There is a gap in our observations of the sun: Part of its atmosphere is effectively invisible to our telescopes. However, images taken from Earth during a solar eclipse are filling in that hole, providing an unprecedented look at 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 and process solar eclipse images so that they reveal some of the sun's best-protected secrets is Czech mathematician Miloslav Druckmüller. His breathtaking renditions of the wispy, white rays and looping magnetic lines that make up the solar corona when it emerges to the human eye from the eclipsed sun are well known in solar physics circles.
Druckmüller spoke to Space.com from his office at the Brno University of Technology in the Czech Republic a few days before his departure to 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: How to photograph the total solar eclipse on April 8, 2024
On eclipse day, 66 cameras fitted with special filters and distributed across three observing locations in Mexico and the U.S. will capture tens of thousands of images during the roughly four-minute eclipse. The researchers hope that from this enormous data set, Druckmüller's expert image processing will be able to tease out some previously unknown information about the sun's otherwise invisible region.
The blind spot
Made of extremely thin, charged gas called plasma, the solar corona is the upper part of the sun's atmosphere. It is a million times dimmer than the underlying photosphere that makes up the sun's visible surface, so it is completely outshined by the light of the star when viewed in visible light.
To observe the corona, astronomers need to obscure the visible solar disk to allow the faint coronal light to appear against the dark cosmos. To do this, they use an instrument called a coronagraph, which is equipped with an occulter that blocks the sun's light.
However, if the occulter were to cover only the visible sun, the diffraction — the bending of light around an obstacle — would let the bright light of the photosphere spill around the occulter's edges and ruin the photographs. Therefore, astronomers use bigger occulters that also cover the inner corona up to the distance of one solar radius from the sun's surface.
The moon, however, being far away, causes no diffraction while being just the right size to perfectly cover the solar disk during a total eclipse.
"Any coronagraph that is trying to mimic a solar eclipse is awfully inferior to an actual solar eclipse," Druckmüller said. "It's too close to the telescope, and that means that it causes diffraction. "Because of that, the occulter must be much larger than the solar disk, which means that we can't see the innermost part of the corona."
Because of these limitations, scientists know very little about the processes that take place in this hidden region.
"If you take images from space telescopes such as the Solar Dynamics Observatory, you can see the sun's surface in extreme ultraviolet light," Shadia Habbal, a professor of solar physics at the University of Hawaii and Druckmüller's collaborator, said during a conference in Brno in November 2022. "You can see a little bit of the corona projected against the surface, but that is clearly not sufficient to understand how this plasma expands into space."
The corona is the source of the solar wind, the constant stream of charged particles that expands across the solar system. Occasionally, it erupts with massive outbursts known as coronal mass ejections, which can cause dramatic geomagnetic storms on Earth. Many of the processes that accelerate this solar wind into space happen in the coronagraph's blind spot: The sun itself.
A chance discovery
In Druckmüller's solar eclipse images, this hidden region emerges in unprecedented detail. The coronal gas — made mostly of ionized hydrogen and helium, with a smattering of heavier elements, such as iron, magnesium, silicon or calcium — loops around the sun's magnetic-field lines. Elsewhere, it streams into space from large sunspots or active regions and flows from coronal holes where the magnetic-field lines are broken.
Druckmüller first experimented with solar eclipse photography in 1999, when, for the first time in 150 years, the moon's shadow visited Central Europe. The Czech Republic was just outside the band of totality, and stormy weather threatened to ruin the experience in many of the nearest viewing locations. Druckmüller set out for Hungary, which provided "a perfectly clear sky."
An expert in mathematical image analysis, Druckmüller spent months perfecting the photographs, trying to create views that would mimic the experience a human observer would have if their eyes could handle the enormous difference in brightness between the parts of the corona close to the sun's surface and those more removed. At that time, he had no idea this newfound "hobby" would turn into a second career.
"I was just playing 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 a paper Habbal and colleagues published in a scientific journal. He didn't know they used his image, and they didn't know it was his.
"It was a complete coincidence," Druckmüller said. "Someone took it from my website and removed the copyright. I only discovered through a friend. I got in touch with them to sort this out, and we started cooperating. Within a year, we went together on an expedition."
Measuring temperature
Since 1999, Druckmüller has taken part in more than a dozen solar eclipse observations. Over the years, the campaigns grew more complicated. For the upcoming eclipse, Druckmüller's team in Brno shipped thousands of pounds of special photographic equipment, which will be spread across the three observing locations.
In addition to capturing the white light of 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 emerge 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 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 has taken a bit of the fun out of the experience. What was once an exciting adventure now looks more like an industrial operation.
"We will spend four days just putting all the equipment together," he said. "The eclipse itself is really stressful for us because everything needs to work just right. I will be relieved when I'm back to my 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 eclipse. To process all of the data, he said, would take perhaps until the next expedition.
