The Solar Eclipse Caused a Bow Wave in Earth’s Atmosphere

This graphic shows atmospheric bow waves forming during the August 2017 eclipse over the continental United States. Image: Shunrong Zhang/Haystack Observatory

It’s long been predicted that a solar eclipse would cause a bow wave in Earth’s ionosphere. The August 2017 eclipse—called the “Great American Eclipse” because it crossed the continental US— gave scientists a chance to test that prediction. Scientists at MIT’s Haystack Observatory used more than 2,000 GNSS (Global Navigation Satellite System) receivers across the continental US to observe this type of bow wave for the first time.

The Great American Eclipse took 90 minutes to cross the US, with totality lasting only a few minutes at any location. As the Moon’s shadow moved across the US at supersonic speeds, it created a rapid temperature drop. After moving on, the temperature rose again. This rapid heating and cooling is what caused the ionospheric bow wave.

The bow wave itself is made up of fluctuations in the electron content of the ionosphere. The GNSS receivers collect very accurate data on the TEC (Total Electron Content) of the ionosphere. This animation shows the bow wave of electron content moving across the US.

The details of this bow wave were published in a paper by Shun-Rong Zhang and colleagues at MIT’s Haystack Observatory, and colleagues at the University of Tromso in Norway. In their paper, they explain it like this: “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. Eclipse passage generated clear ionospheric bow waves in electron content disturbances emanating from totality primarily over central/eastern United States. Study of wave characteristics reveals complex interconnections between the sun, moon, and Earth’s neutral atmosphere and ionosphere.”

The ionosphere stretches from about 50 km to 1000 km in altitude during the day. It swells as radiation from the Sun reaches Earth, and subsides at night. Its size is always fluctuating during the day. It’s called the ionosphere because it’s the region where charged particles created by solar radiation reside. The ionosphere is also where auroras occur. But more importantly, it’s where radio waves propagate.

The ionosphere surrounds the Earth, extending from about 80 km to 650 km. Image Credit:  NASA's Goddard Space Flight Center/Duberstein
The ionosphere surrounds the Earth, extending from about 80 km to 650 km. Image Credit: NASA’s Goddard Space Flight Center/Duberstein

The ionosphere plays an important role in the modern world. It allows radio waves to travel over the horizon, and also affects satellite communications. This image shows some of the complex ways our communications systems interact with the ionosphere.

This graphic shows some of the effects that the ionosphere has on communications. Image: National Institute of Information and Communications Technology
This graphic shows some of the effects that the ionosphere has on communications. Image: National Institute of Information and Communications Technology

There’s a lot going on in the ionosphere. There are different types of waves and disturbances besides the bow wave. A better understanding of the ionosphere is important in our modern world, and the August eclipse gave scientists a chance not only to observe the bow wave, but also to study the ionosphere in greater detail.

The GNSS data used to observe the bow wave was key in another study as well. This one was also published in the journal Geophysical Research Letters, and was led by Anthea Coster of the Haystack Observatory. The data from the network of GNSS was used to detect the Total Electron Content (TEC) and the differential TEC. They then analyzed that data for a couple things during the passage of the eclipse: the latitudinal and longitudinal response of the TEC, and the presence of any Travelling Ionospheric Disturbances (TID) to the TEC.

Predictions showed a 35% reduction in TEC, but the team was surprised to find a reduction of up to 60%. They were also surprised to find structures of increased TEC over the Rocky Mountains, though that was never predicted. These structures are probably linked to atmospheric waves created in the lower atmosphere by the Rocky Mountains during the solar eclipse, but their exact nature needs to be investigated.

This image of GNSS data shows the positive Travelling Ionospheric Disturbance (TID) structure in the center of the primary TEC depleted region. The triangles mark cities in or near the Rocky Mountains. Image: Coster et. al.
This image of GNSS data shows the positive Travelling Ionospheric Disturbance (TID) structure in the center of the primary TEC depleted region. The triangles mark cities in or near the Rocky Mountains. Image: Coster et. al.

