“6 neighborhoods recorded in the Moscow region”: how the Tonga volcano is heard in Russia

– Did the waves from the volcano in the Tonga archipelago really reach Moscow more than once?

– They toured not only Moscow, but the whole world several times. The explosion was very powerful. Do you know that on October 30, 1961, the most powerful 58-megaton thermonuclear bomb was tested on Novaya Zemlya? Relatively speaking, the “voice” went from him and circled the globe twice. And this is not the only case. For example, when the Chelyabinsk meteorite exploded, a worldwide revolution was recorded.

– Why can we only talk about sound conditionally?

– Sound is longitudinal wave oscillations when particles of the medium within the wave oscillate along its propagation. Sound propagates at the speed of sound depending on the density of the medium, temperature and wind. However, powerful explosions produce not only sound but also many other waves in the atmosphere.

For example, internal gravitational waves (not to be confused with astrophysical gravitational waves. – socialbites.ca). Their formation is due to the interaction of Archimedes’ buoyancy with the force of gravity. The oscillations of the particles in them no longer pass along the direction of propagation, but at an angle. The frequency of internal gravitational waves is such that their period is more than five minutes. These waves propagate at speeds much slower than the speed of sound.

– Did the barometric stations in Moscow detect gravitational waves?

– Not. With powerful explosions, intermediate waves are also formed. In science, this is called the “basic mode of oscillation” or colloquially a Lamb wave (in the strict sense, Lamb waves propagate in solid plates or spheres). It is characterized by propagation across the earth’s surface with only zero vertical velocity. It propagates at the speed of sound, but with much longer oscillation periods, it is more than the five-minute limit for sound.

It was this Lamb wave observed at stations around the world that recorded the eruption of the volcano. The gravitational waves that also arose during the explosion, most likely, did not reach Moscow.

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Gravitational waves rippling after a volcano erupts

NASA

– One of the satellite images released by NASA shows wavelets moving through the atmosphere above the volcano. What is it caused by?

– These are just internal gravitational waves, their characteristic appearance. As an analogy, imagine that you throw a pebble into the water and circles emerge from it. Circles are internal waves that exist in water. The same thing happens in the atmosphere, and if there are clouds, the waves will create characteristic ripples in them.

– If internal gravitational waves can be thought of as circles on water, how are Lamb waves?

“You can’t get a house parable for them. They appear when solving the hydrodynamic equation and during powerful explosions in the atmosphere, it is impossible to create their similarity with improvised means.

How many times has a wave of Lambs from a volcano circled the Earth?

– In the Moscow region, the 6 arrivals of the Lamb wave were accurately recorded. Initial arrival corresponds to the direct propagation of the wave from the source to the observation point. The second is the so-called antipodal propagation, that is, the wave propagates in the opposite direction and comes from the other end of the Earth. The third arrival corresponds to a worldwide revolution of the direct wave. The fourth arrival is a return of the antipodal, similar to the fifth and sixth fixations. This is known for sure. Maybe more, but much harder to fix because as it spreads, its amplitude decreases. And the natural noise at low frequencies is very important.

– How did you manage to fix it? Does it look like a “fall” on the barograph chart?

– A single slope is only visible when using a conventional meteorological barograph. Its sensitivity does not allow multiple fluctuations to be seen. If you bought a serious scientific instrument, you would see fluctuations, as in the usual recording of audio signals from distant explosions. The home barometer recorded only a “blurry” picture, an average maximum.

For example, a high-quality signal was given by microbarometers in Dubna – this is the IS43 station of the IMS, the international infrared monitoring system of the implementation of the Comprehensive Nuclear-Test-Ban Treaty.

How often do lamb waves occur?

“With every big bang that shook the entire atmosphere. It’s hard to say the lower limit of the power required, no one has studied it. I can assume a few megatons are needed. In this case, the explosion must occur at an appropriate height so that the power does not travel to Earth and evaporate into space.

– How accurately can the strength of an eruption or volcanic eruption be determined using lamb waves or gravitational waves?

