IN AUGUST 2017, scientists sailed a boat off South Carolina equipped with a weather balloon. The plan was to float it above the clouds for a guaranteed view of an impending total solar eclipse. Then, a terrible storm struck. “They were mostly trying to keep the boat from capsizing,” says , a physicist at Montana State University who leads the .
The team behind this project had launched 55 balloons across the US in total. As these popped and parachuted back to Earth, many got caught in trees. It took weeks to get them back. “This time,” says Des Jardins, “we’re giving everyone a special tree pole.”
After a six-year wait, the next total solar eclipse over the US is almost here. First comes a practice run. On 14 October, an annular solar eclipse will see almost all of the sun blocked by the moon, leaving just a “ring of fire”. Then, on 8 April 2024, the real deal arrives – a total eclipse visible over a narrow strip of North America.
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The latter offers a chance to see part of the sun usually hidden from view: its wispy, mysterious outer atmosphere, known as the corona. This is the birthplace of the solar wind that travels through our patch of space, sometimes causing aurorae and disrupting satellites. But we understand very little about it. The coming eclipses offer a unique, if fleeting, opportunity to study it. Over the past few years, researchers have been diligently preparing so that, when the time comes, they are ready. That time is now.
It is a quirk of our solar system that the moon and the sun can appear to be the exact same size in the sky. On average, the moon is 400 times closer than the sun while also being 400 times smaller. This cosmic coincidence is what gives us the phenomenon of a solar eclipse, when light from the sun is blocked by the moon.
Total solar eclipses
Solar eclipses come in many forms. They can be partial, where only some of the sun is blocked: an annular eclipse is a special kind of partial eclipse. Or they can be total, in which the moon fully covers the sun’s disc blocking its light in its entirety from our point of view. These are the most dramatic to watch… and the most fruitful for research.
For more than 150 years, these events have provided opportunities for scientific discovery. It was while viewing a total solar eclipse in Guntur, India, in 1868 that astronomer Pierre-Jules Janssen noticed a bright yellow light that had to be coming from an element not yet identified on Earth. It was ultimately named helium, after Helios, the Greek god of the sun.
But perhaps the most famous total solar eclipse came in 1919, when astronomers Arthur Eddington and Frank Watson Dyson went to Sobral in Brazil and to Príncipe Island in the then Portuguese colony of São Tomé and Príncipe just off the coast of Gabon. They studied the positions of stars and planets that became visible in the day during the fleeting time that the moon was blocking the sun’s light, confirming that they appeared to shift in the sky. This was the first evidence of gravitational lensing, when large bodies bend light from distant objects, and confirmation of Albert Einstein’s general theory of relativity, which was published just a few years earlier.
A century later, during a total eclipse in 2019, gravity once again took centre stage. This time, researchers were hunting for gravity waves – not to be confused with gravitational waves, which are ripples in space-time. “Gravity waves are perturbations in the atmosphere generated by mountain ranges and by day and night temperature differences,” says Des Jardins.
It was first suggested in the 1970s that the cold, dark shadow of a total solar eclipse would generate visible gravity waves high up in the atmosphere, she says. But this had never been observed in the stratosphere. That changed with the 2019 event, when a . Unfortunately, the researchers didn’t manage to get any images – but that is something Des Jardins is hoping for with the upcoming eclipses. “There’s definitely a chance we’ll be able to see gravity waves,” she says. “We’re hoping the balloons will bob along the gravity waves at 90,000 feet [27,432 metres].”
Expectations are sky high for the April eclipse because totality – when the sun is entirely covered – will last up to 4 minutes and 27 seconds – the longest such period on land for over a decade. “The colder, the darker and the longer the change, the higher chance there is of gravity waves,” says Des Jardins.
But the main reason why solar scientists will be studying April’s event is to view the strangest part of the sun: the corona. Seen fleetingly as a bright halo that appears only during totality, it is a million times dimmer than the rest of the sun in visible light. The corona is also a million degrees hotter than the sun’s surface (the photosphere – which reaches only about 6000°C) and it extends millions of kilometres into the solar system.
The corona is where the sun’s magnetic fields act on charged particles to form complex shapes, known as streamers, loops and plumes, among other names. Understanding the corona will help us predict the solar wind, the stream of charged particles hurled from the sun into space. This is what causes aurorae, but it is also a potential threat to astronauts, satellites and electricity grids.
Eclipse chasers
, a solar researcher at the University of Hawaii Institute for Astronomy, has been chasing solar eclipses for almost 30 years, using special filters and cameras to measure the temperatures of the particles from the innermost part of the corona. It isn’t always glamorous. In 1995, for example, armed with one of the first digital cameras, Habbal and her team arrived at what seemed a hospitable site near Jaipur, India, to view a 42-second eclipse. “We stayed in elaborate, decorated tents each with toilets and sinks,” she says. “But in the morning we discovered that the sewage was right outside the tent.”
Since then, Habbal’s group, now known as the , has travelled to places as far afield as the Marshall Islands, Kenya, Mongolia, the Norwegian archipelago of Svalbard, Antarctica and Libya. At each eclipse, some of which last just a few seconds, Habbal and her team image the corona using their filters. Studying the different wavelengths of light emitted by charged iron particles in the corona lets them tease out temperatures.
Most of the time, solar physicists studying the corona rely on coronagraphs on space-based observatories, which use a disc on a telescope to block the sun. But these devices cover up the innermost part of the corona, the source of towers of plasma called prominences and eruptions called coronal mass ejections.