“… a giant active celestial experiment provided by the sun and moon.” – Phil Erickson, assistant director at Haystack Observatory.

“Since the first days of radio communications more than 100 years ago, eclipses have been known to have large and sometimes unanticipated effects on the ionized part of Earth’s atmosphere and the signals that pass through it,” says Phil Erickson, assistant director at Haystack and lead for the atmospheric and geospace sciences group. “These new results from Haystack-led studies are an excellent example of how much still remains to be learned about our atmosphere and its complex interactions through observing one of nature’s most spectacular sights — a giant active celestial experiment provided by the sun and moon. The power of modern observing methods, including radio remote sensors distributed widely across the United States, was key to revealing these new and fascinating features.”

The Great American Eclipse has come and gone, but the detailed data gathered during that 90 minute “celestial experiment” will be examined by scientists for some time.

Tales From Totality: Standing in the Shadow of the Moon

A brilliant diamond ring punctuates totality. Image credit and copyright: Shahrin Ahmad.

They came, they saw, they battled clouds, traffic and strange charger adapters in a strange land. Yesterday, millions stood in awe as the shadow of the Moon rolled over the contiguous United States for the first time in a century. If you’re like us, your social media feed is now brimming with amazing images of yesterday’s total solar eclipse.

Already, we’ve seen some amazing reader images at Universe Today, with more to come. As a special look at a unique event, we’ve collected reader testimonies from every state along the path of totality of just what the eclipse was like.

Enjoy!

Oregon- Shahrin Ahmad (@Shahgazer)

We drove from Dalles at 3 AM. Nearing the observation spot, we got a flat tire! It was 5:30 AM, and no phone line! I sent a text to the land owner and somehow it reached him and we managed to be there by 6:30 AM. We observed from a secluded spot about 30 miles from Madras, with a 2 minutes and 2 seconds of totality. The sky was really clear during sunrise, but as totality approached we got some thin clouds hovering in the east. Luckily, it was thin enough to not spoil anything. The corona was incredibly beautiful with longer (streamers) jutting out at the 4 and 8 o’clock position. The first and second diamond ring were spectacular with the eye, probably with the help with the thin clouds. We calculated about 7 degree drop in temperature. The shadow was enormous, engulfing Mt Hood from the west and flew past above us towards and towards the Sun. Mesmerizing! 2 minutes simply was not enough, since this is probably my best view of a total solar eclipse so far!

The bright star Regulus, tangled up in the solar corona. Image credit and copyright: Shahrin Ahmad.

(Note: to our knowledge, no one witnessed the brief moments of totality as the umbra of the Moon brushed tiny corners of Montana and Iowa… if you’re reading this and did so, let us know!)

Idaho- Bruce McCurdy (@BruceMcCurdy)

How to describe such a magnificent spectacle in a “brief paragraph”? Our group from Edmonton observed totality under clear skies near Birch Creek, Idaho. After the Moon’s silhouette inexorably progressed & gradually swallowed up an impressive line of sunspots, the pace of dynamic events picked up dramatically in the minutes surrounding totality. The temperature dropped noticeably. Light faded & became “flat” while shadows became better defined & lost their fuzzy edges (penumbrae). The Moon’s onrushing shadow became visible on the mountains to our west, while rapidly-moving shadow bands squiggled on the ground around us. The sky took on an eerie indigo hue as the last vestiges of direct sunlight were obscured. A new & temporary centrepiece emerged in the sky: the black circle of the lunar night side highlighted by a spectacular corona, its far-flung pearly-white streamers contained within sharply defined edges. Around the black limb fiery coral pink prominences added intense colour highlights to the scene. Just beyond the corona gleamed Regulus, closer to the Sun than is possible for any other star of first magnitude or brighter, while off to one side Venus shone brilliantly, far higher in the sky than its customary window of dominance in normal twilight. All too soon the right edge of the lunar silhouette brightened, then blossomed in a brilliant diamond ring that continued to intensify for a couple of glorious seconds until filters again became a must. By now the mountains to our east were in darkness as the umbral shadow receded from our immediate location, leaving a number of our small party in tears from the intensity of the experience.

