuations.
#创作团队:
Compile: M ka
Typography: Wenwen
#参考来源:
https://theconversation.com/tonga-eruption-was-so-intense-it-caused-the-atmosphere-to-ring-like-a-bell-175311
https://www.jpl.nasa.gov/news/tonga-eruption-sent-ripples-through-earths-ionosphere
https://oceanservice.noaa.gov/facts/meteotsunami.html
#图片来源:
Cover image: Alen/Stockvault
首图：Mathew Barlow/University of Massachusetts Lowell