The discovery was made recently after researchers analyzed daily records from the Radio Research Station at Ditton Park near Slough, England. There, routine measurements of the ionosphere were taken from 1933 to 1996, "the longest continuous set of ionospheric measurements in the world," according to the study published Tuesday in the journal Annales Geophysicae
"What they didn't realize at the time was that the [ionsopheric records] actually contain the signatures of the actual war itself," said Chris Scott, study author and University of Reading professor of space and atmospheric physics.
The researchers decided to focus on major bombing raids of German cities by Allied forces and the Royal Air Force between 1943 and 1945, rather than the "London Blitz" earlier in the war. This is because the blitz was ongoing from September 1940 to May 1941, which means the data would be harder to separate from other seasonal changes due to the sun that might affect the ionosphere.
And the German Luftwaffe two-engine planes couldn't carry the much larger bombs the Allied forces could haul in their four-engine planes. For example, the Luftwaffe could carry about 4,440 pounds of bombs. The Allied air forces could carry bombs like the "Tall Boy" and the "Grand Slam," respectively weighing 12,000 and 22,000 pounds.
The Allied bombing raids over German cities were more intense, shorter and occurred during the day. The researchers compared ionospheric data from the Radio Research Station with information concerning 152 large Allied air raids.
In the ionosphere, the sun's radiation collides with gas that breaks into electrons and ions.
What the researchers realized is that the shockwaves produced by the bombs impacting the Earth's surface were able to reach the ionosphere 621 miles above. The shockwaves heated the upper atmosphere and weakened it, decreasing the amount of electrons in the ionosphere. The impacts would last about 24 hours. Given the little we know about the ionosphere, it's difficult to say what effect that may have had on humans or the Earth.
While the ionosphere is affected by solar activity coming from space, things like thunderstorms and other natural events on Earth can impact it, too. But the ionosphere is always changing, which is why there are multiple ongoing missions to study it.
"The images of neighborhoods 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," said Scott. "But the impact of these bombs way up in the Earth's atmosphere has never been realized until now. It is astonishing to see how the ripples caused by man-made explosions can affect the edge of space. 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."
"What we get with these readings is a bit of scientific detective work which allows us a glimpse of not just what was happening on the ground but the effects of the bombing at the edge of space," said Patrick Major, study co-author and University of Reading historian.
This also explains why aircraft dropping bombs in the raids were damaged, even when flying at the recommended altitude to safely avoid shockwaves. But the impact of the shockwaves on the ground was well known.
"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," Major said. "There were even rumors 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. The unprecedented power of these attacks has proved useful for scientists to gauge the impact such events can have hundreds of kilometers above the Earth, in addition to the devastation they caused on the ground."
The results could help scientists to learn more about the events on Earth that impact the ionosphere.
"Because we know the energies involved in these explosions, that gives us a real quantifiable way of assessing how much energy is required to make the ionosphere warmer," and could be applies to the waves generated by earthquakes, volcanoes and even thunderstorms, Scott said. <