Wyoming- Kelly Kizer Whitt (@Astronomommy)

We woke up in the Tetons Monday morning to a sky streaked with clouds. But the hourly weather report showed clearing, so we headed to our spot before 7 AM. We were able to secure parking by our preferred observing location, the Mormon Barn with a view of the iconic Teton range in the background. Looking east, we saw the clouds slink away to the south until skies were blue and clear, despite lingering haze and smoke on the northern horizon from wildfires.

Crescent Suns along with the Tetons. image credit and copyright: Kelly Kizer Whitt.

Having been a science writer for two decades, I was well versed on total solar eclipses even though I’d never seen one first hand. But it didn’t unfold quite as I expected. The sky and air didn’t take on a twilight quality until the Sun was well over halfway obscured. Then when darkness fell, it came fast and the temperature dropped hard. We had on our eclipse glasses and were staring at the Sun, waiting to see bailey’s beads or the diamond ring. But first I glanced down and saw the slithering, wiggling lines of darkness and light known as the shadow bands. They have a truly creepy quality as they dance in the growing dark. Then we looked back up as the sliver of orange disappeared and the Sun winked out from our glasses. Pulling them off, my family let out cries of surprise when they saw the black hole where the Sun had been, surrounded by the long, wispy, intricate corona. The eclipsed Sun and corona took up a much larger space in the sky than I expected, but the photo I took (just like when photographing a full moon) does not give a true representation of what you can see with your eyes.

I only took three photos because I wanted to just enjoy the view. I almost forgot to look for the stars. We saw a plane, Venus, and Sirius. Our eyes never adjusted enough to spot Jupiter or the others and the rosy glow of a false twilight brightened all horizons in a 360-degree ring. So soon it was over. The bailey’s beads and diamond ring we missed as the total eclipse began, and appeared to us instead at the end. These phenomena were a bright and beautiful warning to get our eclipse glasses back on. The world returned to daylight fairly quickly, but the drop in temperature lingered a bit longer. Our memories will last a lifetime.

Nebraska- (@BigBadEd)

Having doubtful cloud forecasts for Scottsbluff & Carhenge,  we met on a foggy morning in Sidney, Nebraska with thoughts of changing plans to Wyoming for clear skies. As the forecast improved,  15 of us set off for Carhenge.  We arrived before 7 AM to plentiful parking & a few hundred people. Towards 9 AM the crowds started to swell, including aliens, welders and the governor of Nebraska. Joined by more people & dogs, I estimate around 3,000 people were at the site. Some clouds went by at mid-coverage, casting a spectacular crescent. Clouds cleared, and cheers rose as we went into totality,  such a beautiful sight some were moved to tears as the diamond ring emerged. A thoroughly wonderful experience shared with friends and spellbound crowd, definitely worth the trip from Florida.

Kansas- Michelle Tevis (@MichelleKTevis)

I saw it (the eclipse) from Weston, Missouri, just northwest of the Kansas-Missouri line. Clouds and rain obscured the sun for most of the eclipse, but the rain subsided during totality and allowed us to get outside for the quick move into darkness. Even though we couldn’t see the eclipse or corona, the atmosphere took on a different feel. There was a change in how things were colored — as if you were looking through darker and darker polarized glasses, and the silence took on a feeling, like a deep vibration.

Missouri- Jeudy Blanco (@Jeudyx)

Totality from Missouri. Image credit and copyright: Jeudy Blanco.

It was amazing. We changed plans last night, instead of going to St Joseph we drove to Columbia. I was really worried the first few minutes of the eclipse because it was cloudy, my PST couldn’t resolve the image of the Sun! But quickly the clouds dispersed. We were on a property from the family of my friend, around 25 people of all ages. When it was around 70% (partial) you could feel in the environment that something was going on. Everything got a lot more quiet and the temperature dropped. Everybody was trying to get pictures of the Sun with their phones on the PST. Then totality started, it was indescribable for me. I was seeing the Sun’s corona with my bare eyes. I was really nervous and anxious, actually. We could see Venus near the Sun. Everybody was super excited, I almost cried. The experience was amazing, a total success, the long trip was worth it.