“Observations during totality are critical,” says Habbal. There is no other way to see the part of the sun’s atmosphere that extends from its surface out to at least five solar radii in a continuous manner. “That’s fundamental to understanding how the solar atmosphere starts at the sun and then extends into interplanetary space,” she says. Only then can accurate computer models be devised that simulate the corona and help in the prediction of space weather.
In the past couple of years, Habbal’s group has made an astonishing discovery. Right now, the sun is heading towards solar maximum in 2025, the most active point in its 11-year cycle, when the solar wind intensifies. Since the corona looks much larger during total solar eclipses at solar maximum, it was thought that the solar cycle and the temperature of the corona are inextricably linked. But it might not be so simple. In 2021, Habbal and her colleagues published research from observations taken during 14 total solar eclipses that suggests . The lines of the sun’s magnetic field can be open, travelling outwards with the solar wind, or closed, which are hotter and form loops. “We found open field lines everywhere regardless of the cycle,” says Habbal. This means the corona has a roughly constant temperature.
Bad weather has prevented observations since 2019. “We had rain in Chile in 2020, clouds at sea in Antarctica in 2021 and there was no eclipse in 2022,” says Habbal. It was during the expedition to Antarctica that team member suggested that next time they could fly a kite equipped with a spectrometer, which separates light into its component wavelengths.
The NASA-funded kite, which has a 6.5-metre wingspan, was successfully tested in Western Australia during a total solar eclipse in April this year. It was launched on a kilometre-long tether attached to a vehicle. “It was pretty miraculous,” says Habbal. Bad weather meant that the team flew it for the first time just 45 minutes before totality. “It was thrilling.”
If the technology works well at the US eclipse next April, the kite will be deployed more in future, probably with cameras added. “It’s much easier and cheaper than using balloons,” says Habbal. But if it doesn’t work, there is always a back-up.
The Solar Wind Sherpas are also planning to observe the eclipse remotely from more than 18 kilometres above the Pacific coast of Mexico, clear of any clouds, thanks to two of NASA’s high-altitude research aircraft. During the total eclipse, two WB-57 planes will follow each other at 740 kilometres per hour, about a quarter of the speed of the moon’s shadow, just south-west of the maximum point of the eclipse. At that speed, totality increases from the 4 minutes 27 seconds for those viewing it from the ground to over 6 minutes. “The WB-57 is perfect for this because in its nose cone is a camera and telescope system that can rotate to point at anything… no matter which way the aircraft is flying,” says , who is in charge of an experiment in the second WB-57 to study the corona in a different way.
Using a stabilised platform, Caspi and his team will capture images of the eclipse using both a visible light camera and a higher-resolution mid-infrared camera developed by NASA. The latter will capture seven different wavelengths of light and help determine which structures in the corona emit their own light and which merely scatter light from the sun’s surface. “We need to be above as much of the atmosphere as we can get to make those observations,” says Caspi. Infrared light is absorbed by Earth’s atmosphere and is hard to observe from ground level.
Caspi is also part of the project, an attempt to make a continuous 60-minute high-resolution movie using 35 teams of citizen scientists in the path of totality, from Texas to Maine, each with exactly the same cameras, telescopes and training so they can make exactly the same kinds of observations. “The teams will be spaced out so that every station is overlapped by its neighbours,” says Caspi. “If one station doesn’t get data, because of clouds or broken equipment, it’s OK.”
He is hopeful the equipment will work, since it was successfully tested earlier this year in Western Australia. “That was the first eclipse I’ve seen,” says Caspi, who only got to see a few brief seconds because he was busy live streaming it on YouTube. “Our equipment couldn’t get online so I spent the whole time holding my phone in front of my face.”
The movie will hopefully allow scientists to study the corona’s complexities, notably its shape and how it changes over a short time. It builds on a CATE project from 2017, which used 68 cameras throughout the path. This time, it will use more sophisticated cameras that are sensitive to different types of polarised light.
“Most of the light that you see during totality is actually light from the surface of the sun that goes up into the corona to scatter off electrons,” says Caspi. This is the K corona, the bright inner part, which overwhelms the light coming only from the corona itself. As the light scatters, it becomes angled, a property called polarisation. “If you can measure the angle of polarisation, then that gives you a 3D structure of the corona, its density and how that changes over time,” he says.
Time is in short supply during a total solar eclipse, so a continuous hour-long video makes it possible to capture processes that take seconds or minutes, like a solar flare or coronal mass ejection, as well as other details. “The corona is permeated by a complicated magnetic field,” says Caspi. “During totality, we don’t see the magnetic field, but instead the hot plasma trapped along it – just like being able to see iron filings around a magnetic field around a magnet.”
Meanwhile, the researchers behind the original CATE project in 2017 are now working on another broadcast. The aim of the Dynamic Eclipse Broadcast (DEB) is to produce a live continuous broadcast of the solar corona. “This time we’re using smaller cameras and telescopes with a wider field of view to image more of the corona,” says
And it isn’t just the sun they are interested in. During next week’s annular eclipse, 48 DEB teams will be studying the shape of mountains on the moon, by measuring how light shines through the peaks as the edge of the moon passes across the sun’s disk. As part of this, 20 teams of girls aged 11 to 13 will be equipped with telescopes and trained how to use them. Even if it is cloudy, they will get to keep the telescopes after.
Still, next week, and again in April, scientists across the US will be hoping for a clear view. Never before has there been a chance to compare what happens during two different kinds of eclipses, one with a deep moon shadow and one where a very large portion of the sun is obscured. “Nature has handed us a wonderful set of eclipses six months apart, in our own country,” says Des Jardins. “No one’s ever done anything like this.”
