[The estimated the blast was around 10 megatons of TNT equivalent,
about 500 times as powerful as the bomb dropped on Hiroshima, Japan,
during World Word II. the wave looked like a ripple produced by
dropping a stone in a pond. ] [[link removed]]

TONGA ERUPTION WAS SO INTENSE, IT CAUSED THE ATMOSPHERE TO RING LIKE
A BELL  
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Kevin Hamilton
January 23, 2022
The Conversation
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_ The estimated the blast was around 10 megatons of TNT equivalent,
about 500 times as powerful as the bomb dropped on Hiroshima, Japan,
during World Word II. the wave looked like a ripple produced by
dropping a stone in a pond. _

The volcano shortly before its eruption., Maxar via Getty Images

 

The Hunga Tonga-Hunga Ha'apai eruption
[[link removed]]
reached an explosive crescendo on Jan. 15, 2022. Its rapid release of
energy
[[link removed]]
powered an ocean tsunami that caused damage as far away as the U.S.
West Coast, but it also generated pressure waves in the atmosphere
that quickly spread around the world.

The atmospheric wave pattern close to the eruption was quite
complicated [[link removed]], but
thousands of miles away it appeared as an isolated wave front
traveling horizontally at over 650 miles an hour
[[link removed]] as it spread
outward.

NASA’s James Garvin, chief scientist at the Goddard Space Flight
Center, told NPR the space agency estimated the blast
[[link removed]]
was around 10 megatons of TNT equivalent, about 500 times as powerful
as the bomb dropped on Hiroshima, Japan, during World Word II. From
satellites watching with infrared sensors above, the wave looked like
a ripple produced by dropping a stone in a pond.

[Animation shows the pulse moving around the world.]Satellite infrared
observations captured the pulse propagating around the world. Mathew
Barlow/University of Massachusetts Lowell

The pulse registered as perturbations in the atmospheric pressure
lasting several minutes as it moved over North America
[[link removed]],
India
[[link removed]],
Europe
[[link removed]] and
many other places around the globe. Online, people followed the
progress of the pulse in real time as observers posted their
barometric observations to social media. The wave propagated around
the whole world and back in about 35 hours.

I am a meteorologist [[link removed]] who has
studied the oscillations of the global atmosphere
[[link removed]] for almost four decades
[[link removed]]. The expansion of the wave
front from the Tonga eruption was a particularly spectacular example
of the phenomenon of global propagation of atmospheric waves, which
has been seen after other historic explosive events, including nuclear
tests.

This eruption was so powerful it caused the atmosphere to ring like a
bell, though at a frequency too low to hear. It’s a phenomenon first
theorized over 200 years ago.

Krakatoa, 1883

The first such pressure wave that attracted scientific attention was
produced by the great eruption of Mount Krakatoa in Indonesia in 1883.
[[link removed]]

The Krakatoa wave pulse was detected in barometric observations at
locations throughout the world. Communication was slower in those
days, of course, but within a few years, scientists
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had combined the various individual observations and were able to plot
on a world map the propagation of the pressure front
[[link removed]]
in the hours and days after the eruption.

The wave front traveled outward from Krakatoa and was observed making
at least three complete trips around the globe
[[link removed]].
The Royal Society of London published a series of maps illustrating
the wave front’s propagation in a famous 1888 report on the
eruption.

[Animation of the maps]Maps from an 1888 report, shown here as an
animated loop, reveal the position every two hours of the pressure
wave from the 1883 eruption of Krakatoa. Kevin Hamilton, based on
Royal Society of London images, CC BY-ND
[[link removed]]

The waves seen after Krakatoa or the recent Tonga eruption are very
low-frequency sound waves. The propagation occurs as local pressure
changes produce a force on the adjacent air, which then accelerates,
causing an expansion or compression with accompanying pressure
changes, which in turn forces air farther along the wave’s path.

In our normal experience with higher-frequency sound waves, we expect
sound to travel in straight lines, say, from an exploding firework
rocket directly to the ear of onlooker on the ground. But these global
pressure pulses have the peculiarity of propagating only horizontally,
and so bending as they follow the curvature of the Earth.

A theory of waves that hug the Earth

Over 200 years ago, the great French mathematician, physicist and
astronomer Pierre-Simon de Laplace
[[link removed]]
predicted such behavior.

Pierre-Simon de Laplace, 1749-1827. Wikimedia

Laplace based his theory on the physical equations governing
atmospheric motions on a global scale. He predicted that there should
be a class of motions in the atmosphere that propagate rapidly but hug
the surface of the Earth. Laplace showed that the forces of gravity
and atmospheric buoyancy favor horizontal air motions relative to
vertical air motions, and one effect is to allow some atmospheric
waves to follow the curvature of the Earth.

For most of the 19th century, this seemed a somewhat abstract idea.
But the pressure data after the 1883 eruption of Krakatoa showed in a
dramatic way that Laplace was correct and that these Earth-hugging
motions can be excited and will propagate over enormous distances.

Understanding of this behavior is used today to detect faraway nuclear
explosions
[[link removed]].
But the full implications of Laplace’s theory for the background
vibration of the global atmosphere have only recently been confirmed
[[link removed]].

Ringing like a bell

An eruption that sets the atmosphere ringing like a bell is one
manifestation of the phenomenon that Laplace theorized. The same
phenomenon is also present as global vibrations of the atmosphere.

These global oscillations, analogous to the sloshing of water back and
forth in a bathtub, have only recently been conclusively detected
[[link removed]].

The waves can connect the atmosphere rapidly over the whole globe,
rather like waves propagating through a musical instrument, such as a
violin string, drum skin or metal bell. The atmosphere can and does
“ring” at a set of distinct frequencies.

[Aerial image shows the top of the eruption and outlines of nearby
islands]Images from a weather satellite captured the volcanic eruption
on Jan. 15, 2022. Japan Meteorology Agency via AP
[[link removed]]

In 2020, my Kyoto University colleague Takatoshi Sakazaki
[[link removed]] and I
were able to use modern observations
[[link removed]] to confirm implications of
Laplace’s theory for the globally coherent vibrations of the
atmosphere
[[link removed]].
Analyzing a newly released dataset
[[link removed]]
of atmospheric pressure every hour for 38 years at sites worldwide, we
were able to spot the global patterns and frequencies that Laplace and
others who followed him had theorized.

These global atmospheric oscillations are much too low-frequency to
hear, but they are excited continuously by all the other motions in
the atmosphere, providing a very gentle but persistent “background
music”
[[link removed]]
to the more dramatic weather fluctuations in our atmosphere.

Laplace’s work was a first step on the road to our modern computer
forecasting of the weather
[[link removed]].

[_Over 140,000 readers rely on The Conversation’s newsletters to
understand the world._ Sign up today
[[link removed]].][The
Conversation]

Kevin Hamilton
[[link removed]],
Emeritus Professor of Atmospheric Sciences, _University of Hawaii
[[link removed]]_

This article is republished from The Conversation
[[link removed]] under a Creative Commons license. Read
the original article
[[link removed]].

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