Illinois- The Universe Today expedition to the Prairie State led by Publisher Fraser Cain also managed to catch a brief glimpse of totality through a gap in the clouds:

Kentucky- Mike Weasner (@Mweasner)

Earthshine (!) on the Moon, seen during totality. Image credit and copyright: Mike Weasner.

About 400 eclipse enthusiasts from around the world including me were part of a Sky and Telescope tour group. We were at Hopkinsville Community College located in Hopkinsville, Kentucky, where totality lasted 2 minutes and 40 seconds, which was too short. We arrived at the viewing site about 4.5 hours before First Contact. Traffic was surprisingly light. There were a few thin clouds but nothing significant. Anticipation was high. Many of us set up cameras and were ready well before First Contact. First Contact occurred with a clear sky, and the sky stayed mostly clear until about 30 minutes before Second Contact. Then a large cloud covered the Sun. Fortunately the cloud moved on within a couple of minutes and the sky was mostly clear through Fourth Contact. Totality was beautiful. Most people saw Venus, some saw Jupiter too, but no one seems to have seen any stars although it did get dark at the site. Many people in the group left soon after totality ended, but I and several others stayed to view and photograph the eclipse through Fourth Contact. 

Tennessee- (Terry Horne @CapH_1)

My wife and I viewed the event from Sheep Barn Ridge, which is a few miles from Kingston, TN. We began the planning in late 2015 when we realized the shadow path was adjacent to our property near my folks in TN. Our location delivered 2 minutes and 29 seconds of totality, with clear skies, a valley pasture view among new friends, goats, llama, ducks, chickens and a few hounds.

An amazing expample of the “Diamond Ring” effect. Image credit and copyright: Phyllis Horne @sahgma

We experienced every awe & oddity we had studied during the ramp up to the event. My wife did an excellent job with her photo efforts. She balanced her personal viewing time and planned equipment duties well. This was a source of much worry and discussion during the months prior.

I’ll mention a few surprises. I was impressed by the amount of light cast on the landscape with barely a sliver of the Sun remaining. I suspect the ambient sunlight to the south east was the major source. The rapid transition to peak darkness was dramatic.

In contrast, I noticed a clear reduction of heat radiation on my skin with about 50% coverage. It was a hot day. I wished I’d had more time to observe the animals.

I found it somewhat humorous how many folks took all of the important PSA’s about retina damage to heart. Before totality they bowed their heads to the ground when they did not have their gasses on while walking, standing and sitting.

What I learned most was, to the inexperienced, East Tennessee Moonshine travels faster than the Moon’s shadow.

Be careful!

Georgia- Jeannette Iriye (@i_fridrich)

We found a lovely scenic overlook facing west in Sky Valley, just outside Dillard, Georgia. Skies were clear with only minimal cloud cover until about 13:30, when heavy cloud cover began to build in the south/southeast. The clouds obfuscated the remainder of our view of the eclipse directly. It did get much cooler, windy, and the crickets were singing just prior to and during totality.

A partially eclipsed Sun versus clouds. Image credit and copyright: Jeannette Iriye.

South Carolina- Terri (@wizbee1)

We didn’t make it to South Carolina, and had to turn the plane back because of weather. Watched instead from Saint Mary’s Georgia. Did feel the temperature drop and experienced darkening but not in totality.

And us? We watched from the Pisgah Astronomical Research Institute in North Carolina as the shadow of the Moon draped over the landscape. The rolling afternoon clouds afforded only brief glimpses of the partially eclipsed Sun. Then, just prior to totality, we caught the final moments as the Sun withered to a brief diamond ring flash… and was gone. Magic! Unfortunately, the corona remained hidden behind high clouds for the 107 seconds of darkness, though we were treated to an unworldly 360 degree sunset below the cloud deck. Nocturnal mosquitoes, fooled by the false dusk, began their rounds, as a light “eclipse wind” kicked up.

