Like a boat sailing through water, the moon’s shadow created rippling waves in the upper layers of Earth’s atmosphere during the momentous total solar eclipse of 2017.
On August 21, the total eclipse raced across the continental United States, the first such eclipse in nearly a century to traverse the country from coast to coast. While a total solar eclipse happens somewhere on Earth every year or two, on average, this event was rare because it fell over so much populated land.
That meant scores of professional and amateur sky-watchers were able to train their instruments on the August eclipse, hoping to study one of the many amazing sights that can only be witnessed during a solar eclipse. That included researchers from MIT's Haystack Observatory and the University of Tromsø in Norway, who were hoping to find evidence for something called the bow-wave phenomenon.
Named after the V-shaped ripples that emanate from the bow of a ship as it moves in water, the bow-wave theory says that a total solar eclipse will create pockets of high-pressure air under the moon’s shadow that then plow through low-pressure air as the shadow zooms across the planet. These air pockets, in turn, should generate small but detectable bow waves in the atmosphere.
The theory emerged in the 1970s, but scientists had trouble confidently documenting it—until now. Thanks to the 2017 eclipse, the MIT-Tromsø team reports that they have the "first unambiguous evidence" of those weak waves, which they recently published in the journal Geophysical Research Letters.
During the August eclipse, the team collected data from about 2,000 satellite receivers placed throughout North America. They detected the tiny bow waves in Earth’s ionosphere, an upper region that starts about 37 miles above the surface. Atoms in this zone are electrically charged due to solar and cosmic radiation.
Though the effects of eclipses on the upper atmosphere have been studied for a few decades, this was the first time there was unprecedented satellite coverage that could capture "amazing details of the ionospheric bow waves," says lead author Shun-Rong Zhang of MIT.
The new study "indicates that only certain types of ionospheric waves are associated with the eclipse," he added, noting that previous studies of the phenomenon need to be revisited. "To quantitatively understand our observations, more sophisticated modeling work is highly needed," he says.
The charged particles in the ionosphere reflect radio waves, supporting our communications systems. But Zhang told Gizmodo that the bow waves from the August eclipse weren't intense enough to have an effect on those systems or the electrical grid.
Instead, the study authors write, the importance of the research is that it "reveals complex interconnections between the sun, moon, and Earth's neutral atmosphere and ionosphere," a relationship scientists are still working to understand.
U.S. scientists hoping to confirm the discovery and continue unraveling the mysteries of the ionosphere will have their next chance in April 2024, when another total solar eclipse will sweep from Texas to New England.