Cologne, Germany, on April 24, 1945.
Image: US Department of Defense

As if the devastating effects of bombs dropped on European cities during the Second World War weren’t terrible enough, a surprising new study shows that the shockwaves produced by these bombing raids reached the edge of space, temporarily weakening the Earth’s ionosphere.

Large Allied bombing raids during World War II produced shockwaves powerful enough to briefly reduce the concentration of electrons in our planet’s ionosphere, according to new research published today in the science journal Annales Geophysicae. This weakening occurred above the bombing sites, and as far as 600 miles (1,000 kilometers) away, according to the new research. The effect was both temporary and not dangerous, but a weakened ionosphere—the layer of the Earth’s atmosphere that is ionized by solar and cosmic radiation—could have interfered with low-frequency radio transmissions during the war.

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“The images of neighbourhoods across Europe reduced to rubble due to wartime air raids are a lasting reminder of the destruction that can be caused by man-made explosions,” Chris Scott, the lead author of the new study and a professor of space and atmospheric physics at Reading University, said in a statement. “But the impact of these bombs way up in the Earth’s atmosphere has never been realized until now.”

Artist’s depiction of the ionosphere, shown in purple (it’s invisible to the naked eye).
Image: NASA’s Goddard Space Flight Center/Duberstein

The integrity of our ionosphere, a thick band that sits 50 to 360 miles (80 to 580 kilometers) above Earth’s surface, is heavily influenced by solar activity, including phenomena such as coronal mass ejections, high-speed solar wind streams, and energetic particle events. Today, disturbances to the ionosphere affect technology such as radio and GPS. But the effects of bombing raids on radio communications made during WWII remain poorly understood.

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The purpose of the new study was to “examine unique ionospheric measurements” made above the Radio Research Center in Slough, United Kingdom, from 1943 to 1945, in order to “determine whether any of the observed variability could be attributed to the major bombing campaigns across Europe,” the authors write in the new study. Hunting for a signature in the UK’s ionospheric records may seem strange given that the bombings took place on the European continent (primarily in Nazi Germany), but there were some good reasons for this, as the authors make clear in their study:

While the bombing of London by the Luftwaffe between September 1940 and May 1941 (popularly known as the “London Blitz”) would have generated explosions at a closer proximity to the ionospheric measurements being made above Slough, this bombing was more or less continuous, making it difficult to separate the impact of wartime raids from those of natural seasonal variability.

At the same time, the four-engine planes used by the Allies could drop much heavier bombs than the twin-engine German planes. Thus, the Allied bombing raids were both stronger and more discernible, making them a more useful variable to study.

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Bombing of a factory at Marienburg, Germany, on 9 October 1943.
Image: US Air Force

Looking at the daily reports collected by the Slough station, Scott was able to document a series of frequent radio pulses over a range of shortwave frequencies. These reached as high as 100 to 300 kilometers above the Earth’s surface. In addition to height, the data also revealed the electron concentration of ionization within the upper atmosphere. Scott and his colleagues then analyzed the ionosphere response records that corresponded to the timing of 152 large Allied raids in Europe, in which 100 to 900 tons (220,000 to 1.8 million pounds) of explosives were dropped onto cities, factories, and other military targets. Looking at the data, Scott could see that electron concentration in the ionosphere “significantly decreased” due to the shockwaves caused by the bombs. The powerful shockwaves heated the upper atmosphere, causing the loss of ionization, the researchers say.

Normally, a temporary reduction in the concentration of electrons is caused by the Sun, but it can also be caused from below, namely from lightning, volcanic eruptions, and earthquakes. The new study shows how modern warfare is a force of nature unto itself. As the researchers point out, one metric ton of TNT has an explosive energy equivalent to a single lightning strike.

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“It is astonishing to see how the ripples caused by man-made explosions can affect the edge of space,” said Scott. “Each raid released the energy of at least 300 lightning strikes. The sheer power involved has allowed us to quantify how events on the Earth’s surface can also affect the ionosphere.”

Patrick Major, a co-author of the study and a historian at the University of Reading, said the strength of these bombs wasn’t lost on those who were there to witness it.

“Aircrew involved in the raids reported having their aircraft damaged by the bomb shockwaves, despite being above the recommended height,” he said. “Residents under the bombs would routinely recall being thrown through the air by the pressure waves of air mines exploding, and window casements and doors would be blown off their hinges. There were even rumours that wrapping wet towels around the face might save those in shelters from having their lungs collapsed by blast waves, which would leave victims otherwise externally untouched.”

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This pioneering work is just the beginning, and it hints at the kinds of WWII research that might still be possible. Scott and Major are now asking the public to help with the digitization of early atmospheric data, which would allow them to assess the impact of the hundreds of smaller raids that transpired during the war. Using these “less extreme examples,” the researchers say they’ll be able to “determine the minimum explosive energy required to generate a detectable ionospheric response.”

At a more philosophical level, this study presents a potent reminder of how powerful our military technologies have become. War, as it has been said so many times, is hell.

[Annales Geophysicae]

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