A Last-Minute Addition to the Solar Orbiter Allows it to See More Deeply into the Sun’s Atmosphere

Spacecraft instruments are highly specialized and can take years to design, build, and test. But a last-minute hack to one of the instruments on the ESA’s Solar Orbiter has allowed the spacecraft to take some difficult observations it would otherwise have been unable to take.

It’s all because of one astronomer and an instrument door.

The ESA launched the Solar Orbiter in February 2020 to study the Sun’s inner heliosphere and polar regions in high resolution. Every six months, it makes a close approach to the Sun, and it studies the same region repeatedly. The idea is to monitor the build-up of magnetic activity that’s a precursor to solar flares and eruptions. Within that, it has more specific scientific objectives.

The spacecraft carries a suite of instruments to achieve its objectives, and one of them is the Extreme Ultraviolet Imager (EUI.) Belgium contributed the EUI to the mission, and its job is to image the layers and structures in the Sun’s atmosphere from the chromosphere to the corona.

The EUI is actually three instruments in one: A full Sun imager and two high-resolution telescopes. The full-Sun imager shows the Sun’s overall structure at coronal temperatures, and the pair of telescopes works in selected wavelength bands. They reveal the finer structures that are visible in the close-up views of the Sun the spacecraft gets during its perihelion phase.

The challenge all Sun-observing spacecraft and instruments have to overcome is the overwhelming light coming from the star. The Sun’s powerful light obscures critical details. The EUI is no exception. Telescopes often use coronagraphs to block out the powerful light and make details in the Sun’s atmosphere visible.

During the instrument’s final construction phase, one Solar Orbiter team member got a brilliant idea. The EUI has its own safety door to protect it during spaceflight and when it’s not in use. What if that door could be modified to improve the imager’s functioning?

“It was really a hack,” says Frédéric Auchère, Institut d’Astrophysique Spatiale, Université Paris-Sud, and a member of the EUI team. “I had the idea to just do it and see if it would work. It is actually a very simple modification to the instrument.”

Spacecraft weight is calculated precisely, but Auchère figured a few grams wouldn’t matter. He added a small, protruding ‘thumb’ to the door that could cover the Sun’s bright disc during operations when the door was halfway opened rather than fully opened. With the overpowering light from the Sun’s disc obstructed by the little thumb on the door, the EUI can effectively detect the UV light coming from the surrounding corona, which is millions of times more faint than the disc. This means the instrument can see deeper into the Sun’s atmosphere.

“Physics is changing there, the magnetic structures are changing there, and we never really had a good look at it before. There must be some secrets in there that we can now find.”

David Berghmans, Royal Observatory of Belgium, EUI Principal Investigator.

The team calls this thumb and half-opened door the occulter mode of operation. It’s basically a new type of instrument. It combines the coronagraph and camera in one instrument instead of requiring a separate coronagraph.

They’ve been testing the EUI in this mode since 2021 and have published a paper in Astronomy and Astrophysics explaining the results. Frédéric Auchère, the astronomer who came up with the idea, is the lead author.

“We’ve shown that this works so well that you can now consider a new type of instrument that can do both imaging of the Sun and the corona around it,” says Daniel Müller, ESA’s Project Scientist for Solar Orbiter.

Auchère’s little addition to the EUI’s instrument door solves a problem that plagues extreme UV imagers. The depths of the Sun’s atmosphere are beyond the view of most of them, and traditional separate coronagraphs usually totally block this region out of necessity. But the EUI’s new occulter can more easily image this difficult region than ever before.

“Physics is changing there, the magnetic structures are changing there, and we never really had a good look at it before. There must be some secrets in there that we can now find,” says David Berghmans, Royal Observatory of Belgium, and the EUI Principal Investigator.

The small thumb added to the door was a late addition to the EUI instrument. So it’s in a sub-optimal position. That means that, among other limitations, it needs longer exposures to work. Some of the test images in the paper required 1000-second exposures.

But now that the effectiveness of this design is proven, it can be improved for future instruments on future missions. “With minor modifications, the efficiency of an FSI-based coronagraph could be increased by two orders of magnitude, which would allow images similar to those presented here to be acquired in 10 seconds,” the authors write in their conclusion.

This means that EUV instruments can start to take over from Visible Light instruments and coronagraphs. “Compared to a VL coronagraph, an EUV instrument offers several advantages,” the authors explain. “There is no background emission from scattering off dust,” they write. Not only is stray light from the instrument easier to control, but it also “… makes an EUV coronagraph less demanding in terms of platform pointing accuracy and stability.”

It looks like our view of the Sun is about to take a leap forward.