The ambitious plans to study the sun during April’s solar eclipse

Across North America, solar scientists will be studying April’s total solar eclipse 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, or 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.

Expectations are sky high for the total solar eclipse on 8 April 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. Here are a few of the experiments that will be taking place.

The solar wind sherpas

Shadia Habbal, a solar researcher at the University of Hawaiʻi 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.

Habbal’s group, now known as the Solar Wind Sherpas, 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 from 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 5 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 corona’s temperature isn’t dependent on the solar cycle. 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.

The high fliers

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 Benedikt Justen 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 2023. 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 upcoming eclipse, 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 backup.

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 Amir Caspi at the Southwest Research Institute in Boulder, Colorado, 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.

The live streamers

Caspi is also part of the Citizen Continental-America Telescopic Eclipse (CATE) 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 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 last 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.”

Solar Eclipse 2024

On 8 April a total solar eclipse will pass over Mexico, the US and Canada. Our special series is covering everything you need to know, from how and when to see it to some of the weirdest eclipse experiences in history.

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.”

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