Moon’s Shadow Ruffles Ionosphere During Last Year’s Solar Eclipse

Row, row row your boat. The waves look just like a wake and follow the moon’s shadow from the central U.S. eastward. The red line is the time of local noon. MIT

The 2017 total solar eclipse drew so many of us together in a single, grand cosmic moment. While we took in the sight, took pictures or both, scientists from MIT’s Haystack Observatory in Westford, Mass. were busy investigating the effects of the moon’s middle-of-the-day shadow on the Earth’s atmosphere.

Our atmosphere is made of layers like lasagna. Clouds and weather happen in the lowest part called the troposphere. Air in the ionosphere is extremely thin and gradually grades into outer space. NASA

Earth’s upper atmosphere is interlocked with the sun’s activity. Events like solar flares or coronal mass ejections — instigators of the northern lights — can have serious effects on radio signal propagation and satellite communications which then affect navigation and GPS satellite services. We’re basically talking about the ionosphere, a region of the atmosphere that extends from 37 mi (60 km) to 620 miles (1,000 km) altitude.

It gets its name from “ion.” An ion is an atom that’s gained  or lost an electron. Instead of being neutral it now has a charge. Solar UV light and cosmic rays strike atoms in the ionosphere, prying their electrons free, ionizing them. By studying eclipses’ effects on the electron content of the upper atmosphere, scientists are learning more about how the ionosphere works. Eclipse shadows in particular can send ripples through the atmosphere they travel across the planet.

A barge creates a bow wave as it moves through a canal. The moon’s shadow did something very similar to the air in the ionosphere during last year’s total solar eclipse. Arnold Paul / CC BY-SA 2.5

One kind of wave, known as an ionospheric bow wave, has been predicted for more than 40 years to exist in the wake of an eclipse passage. Researchers at MIT’s Haystack Observatory and the University of Tromsø in Norway confirmed their existence definitively for the first time during the August 2017 eclipse. The team studied how where and how many electrons were bouncing around up there from data collected by a network of more than 2,000 Global Navigation Satellite System receivers spread across the nation in touch with multiple orbiting satellites.

Frame grab from the video showing the ionospheric wake created by the moon’s shadow. You can read more about the study in this article. Haystack Observatory / MIT

The observed ionospheric bow waves are a lot like those formed by a ship as it slices through water only in this case, the waves are caused by the sudden change in temperatures as the moon’s shadow travels quickly across a slice of the atmosphere. That shadow moved fast, averaging easily over 1,000 mph or faster than the speed of sound. The air in its path cooled rapidly and then heated back up just as quickly. Those who saw the eclipse won’t soon forget how quickly the temperature dropped in the minutes before totality.

The team found that the waves started in the lower atmosphere and worked their way up to the thin air in the ionosphere mostly over the central and eastern U.S. What does it all mean? That we’re more connected than we knew to the habits of the moon and sun even when it comes to seemingly small things like a passing shadow.

4 Responses

    1. astrobob

      Hi Jake – yes, it is. It’s the local noon meridian (Daylight Saving Time noon or ~ 1 p.m. local time).

      1. Jake Straw-walker

        Thanks. It is a rather amazing use of GPS. I imagine there was no small amount of computing power to wean out of all those receivers the distortions due to the eclipse. By the way, I was in Fairmont, Nebraska for the show.

        1. astrobob

          Hi Jake,

          Yes, I think of the computing that had to be done to combine all the data into a moving signal. I was in Nebraska, too — about 10 miles south of Alliance.

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