On the trail of the solar wind drive

The semicircular dark area is a coronal hole in the sun. © ESA/olar Orbiter/EUI; Science, Chitta et al.

How does the sun blow such powerfully charged particles into space? Mini plasma streams that astronomers have now discovered in a coronal hole in the Sun could be responsible for the mysterious drive of the solar wind. The images taken by ESA's Solar Orbiter spacecraft show that these relatively small and short-lived structures are rapidly hurtling into space and could therefore collectively produce the solar wind. According to the scientists, it would actually not be a uniform stream of particles, but rather a river that is created from many small streams, so to speak.

It gives us heat and light - but the sun gives us even more: it constantly emits a stream of charged particles such as protons and electrons. The strength of this so-called solar wind fluctuates depending on solar activity and can develop into real storms. The fastest particles reach speeds of more than 500 kilometers per second. As for the sources of the solar wind, previous research has already pointed to the so-called coronal holes, which occur particularly near the solar poles. In images of the solar corona in ultraviolet light, these structures appear as dark areas. It has already been shown that the field lines of the solar magnetic field in the coronal holes do not point back to the sun in an arc, but extend into space. But what exactly happens in these sources of solar wind?

“Hot” view of the solar south pole

“How the sun manages to accelerate the solar wind into space at high speeds was previously unclear,” says first author Lakshmi Pradeep Chitta from the Max Planck Institute for Solar System Research (MPS) in Göttingen. The new insights have now been made possible by ESA's Solar Orbiter space probe. When she reached the point of her mission closest to the sun on March 30, 2022, she was able to record a coronal hole at the solar south pole in unprecedented detail and rapid image sequence. “The unique images gave us the opportunity to look more closely than ever before at the source regions of the solar wind and thus understand the processes better than before,” says Chitta.

As the team reports, they discovered a previously unknown phenomenon in the recordings: many small streams could be seen moving along

Recordings of various pico flare currents. The dimensions of each individual image are 6,000 kilometers by 6,000 kilometers. © ESA/Solar Orbiter/EUI; Science, Chitta et al.

moving away from the sun at speeds of a few hundred kilometers per second. The plasma structures have an elongated or Y-shaped shape, are only about 100 kilometers wide and are very short-lived. Because they fade again after about 20 to 100 seconds, the image sequences showed. These so-called pico-flare currents are probably caused by local restructuring of the solar magnetic field, say the scientists.

It's the sum that matters

By earthly standards, their estimated amount of energy appears large: during their short existence, they release quantities that around 10,000 households in Germany consume in the course of a year. But compared to other ejections from the sun, this is modest: they provide about a trillionth of the energy released by the sun's largest radiation bursts. But the bottom line is that something really comes together, according to the researchers' models: In total, the pico flare currents probably provide a large part of the energy that accelerates solar wind particles so much. “The currents that we have now discovered are small and only occur sporadically. “But they are apparently a common phenomenon and almost omnipresent in the coronal hole under consideration,” says Chitta.

According to the researchers, the discovery could now change the fundamental idea of ​​the solar wind. Until now, despite its fluctuations in intensity, it has been viewed as a homogeneous particle stream. However, the new observations now show that it fluctuates irregularly, at least at the beginning: the solar wind is apparently based on many tiny currents, which then form one large one - similar to how rivers are fed by a large number of small streams and tributaries. “The more closely we look into the sun’s corona with Solar Orbiter, the more we realize what a crucial role the smallest structures and processes play in understanding our star,” says co-author Hardi Peter from MPS.

The scientists now hope to learn more about the pico flare currents as the Solar Orbiter mission progresses. There could soon be even better perspectives. In the coming years, the probe will increasingly move into an orbit that will enable a better view of the sun's polar regions and thus of the coronal holes and their exciting details.

Source: Max Planck Institute for Solar System Research, specialist article: Science, doi: 10.1126/science.ade5801

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