Author and wife (@MyschaTheriault) standing in the shadow of the Moon, plus our view from the Pisgah Astronomical Research Institute (PARI) just before totality. Thanks to @Dayveesutton for snapping the pic!

Then, it was over. Got the eclipse bug? Well, another total solar eclipse crosses the U.S. in 2024… but you don’t have to wait that long, as we’ve got one coming right up crossing Argentina and Chile on July 2nd, 2019…

I’ll see you there!

Prelude to Totality: A Final Look at the Total Solar Eclipse

corona
totality
Totality! The view during the November 2012 total solar eclipse. Image credit and copyright: Sharin Ahmad (@Shahgazer)

It’s hard to believe: we’re now just one short weekend away from the big ticket astronomical event for 2017, as a total solar eclipse is set to cross over the contiguous United States on Monday, August 21st.

Celestial mechanics is a sure thing in this Universe, a certainty along with death and taxes that you can bet on. But there are still a few key question marks leading up to eclipse day, things that we can now finally make intelligent assumptions about a few days out.

Although totality slices through the U.S., partial phases of the eclipse touch on every continent except Antarctica and Australia. Credit: Michael Zeiler/The Great American Eclipse.

First up is solar activity. If you’re like us, you’ll be showing off the Sun in both visible and hydrogen alpha as the Moon begins making its slow hour long creep across the disk of Sol. First, the good news: sunspot active region AR 2671 made its Earthward debut on Tuesday August 15th, and will most likely stick around until eclipse day. The bad news is, it most likely won’t have lots of friends, as solar cycle #24 begins its long slow ebb towards the solar minimum in 2019-2020. Likewise, I wouldn’t expect to see any magnificent sprouting red prominences in the solar chromosphere in the seconds bracketing totality, though we could always be pleasantly surprised.

sunspot
The Earthward face of Sol as of August 17, four days before totality. Sunspot AR 2671 is robust and growing in complexity. Credit: NASA/SDO/HMI

How will the white hot corona appear during totality? This is the signature climax of any total solar eclipse: veteran umbraphiles can actually glance at a photo of totality and tell you which eclipse it was from, just on the shape of the corona. The National Solar Observatory released a model of what that Sun’s magnetosphere was doing one Carrington rotation (27 days) prior to the eclipse on July 25th, a pretty good predictor of the corona might look like during those fleeting moments of totality:

Solar Corona
The shape of the field lines of the solar corona, one rotation prior to the August 21st total solar eclipse. Credit: The National Solar Observatory.

NASA will be chasing the umbra of the Moon with two converted W-57 aircraft during the eclipse, hoping to unlock the “coronal heating paradox,” image Mercury in the infrared, and hunt for elusive Vulcanoid asteroids near the eclipsed Sun.

corona
The view of the corona during totality? This computational model was derived from NASA SDO data during the last solar rotation. Credit: Predictive Science Inc.

The corona is about twice as bright as a Full Moon, and its interface with the solar wind extends out past the Earth. The very onset of totality is like the footstep of a giant passing over the landscape, as the door of reality is suddenly ripped open, revealing the span of the glittering solar system at midday. Keep your eyes peeled for Mercury, Venus, Mars, Jupiter and twinkling Regulus tangled up in the corona, just a degree from the Sun-Moon pair:

The line up of the planets, bright stars and the eclipsed Sun during totality at 2:37 PM EDT as seen from Franklin, North Carolina. Credit: Stellarium.

Also, be sure to scan the local horizon for a strange 360 degree sunset as you stand in the umbra of the Moon. An “eclipse wind” may kick up, as temperatures plummet and nature is fooled by the false dawn, causing chickens to come home to roost and nocturnal animals to awaken. I dare you to blink. Totality can affect the human heart as well, causing tears to cries of surprise.

