How photos of the April 8 solar eclipse will help us understand the sun’s atmosphere

By | April 2, 2024

There is a gap in our observations of the Sun: part of the atmosphere is effectively invisible to our telescopes. However, images taken from Earth during a solar eclipse fill that gap and provide unprecedented insight into our star’s hidden layer.

But capturing these images requires expertise, special equipment and a lot of patience. . One of the few people who can take it and process it images of solar eclipses so they reveal some of the sun’s best-protected secrets, is the Czech mathematician Miloslav Druckmüller. His breathtaking depictions of the wispy white rays and loops of magnetic lines that make up the solar corona as it emerges from the eclipsed Sun to the human eye are well known in solar physics circles.

Druckmüller spoke to from his office at the Technical University of Brno in the Czech Republic a few days before his departure for Mexico. There, in collaboration with researchers from the University of Hawaii, he will oversee a complex imaging campaign during the total solar eclipse on April 8.

Related: This is how you photograph the total solar eclipse on April 8, 2024

On the day of the eclipse, 66 cameras equipped with special filters and spread across three viewing locations in Mexico and the US will capture tens of thousands of images during the approximately four-minute eclipse. The researchers hope that Druckmüller’s expert image processing from this enormous data set will be able to uncover previously unknown information about the otherwise invisible region of the Sun.

The blind spot

The solar corona is made of extremely thin, charged gas called plasma and is the top part of the solar cell the atmosphere of the sun. It is a million times fainter than the underlying photosphere that forms the Sun’s visible surface, and so is completely eclipsed by the star’s light when viewed in visible light.

To observe the corona, astronomers must darken the visible solar disk so that the faint coronal light can appear against the dark cosmos. To do this, they use an instrument called a coronagraphwhich is equipped with an occulter that blocks the sunlight.

But if the more occult were to cover only the visible Sun, the diffraction – the bending of light around an obstacle – would cause the bright light of the photosphere to flow around the edges of the occult and ruin the photos. Therefore, astronomers use larger occulters that also cover the inner corona up to a distance of one solar radii from the Sun’s surface.

However, because the moon is far away, it does not cause diffraction, even though it is just the right size to perfectly cover the solar disk during a total solar eclipse.

“Any coronagraph that tries to simulate a solar eclipse is terribly inferior to an actual solar eclipse,” Druckmüller said. ‘It’s too close to the telescope and that means it causes diffraction. Therefore, the occult must be much larger than the solar disk, which means we cannot see the inner part of the corona.”

Because of these limitations, scientists know very little about the processes taking place in this hidden area.

“When you take images from space telescopes like the Solar Dynamics Observatory“You can see the surface of the Sun in extreme ultraviolet light,” said Shadia Habbal, professor of solar physics at the University of Hawaii and Druckmüller collaborator, at a conference in Brno in November 2022. “You can see a little piece of the corona . projected against the surface, but that is clearly not sufficient to understand how this plasma expands in space.”

The corona is the source of the solar wind, the constant stream of charged particles spreading across the solar system. Occasionally it erupts in huge eruptions known as coronal mass ejections, which can cause dramatic geomagnetic storms on Earth. Many of the processes that accelerate this solar wind into space take place in the coronagraph’s blind spot: the sun itself.

a black circle in the center emits blinding arrays of white light

a black circle in the center emits blinding arrays of white light

An accidental discovery

In Druckmüller’s solar eclipse images, this hidden area emerges in unprecedented detail. The coronal gas – made largely of ionized hydrogen and helium, with a few heavier elements, such as iron, magnesium, silicon or calcium – orbits the Sun’s magnetic field lines. Elsewhere it flows into space from large sunspots or active regions and flows from coronal holes where the magnetic field lines are interrupted.

Druckmüller first experimented with solar eclipse photography in 1999, when the moon’s shadow visited Central Europe for the first time in 150 years. The Czech Republic was just outside the totality band, and stormy weather threatened to ruin the experience at many of the closest viewing locations. Druckmüller left for Hungary, which provided “a perfectly clear sky”.

Druckmüller, an expert in mathematical image analysis, spent months perfecting the photographs, attempting to create images that would mimic the experience a human observer would have if their eyes could process the vast difference in brightness between parts of the corona that are close to the sun’s surface and those that are more distant. At the time, he had no idea that this new “hobby” would become a second career.

“I just played with it,” Druckmüller said. “My goal was to create beautiful, high-quality solar eclipse images. That was it.”

In the early 2000s, he shared his creations on a personal website. From there, one of the images found its way into an article that Habbal and colleagues published in a scientific journal. He didn’t know they were using his image, and they didn’t know it was his.

“It was a complete coincidence,” Druckmüller said. “Someone took it off my website and removed the copyright. I only found out through a friend. I contacted them to sort it out and we started working together. Within a year we went on an expedition together.”

a row of photographers stands in the snow, with mountains in the distance on the right.  a darkening sky contains a small black point surrounded by brilliant light.a row of photographers stands in the snow, with mountains in the distance on the right.  a darkening sky contains a small black point surrounded by brilliant light.

a row of photographers stands in the snow, with mountains in the distance on the right. a darkening sky contains a small black point surrounded by brilliant light.

Measure temperature

Since 1999, Druckmüller has participated in more than a dozen solar eclipse observations. Over the years, the campaigns became more complicated. For the upcoming solar eclipse, Druckmüller’s team in Brno has shipped thousands of kilos of special photographic equipment, which will be distributed across the three observing sites.

In addition to capturing the white light from the corona, the researchers can now visualize spectral lines of various energetic ions present in the coronal gas. This opens the door for more exciting science.

“Different types of ions are formed at different temperatures, so by imaging spectral lines of different types of ions we can measure the temperature in different parts of the corona,” Druckmüller said. “That’s very difficult to do in any part other than the visible part of the spectrum and can’t really be done by probes in space, which don’t see the visible light.”

Druckmüller admitted that the increasing complexity of the observations took the fun out of the experience. What was once an exciting adventure now looks more like an industrial operation.

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“We will spend four days assembling all the equipment,” he said. “The eclipse itself is very stressful for us because everything has to work just right. I’ll be relieved when I get back to the office and have the hard drive on my desk to just work with.”

Druckmüller hopes to have the first images ready to share with the world and his scientific colleagues in Hawaii two weeks after the solar eclipse. According to him, processing all the data might take until the next expedition.

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