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Satellite image of the eruption of the Hunga-Tonga-Hunga-Haapai underwater volcano

CIRA/NOAA/Reuters

– In terms of geophysics – very true. It’s worth noting that this only measures the energy entering the air, not the full power of the explosion. Neither in soil nor in water. In other words, the strength of an equivalent explosion that would explode in the atmosphere and produce the same vibrations is specified. The equivalent power of the eruption of this volcano reached about 200 megatons. This is several times larger than the largest nuclear explosion.

– Impossible to repair without tools?

You cannot hear such a wave. A person can hear relatively up to 20 hertz, and the frequency of such waves is one thousandth of hertz.

– The recording of a volcanic eruption in the Tonga archipelago is not the only recent case where sound or other waves were recorded at an abnormal distance. An article was recently published in the journal Science on how Americans use the microphone in a balloon. To listen By performing more than 400 kilometers of rocket takeoff, they announced that they had opened a new atmospheric channel where sound travels hundreds of kilometers. Is this really a discovery?

– What is described here has nothing to do with the Lamb wave and furthermore with the internal gravitational waves. The idea is simple: you have an acoustic ocean waveguide underwater. It exists because the speed of sound is different at different depths due to differences in density and salinity. At a certain depth, a zone with the minimum speed of sound is formed, in which it acts as a horizontal waveguide that allows you to hear the explosion of one kilogram of TNT at a distance of thousands of kilometers, but only the source and receiver are located at this minimum depth. Imagine an acoustic beam rising at a small angle from the explosion inside the waveguide. Since the speed of sound is higher at the top, the beam will bend downward due to refraction and eventually rotate downward. And since the speed of sound also drops below the minimum, the beam will bounce back many times in the direction of the level where the source is.

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underwater sound channel

WHY

Researchers from America write that they believed for a long time in the existence of such a waveguide in the atmosphere and they finally found it… Although it is well known that there are many areas of sound speed minimums in the atmosphere. So there are many waveguides in the atmosphere similar to ocean waveguides.

Note that such waveguides only work effectively if the sound source and receiver are on the waveguide axis, i.e. at the minimum horizon of the sound speed.

There are at least three types of spherical waveguides in the atmosphere that operate when the sound source and receiver are located on the earth’s surface.

In a waveguide of the first type, the lower limit is the earth’s surface, and the upper limit is the maximum horizon of the sound speed, located at the upper limit of the temperature inversion (increase), usually at an altitude of up to one m. kilometer. However, because they work with the concept of effective sound velocity in atmospheric acoustics (sound velocity plus wind velocity in the propagation direction), the upper limit of the surface waveguide is highly dependent on the wind velocity.

The second type of waveguide also has a lower limit at the earth’s surface and an upper limit at the horizon of the maximum effective speed of sound. Usually these are altitudes of 20-50 kilometers.

A third type of waveguide has an upper limit in the thermosphere at altitudes above 100 km.

Such waveguides have been known since the end of the last century, when experiments were carried out by recording sound from volcanoes. And in these experiments, the stratosphere was discovered – a zone of temperature increase at altitudes of more than 10 km.

– Are phenomena possible, where people can hear sounds far away? For example, a small explosion of TNT a thousand kilometers away?

“I can tell you about my personal experience. In 1981 in Kazakhstan, during an experiment on the study of magnetospheric-atmospheric relations during seismic events, I was 200 kilometers away from an explosion with a capacity of 260 tons of TNT, and I heard it perfectly with my ears. Of course, this was due to the atmospheric waveguide. The sound bounced off the upper atmosphere at an altitude of about 100 kilometers and returned to Earth.

– Finally: Do lamb waves or gravitational waves have any practical effect on our lives?

– Life is affected by fluctuations in atmospheric pressure. These waves are accompanied by fluctuations in atmospheric pressure. We had an article where we researched what you wanted to ask. According to our data, there is some correlation between pressure fluctuations and the frequency of hospital admissions of cardiovascular patients.