The blast that shook the ionosphere

Just after 6 p.m. local time (15.00 UTC) on August 4, 2020, more than 2,750 tons worth of unsafely stored ammonium nitrate exploded in Lebanon's port city of Beirut, killing around 200 people, making more than 300,000 temporarily homeless, and leaving a 140-metre-diameter crater in its wake. The blast is considered one of the most powerful non-nuclear, human-made explosions in human history.

Now, calculations by Hokkaido University scientists in Japan have found that the atmospheric wave from the blast led to electron disturbances high in Earth's upper atmosphere. They published their findings in the journal Scientific Reports.

The team of scientists, which included colleagues from the National Institute of Technology Rourkela in India, calculated changes in total electron content in Earth's ionosphere: the part of the atmosphere from around 50 to 965 kilometres in altitude. Natural events like extreme ultraviolet radiation and geomagnetic storms, and human-made activities like nuclear tests, can cause disturbances to the ionosphere's electron content.

"We found that the blast generated a wave that travelled in the ionosphere in a southwards direction at a velocity of around 0.8 kilometres per second," says Hokkaido University Earth and Planetary scientist Kosuke Heki. This is similar to the speed of sound waves travelling through the ionosphere.

The team calculated changes in ionospheric electron content by looking at differences in delays experienced by microwave signals transmitted by GPS satellites to their ground stations. Changes in electron content affect these signals as they pass through the ionosphere and must be regularly taken into consideration to accurately measure GPS positions.

The scientists also compared the magnitude of the ionospheric wave generated by the Beirut blast to similar waves following natural and anthropogenic events. They found that the wave generated by the Beirut blast was slightly larger than a wave generated by the 2004 eruption of Asama Volcano in central Japan, and comparable to ones that followed other recent eruptions on Japanese islands.

The energy of the ionospheric wave generated by the Beirut blast was significantly larger than a more energetic explosion in a Wyoming coal mine in the USA in 1996. The Beirut blast was equivalent to an explosion of 1.1 kilotons of TNT, while the Wyoming explosion was equivalent to 1.5 kilotons of TNT. The total electron content disturbance of the Wyoming explosion was only 1/10 of that caused by the Beirut blast. The scientists believe this was partially due to the Wyoming mine being located in a somewhat protected pit.