Here’s an interesting, though remote, possibility. Could a sungrazing “eclipse comet” photo bomb the show? Karl Battams (@SungrazerComets) raises this question on a recent Planetary Society blog post. Battams works with the Solar Heliospheric Observatory (SOHO), which has discovered an amazing 3,358 comets crossing the field of view of its LASCO imagers since 1995. Comets have been discovered during eclipses before, most notably in 1882 and 1948. To be sure, it’s a remote possibility this late in the game, but Battams promises to give us one last quick look via SOHO the morning of the eclipse on his Twitter feed to see if any cometary interlopers are afoot.

The possible search area for Kreutz group sungrazers during the August 21st eclipse. Credit: Karl Battams.

Now, on to the biggest question mark going into this eclipse weekend: what’s the weather going to be like during the eclipse? This is the ever-dominating factor on everyone’s mind leading up to eclipse day. Keep in mind, the partial phases are long; even a partly cloudy sky will afford occasional glimpses of the Sun during the partial phases of an eclipse. Totality, however, is fleeting – 2 minutes and 40 seconds near Hopkinsville, Kentucky and less for most – meaning even a solitary cumulus cloud drifting across the Sun at the wrong moment can spoil the view. No weather model can predict the view of the sky to that refined a level. And while best bets are still out west, lingering forest fires in Oregon are a concern, along early morning fog on the western side of the Cascade Mountains. Michael Zeiler over at The Great American Eclipse has been providing ESRI models of the cloud cover over the eclipse path for Monday… here’s the outlook as of Thursday, August 17th:

A look at cloud cover prospects over the eclipse path as of August 17. Credit: Michael Zeiler/Great American Eclipse/ESRI.

Computer models should begin to come into agreement this weekend, a good sign that we know what the weather is going to do Monday. Needless to say, a deviation from the standard climate models could send lots of folks scrambling down the path at the last minute… I’ve heard of folks with up to 5 (!) separate reservations along the path of totality, no lie…

The NOAA also has a fine site dedicated to weather and cloud coverage across the path come eclipse day, and Skippy Sky is another great resource aimed at sky viewing and cloud cover.

Clouded out? The good folks at the Virtual Telescope have got you covered, with a webcast for the total solar eclipse starting at 17:00 UT/1:00 PM EDT:

Credit: The Virtual Telescope Project.

Of course, you’ll need to use proper solar viewing methods during all partial phases of the eclipse. This means either using a telescope with a filter specifically designed to look at the Sun, a pin hole projector, or certified ISO 12312-2 eclipse glasses. If you’ve got an extra pair, why not convert them into a safe filter for those binoculars or a small telescope as well:

Also be wary of heatstroke, standing out showing folks the partially eclipsed Sun for an hour or more. It is August, and heat exhaustion can come on in a hurry. Be sure you have access to shade and stay cool and hydrated in the summer Sun.

Finally, eyes from space will be watching the eclipse from the International Space Station as well. Looking out at Monday, the ISS will pass through the penumbra of the Moon and see partial phases of the eclipse three times centered on 16:32, 18:20, and 20:00 Universal Time. The center time is particularly intriguing, as astros have a chance to see the dark umbral shadow of the Moon crossing the central U.S. This also means that eclipse viewers on planet Earth around southern Illinois might want to glance northward briefly, to spy the ISS during totality. Also, viewers along a line along southern central Canada will have a chance to catch an ISS transit across the face of the partially eclipsed Sun around the same time. Check CALSky for details.

Three passes of the International Space Station versus the path of of totality. The inset shows the view of the partially eclipsed Sun as seen from the ISS. Credit: NASA/JSC.

We’ll be at the Pisgah Astronomical Research Institute in southwestern North Carolina, for a glorious 104 seconds of totality. We expect to be out of wifi range come eclipse day, but we’ll tweet out key eclipse milestones as @Astroguyz. We also plan on writing up the eclipse experience with state-by-state testimonials post eclipse.

Perhaps, the August 21st total solar eclipse will bring us all together for one brief moment, to witness the grandest of astronomical spectacles. We’re lucky to share a small patch of time and space where total solar eclipses are possible.  Good luck, clear skies, and see you on the other side early next week!