Even though the “Great American Eclipse” is now many months behind us, we are still learning new things about how the passage of the Moon’s shadow affected our atmosphere.
Earth scientists are particularly interested in studying the electrified layers situated 80 to 1,000 km (50 to 600 miles) above the ground, known as the ionosphere. It experienced a drastic shock as the Moon’s umbra came along and briefly shut off its one source of energy: the Sun.
Ultraviolet light from the Sun is what breaks apart molecules in the outermost layer of Earth’s atmosphere. Even though these molecules are few and far between at such high altitudes, when they’re ionized they have measurable effects, such as bouncing radio waves back to Earth. (We wouldn’t have radio reception if not for the ionosphere.) Yet during nighttime, without the Sun’s energy input, some layers of the ionosphere disappear altogether.
During last summer’s solar spectacular, for the very first time, the effect of the change in sunlight, in the form of ionospheric bow waves, was observed. This phenomenon had long been suspected but never actually observed.
While total solar eclipses are far from unique celestial events — occurring on average about once every 18 months somewhere on Earth — last August’s coast-to-coast totality path across the contiguous U.S. was the first such circumstance since 1918. It was this unusual shadow trajectory that allowed researchers at MIT's Haystack Observatory and the University of Tromsø, Norway, to make definitive observations of eclipse-induced bow waves.
The teams made use of the U.S.-owned Global Positioning System (GPS), a constellation of satellites that can locate a receiver anywhere in the world — as well as provide accurate, high-resolution data on the total electron content of the ionosphere. Last August, during totality’s 4,000-kilometer, 91-minute trek across 14 states, researchers studied ionospheric electron content data collected by a vast network of more than 2,000 of extremely sensitive receivers in place across the nation.
The eclipse generated clear ionospheric bow waves in electron content disturbances resulting from totality, observed most clearly over the central and eastern U.S.
Bow waves are observed whenever an object shoots through a medium more quickly than waves in that medium can travel. A speedboat, for example, will build up water along its bow that moves more quickly than waves within the water can travel.
Likewise, when a jet flies, it builds up invisible pressure waves in front of it. As the jet flies faster and faster, the pressure waves can’t get out of the way of each other. Eventually, when the jet reaches supersonic speeds, the waves compress together into a single bow wave. All those in a narrow path below the jet’s flight path will be able to hear the sonic boom as it passes overhead.
During the solar eclipse, it was the Moon’s shadow that moved at supersonic speeds. In an article published last December in the journal Geophysical Research Letters, Haystack’s Shunrong Zhang and his colleagues wrote:
“The eclipse shadow has a supersonic motion which [generates] atmospheric bow waves, similar to a fast-moving river boat, with waves starting in the lower atmosphere and propagating into the ionosphere. [Such] study of wave characteristics reveals complex interconnections between the Sun, Moon, and Earth's neutral atmosphere and ionosphere.”
No doubt scientists are already looking forward to repeating this fascinating experiment at the next total solar eclipse that will pass over the United States on April 8, 2024. While the Moon’s shadow will not go coast-to-coast, its 3,400-km- long path, nearly 80 km wider than 2017’s path of totality, will pass over the central and eastern U.S., from Texas to Maine.