First Proba-3 science: Surprisingly speedy solar wind found in inner corona

Since July 2025, the European Space Agency’s pair of Proba-3 satellites has already created 57 artificial solar eclipses. So far, the mission has collected more than 250 hours of high-resolution videos of the sun’s atmosphere, called the corona. That’s the same amount of observing time as about 5,000 total solar eclipse campaigns carried out on Earth.

But the science is even more exciting. For the first time, we can carefully track how material from the sun moves through the inner corona, where space weather is born. The first results, recently published in The Astrophysical Journal Letters, show that solar wind structures in the inner corona can travel three to four times faster than scientists thought.

Before Proba-3, a total solar eclipse seen from Earth was the best way to see the sun’s inner corona. When the moon blocks out the sun’s direct light, expert photographers can capture beautiful details in the atmosphere around the sun. But total solar eclipses happen on average only once every 18 months and totality lasts at most a few minutes.

Proba-3 creates artificial total solar eclipses by flying its two spacecraft in an extremely precise formation. For around five hours at a time, the Occulter spacecraft acts like an artificial moon and blocks the sun’s direct light so the other spacecraft, the Coronagraph, can see the sun’s corona.

Proba-3’s ASPIICS coronagraph instrument can see down to 70 000 km from the sun’s surface, one tenth of the sun’s radius. No other space-based coronagraph can observe the light scattering off particles in the sun’s corona this close to the sun.

ASPIICS takes one or two images per minute. These are combined into videos that reveal never-before-seen movement in the hard-to-observe inner corona. “These intricate movements have never been observed in optical wavelengths so low in the sun’s inner corona,” notes Joe Zender, ESA’s Proba-3 project scientist.

‘Slow’ solar wind seen speeding close to the sun

Besides light, the sun sends out a stream of particles called the solar wind. “We can track how solar wind speeds up close to the sun, we see it all over Proba-3’s field of view, and we have already seen speeds and accelerations that surprised us,” says Zender.

Just like wind on Earth, solar wind can be fast or slow, smooth or gusty. Fast solar wind usually flows in a smooth current from magnetic structures called coronal holes. In contrast, slow solar wind is variable and gusty, making understanding how it works more difficult.

Scientists think that slow solar wind is generated by the sun’s magnetic field lines changing how they are connected, merging and separating again. This process pushes out blobs of plasma (electrically charged gas) in so-called “streamers”: large, bright rays in the corona.

“In the inner corona, a region very difficult to observe, we saw slow solar wind gusts moving three to four times faster than expected,” says Andrei Zhukov of the Royal Observatory of Belgium, the principal investigator of Proba-3’s ASPIICS instrument and the lead author of the study.

Previously, scientists found that close to the sun’s surface, slow solar wind should have speeds around 100 km/s. Instead, Zhukov’s team tracked some blobs of plasma moving at 250–500 km/s.

Each arrow in the graph from Zhukov’s team shows how a single blob of plasma moving through the sun’s inner corona changes its speed as it moves away from (right-pointing arrow) or toward (left-pointing arrow) the sun. Arrows angled up show plasma blobs speeding up as they move, while down-pointing arrows show blobs slowing down. The shaded regions show uncertainties in the measured speeds and directions.

Overall, the wide range of speeds, accelerations and movement directions in the data shows why slow solar wind is so hard to understand. Zhukov explains, “Slow solar wind is naturally not uniform, involving lots of small-scale structures in the sun’s magnetic field that we can see thanks to ASPIICS.”

“This first dataset is just the beginning of the much longer journey to fully understand what’s happening. Now it’s up to theoretical experts to compare this to models of the magnetic field and plasma acceleration in the sun’s corona,” says Zender.

Looking forward to much more science

Excitingly, most of the data collected by Proba-3 so far is yet to be analyzed. Scientists are invited to use ASPIICS coronagraph data to investigate the workings of the sun’s corona and space weather.

Key open questions to answer are: What accelerates the solar wind? How does the sun fling out material in coronal mass ejections? And why is the solar corona so much hotter than the sun itself?