A volcanic eruption off the coast of Tonga has caused mysterious concentric ripples in the atmosphere. Scientists have never seen anything like this before. Wave fluctuations were recorded both at the surface and high above the Earth in the ionosphere. The unique phenomenon was explained by gravitational waves, but everything turned out to be more complicated.
Volcano and atmospheric gravity waves
The Hunga-Tonga-Hunga-Haapai volcano off the coast of the island kingdom of Tonga woke up in December 2021, and a month later there was a powerful explosion. It generated earthquakes and tsunamis that reached the coast of Peru on the other side of the Pacific Ocean. A huge cloud of ash rose twenty kilometers into the stratosphere. The sound of the explosion was heard thousands of miles away, in the Yukon Territory of Canada, and infrasonic waves – below the threshold of human hearing – were recorded by instruments around the globe.
In addition, the eruption caused massive fluctuations in the atmosphere – the so-called atmospheric gravity waves. They were discovered by NASA’s Aqua satellite a few hours later. His images show dozens of concentric circles, each representing a fast-moving wave.
Acoustic-gravity waves (AGWs) are well known to atmospheric physicists, but never before have they been so clearly recorded during volcanic eruptions. Typically, strong AGWs are associated with earthquakes, tsunamis, and some artificial events such as rocket launches or explosions. Smaller waves arise due to a variety of phenomena – the movements of atmospheric fronts, thunderstorms, geomagnetic storms, solar flares, even diurnal fluctuations in the atmosphere.
“There is nothing unusual about these waves,” says Sergey Pulinets, chief researcher at the Space Research Institute of the Russian Academy of Sciences, Doctor of Physical and Mathematical Sciences. “In fact, these are sound waves, only with very low vibrations, so we don’t hear them. Like any sound, they appear during compression-expansion of air, when atmospheric masses come into motion.
Waves from the explosion of the volcano circled the globe several times, and barometers in different parts of the world recorded several small bursts – about 1.5 millibars – of pressure increase. In Seattle, on the West Coast of the United States, the surge was so strong that it dispelled the traditional local fog, the local office of the National Weather Service said. In Great Britain, at a distance of about 16,500 kilometers from the islands of Tonga, the first wave was caught 14 hours after the eruption, which made it possible to determine its speed – about 330 meters per second. This roughly corresponds to the speed of sound. Subsequent waves were recorded by especially sensitive barometers for another day.
The initial wave was felt all over the world. It was recorded by all 53 stations of the infrasound monitoring system of the Comprehensive Nuclear-Test-Ban Treaty Organization located at a distance of 1,800 to 18,000 kilometers from the volcano. For comparison: atmospheric waves from the Chelyabinsk meteorite that shook the Earth in 2013 caught only half of the points of the network.
Not just a wave
Theoretically, a fast upward flow of hot air and ash from an erupting volcano into the upper atmosphere can cause acoustic-gravity waves on a larger scale. However, what scientists observed after the Hunga-Tonga-Hunga-Haapai eruption did not fit into the scheme both in terms of scale and pattern. In the pictures, the oscillations looked like a mixture of waves of different types and sizes.
“The peculiarity is that this is an underwater eruption,” Sergey Pulinets notes. “The sound wave from the explosion itself and the atmospheric wave from a powerful ash emission superimposed. Another tsunami caused. Different scales created a rich pattern of oscillations of different frequencies. in the middle of the ocean, a circular wave went, and not reflected, as in an eruption on the coast.
The explosive speed of the eruption was also unusual. Usually volcanoes pour out lava and throw it out for several days, and sometimes weeks. Here, everything happened in a matter of minutes – as a result of one strong impulse.
NASA experts estimated the power of the Hunga-Tonga-Hunga-Haapai explosion at ten megatons of TNT. This is 500 times stronger than the explosion of the atomic bomb dropped on Hiroshima. The colossal detonation, as well as the underwater collapse and the associated tsunami, generated a whole spectrum of waves propagating both in the lower and upper layers of the atmosphere. In addition to acoustic-gravitational – infrasonic, Lamb waves and electromagnetic oscillations in the ionosphere. This, according to scientists, is the uniqueness of the atmospheric events that led to the eruption.
Satellite imagery shows waves rippling like a stone thrown into a pond from where an ash plume pierced the lower atmosphere. The static discharges within him unleashed a flurry of volcanic lightning. Satellites recorded more than 60,000 strikes in the fifteen minutes after the volcano’s initial explosion, corresponding to nearly 70 lightning strikes per second.
After that, an atmospheric shock wave began to diverge in all directions from the plume, which provoked a jump in atmospheric pressure. Using data from NASA’s GOES-West satellite available online, Matthew Barlow, Professor of Environmental, Earth and Atmospheric Sciences at the University of Massachusetts Lowell, built a composite image of the original wave’s movement through Earth’s atmosphere.
The sequence of events looks like this: the shock wave generates a high-frequency acoustic wave, which after a while passes into a low-frequency one, and then into an infrasonic one. In the upper layers of the atmosphere, where gases are ionized, split into charged particles – ions and electrons – waves already cause electromagnetic oscillations.
Using data from several ground and space sensors, British physicist Katherine Mitchell of the University of Bath created the video. It shows how sky waves propagate from the volcano towards New Zealand within a few hours after the eruption. They are seen as a change of positive and negative deviations of the total electron content. According to the American Geophysical Union, the waves took five hours to reach the coast of the United States.
Scientists constantly monitor, using satellite observations, electromagnetic waves propagating in the ionosphere – the region of the atmosphere in the range from 60 to 1000 kilometers from the earth’s surface. Within this zone (at a distance of about 400 kilometers from the Earth) the manned orbital station ISS rotates. For her and other spacecraft, ionospheric disturbances do not pose a danger. But they can affect the operation of satellite navigation systems such as GPS or GLONASS. The satellites themselves are located much farther – from 20 to 26 thousand kilometers from the Earth, but their signal can deviate, falling into the ionosphere.
“The wave from the satellite reaches the Earth with a certain delay. Additional inhomogeneities in the ionosphere introduce errors into the estimated signal delay, which reach tens of meters. Special services track them and remove them – errors in navigation can lead to accidents,” explains Pulinets.
Surprisingly, seven days after the eruption, acoustic-gravity waves were still propagating, having circumnavigated the globe for the tenth time. They were recorded in the infrared range by geostationary satellites GOES-16 and GOES-17.