Solar Orbiter discovers plasma jets that could fuel the production of solar wind

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Recently, the joint European Space Agency (ESA)/NASA Solar Orbiter spacecraft discovered an array of tiny jets of material being ejected from the Sun’s outer atmosphere. Ejecting plasma at 100 kilometers per second, the jets last for just 20 to 100 seconds, and could be the source of solar wind.

Solar wind is made up of plasma, or charged particles, that continuously flow away from the Sun. As solar wind moves farther and farther from the Sun, it will collide with anything in its path, such as Earth’s magnetic field, which — in turn — creates the famous aurora borealis. Scientists have long theorized how solar wind is created, but have never been able to confirm their ideas. However, Solar Orbiter’s recent findings may finally provide scientists with the answer they’ve been searching for for decades.

The Sun boasts many extreme and unique characteristics that have stumped scientists for decades. Solar wind is one of the most prominent and fundamental features of the Sun. In fact, the potential causes of solar wind have been the focal point of thousands of heliophysical studies.

Solar Orbiter launched in February 2020 from Florida atop a United Launch Alliance Atlas V rocket. Just a few short months later, the spacecraft was already sending back some of the closest images of the Sun ever taken and has since provided scientists with data that has allowed them to better understand our Sun and the ways in which it affects our lives, Earth, and our solar system as a whole.

Solar Orbiter’s recent discovery of the plasma jets was made using the spacecraft’s Extreme Ultraviolet Imager (EUI) instrument. On March 30, 2022, the EUI imaged the Sun’s south pole in the extreme ultraviolet channel of the instrument’s high-resolution imager and sent the data back to Earth for analysis. More specifically, the images were taken at a wavelength of 17.4 nanometers.

“We could only detect these tiny jets because of the unprecedented high-resolution, high-cadence images produced by EUI,” says lead author Lakshmi Pradeep Chitta of the Max Planck Institute for Solar System Research in Germany.

In the images, scientists quickly noticed a group of small, faint, and seemingly short-lived features that would later be confirmed to be plasma jets in the Sun’s outer atmosphere. Further analysis of the images showed that the plasma jets are caused by the expulsion of plasma from within the solar atmosphere.

Scattered throughout the Sun’s magnetic field are magnetic structures called coronal holes. These structures are where regions of the magnetic field do not turn back down toward the Sun and, instead, extend out into the solar system. Scientists have long known that coronal holes are associated with a significant fraction of solar wind that is ejected from the Sun. Plasma can flow along the open magnetic field lines in coronal holes and thus be thrown out into space — creating solar wind.

However, how does the plasma get launched into the coronal holes?

For decades, scientists have assumed that the extreme heat of the corona, or the Sun’s outer atmosphere, would cause it to naturally expand and contract. When the corona expands, plasma within it escapes via the magnetic lines of coronal holes, which would explain the source of the plasma within the solar wind.

However, the plasma jets Solar Orbiter discovered are situated within a coronal hole at the Sun’s south pole. If these jets do in fact fuel solar wind, the discovery of the plasma jets would directly challenge scientists’ assumption that solar wind is only produced via a steady and continuous flow of plasma into space.

“One of the results here is that to a large extent, this flow is not actually uniform, the ubiquity of the jets suggests that the solar wind from coronal holes might originate as a highly intermittent outflow,” said co-author Andrei Zhukov of the Royal Observatory of Belgium.

Though these plasma jets could be a source of solar wind, each individual jet is small and doesn’t expel a high amount of energy. The jets are even less energetic than nanoflares, which are considered to be the least energetic of all known coronal phenomena. Nanoflares themselves are around a billion times less energetic than X-flares, the most energetic coronal phenomena. The plasma jets at the Sun’s south pole are approximately a thousand times less energetic than nanoflares.

A mosaic of Solar Orbiter images showing the plasma jets discovered at the Sun’s south pole. (Credit: ESA/NASA/Solar Orbiter/EUI Team/Lakshmi Pradeep Chitta/Max Planck Institute for Solar System Research)

Given the amount of plasma jets found at the south pole, scientists believe that the jets expel a significant amount of material that is found in solar wind. However, not all solar wind material is thrown out by the jets, so there is a possibility for there to be even smaller, more frequent coronal events that are powering solar wind.

“I think it’s a significant step to find something on the disc that certainly is contributing to the solar wind,” said David Berghmans, principal investigator of the EUI instrument, of the Royal Observatory of Belgium.

While Solar Orbiter’s recent observations have revealed a lot about coronal phenomena at the south pole, the spacecraft was only taking images of the south pole from an edge-on angle. Currently, Solar Orbiter is orbiting around the Sun’s equator, and thus can only perform observations of the Sun’s extremities at an angle.

“It’s harder to measure some of the properties of these tiny jets when seeing them edge-on, but in a few years, we will see them from a different perspective than any other telescopes or observatories so that together should help a lot,” said Solar Orbiter Project Scientist Daniel Müller.

Throughout the next few months and years of its mission, Solar Orbiter will gradually begin to incline its orbit more and more. By the time Solar Orbiter has completed its inclination change, the spacecraft will have unprecedented views of the Sun’s polar regions. What’s more, the Sun will continue to move through a solar cycle during Solar Orbiter’s inclination change, meaning that the spacecraft will be able to observe new coronal holes and other solar phenomena.

Given that the Sun is the only star we can observe in extreme detail, Solar Orbiter’s results will allow astrophysicists, planetary scientists, cosmologists, and more to better understand the internal and external functions of a star. There are likely trillions of stars in the universe that are just like our Sun, so understanding how they function and their characteristics will allow us to better understand our universe and the solar systems that fill it.

Chitta et al.’s results were published in the journal Science on Aug. 24, 2023.

(Lead image: Artist’s depiction of Solar Orbiter at perihelion. Credit: ESA/Medialab)

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