– Did the waves from the volcano in the Tonga archipelago really reach Moscow more than once?
– They traveled across Moscow and the entire globe, several times. The eruption was incredibly powerful. It is known that on October 30, 1961, the 58-megaton thermonuclear test at Novaya Zemlya produced a shock so far reaching that the airwave circled the planet twice. This is not an isolated instance. For example, when the Chelyabinsk meteorite exploded, its atmospheric impact was felt worldwide, triggering a measurable shift in data streams globally.
– Why is sound only a partial description of what happened?
– Sound is a longitudinal wave, with particles in the medium oscillating along the direction of travel. Its speed depends on density, temperature, and wind. Yet extremely strong explosions generate a chorus of waves that travel through the atmosphere in different ways.
One type is internal gravitational waves, distinct from astrophysical gravitational waves. They arise from the interaction of buoyancy forces with gravity. In these waves, particle movement is not simply along the travel path but at angles, with frequencies that yield periods longer than five minutes. Their speeds are far slower than the speed of sound.
– Did barometric stations in Moscow detect gravitational waves?
– Not directly. Powerful explosions also create intermediate waves. Scientists call this the basic mode of oscillation, often referred to as a Lamb wave. In the strict sense, Lamb waves propagate through solid plates or spheres, and on the Earth’s surface they move with essentially zero vertical velocity. They travel at the speed of sound but exhibit much longer periods, well beyond a five‑minute threshold for ordinary sound.
It was this Lamb wave pattern observed at stations worldwide that signaled the volcano eruption. Gravitational waves that might arise during the explosion likely did not reach Moscow.
– NASA released satellite images showing wavelets traveling through the atmosphere above the volcano. What causes these?
– Those are internal gravitational waves, visible in their characteristic form. Think of throwing a pebble into water and watching circles spread outward; a similar pattern appears in the atmosphere as long as clouds exist, producing ripple-like structures.
– If internal gravitational waves resemble circles on water, how are Lamb waves different?
There is no simple house analogy for Lamb waves. They emerge when solving the hydrodynamic equations and, during powerful atmospheric explosions, do not resemble anything easily created with improvised means.
How many times did a Lamb wave from a volcanic eruption circle the Earth?
– In the Moscow region, researchers accurately recorded six arrivals of the Lamb wave. The first arrival is the direct wave reaching the observation point. The second is antipodal propagation, where the wave travels the opposite direction and arrives from the far side of the planet. The third arrival reflects a worldwide circulation of the direct wave. The fourth repeats the antipodal path, with the fifth and sixth arrivals representing subsequent cycles. The amplitude diminishes with each pass, and ambient low‑frequency noise becomes significant.
– How were these events fixed? Do they resemble a falling slope on a barograph chart?
– A single slope is visible on a standard meteorological barograph, but its sensitivity hides multiple fluctuations. A serious scientific instrument reveals a pattern similar to audio recordings of distant explosions; a home barometer shows only a blurred, averaged peak.
For example, high‑quality signals were captured by microbarometers in Dubna, the IS43 station of the IMS, part of the international monitoring system for the Comprehensive Nuclear‑Test‑Ban Treaty.
How often do Lamb waves recur?
– With every major atmospheric disturbance. Pinning down a lower limit is tricky, as no exhaustive study exists. It is plausible that a few megatons are needed, with the explosion occurring at an altitude that preserves energy in the air rather than venting it into space.
– How precisely can the strength of an eruption be inferred from Lamb or gravitational waves?
– Geophysically, these measures are reliable for the energy that enters the air, not the total power of the explosion itself. The comparison is about the equivalent airburst strength that would generate the same atmospheric vibrations. In this case, the eruption’s equivalent power is estimated around 200 megatons, considerably larger than the largest historic nuclear test.
– Is repair possible without specialized tools?
– A human cannot hear such waves. The audible range tops out around 20 hertz, while these waves operate at frequencies near one thousandth of a hertz.
– The Tonga eruption is not the only recent case where sounds or other waves were detected at remarkable distances. A Science magazine article noted that American balloon microphones could detect the sound of rockets launching more than 400 kilometers away, suggesting a new atmospheric channel for sound transmission. Is this a breakthrough?
– The explanation here does not involve Lamb or gravitational waves. The idea resembles an underwater acoustic ocean waveguide, where sound speed varies with depth due to density and salinity differences. At a particular depth, a minimum sound speed creates a horizontal waveguide that could carry the roar of one kilogram of TNT across thousands of kilometers, provided the source and receiver lie on the axis of that minimum speed. The sound beam would bend and reflect, staying within the guiding path as it travels.
Researchers in the United States have long suspected such a waveguide exists in the atmosphere and have identified zones where it functions. There are several atmospheric waveguides, each governed by wind and temperature profiles. These waveguides have been known since the late twentieth century when volcano recordings first revealed a stratified atmospheric layer, the stratosphere, with a temperature increase at higher altitudes.
– Can such phenomena allow people to hear distant sounds like a small TNT blast a thousand kilometers away?
– A personal account from 1981 in Kazakhstan describes hearing a 260‑ton TNT explosion 200 kilometers away. The observer attributed the sound to an atmospheric waveguide, with the sound bouncing off the upper atmosphere at about 100 kilometers and returning to Earth.
– Do Lamb waves or gravitational waves have practical effects on daily life?
– They influence atmospheric pressure fluctuations, which in turn correlate with broad patterns of health data. A recent study explored how pressure changes relate to hospital admissions for cardiovascular patients, suggesting a tangible, though indirect, impact on daily life.
In sum, the study of atmospheric waves from volcanic and explosive events reveals a complex mix of wave types that traverse the globe in surprising ways. These waves include direct sound, Lamb waves, and internal gravitational waves, each with distinct propagation rules and practical implications for science and public health alike.