Exciting discovery: Astronomers have discovered a never-before-seen structure in our sun’s corona – a dynamic web of interwoven plasma filaments. This structure, visible only in UV light, lies in the middle layer of the solar atmosphere and provides crucial clues to the origin of the slow solar wind, as researchers report in “Nature Astronomy”. According to them, the interaction of this plasma network releases magnetic energy, which accelerates the solar wind particles.
The solar wind shapes the entire solar system: it can tear entire atmospheres out into space, chemically change planetary surfaces and deform their magnetic fields. At the same time, the steady wind of charged particles together with the solar magnetic field forms the protective heliosphere around our solar system. But as important as the solar wind is for our cosmic homeland, its origin is not clear.
According to recent measurements and models, the fast, particularly energetic part of the solar wind appears from particularly hot, magnetized places in the chromosphere, the lower atmosphere of the Sun. In these zones, the solar corona appears darker in UV light. However, where the slower, steady portion of the solar wind comes from was previously unclear.
First look into the middle corona
Now, a discovery in the solar corona is providing the first clues to the origin of the slow-moving solar wind. This was made possible because the researchers led by Pradeep Chitta from the Max Planck Institute for Solar System Research (MPS) in Göttingen were able to image and examine the entire middle corona in UV light for the first time. This layer, which begins around 350,000 kilometers above the sun’s surface, has so far been a blind spot, because space probes and solar observatories can only look into lower or higher layers.
Only three new weather satellites of the US GOES system have now made it possible to take a closer look at the mean solar corona for the first time. Because they also carry UV cameras on board aimed at the sun to predict the weather in space. Chitta and his team have now evaluated the data collected by these instruments during a special measurement campaign.
Interwoven network of plasma threads
The images revealed for the first time a complex, dynamic structure in this middle layer of the solar corona. Filamentous, intertwined plasma structures were revealed in the corona above areas where dark coronal holes border zones of high magnetic activity. “We observed constant interactions and persistent remodeling in this coronal web, as well as the formation of highly structured and variable streams of slow-moving solar wind over their tops,” the team reports.
These observations, combined with data from NASA’s solar observatories SOHO and Solar Dynamics Observatory (SDO), also revealed that the plasma filaments in this coronal web appear to follow magnetic field lines – and that these continually cross and interact with each other. “Magnetic structures that appear open at first approach one another and then form closed loops,” report Chitta and his colleagues. Such magnetic reconnections release energy, as models have shown before.
Motor of the slow solar wind found?
The astronomers could thus have discovered an important engine of the slow solar wind. Because their observations suggest that the newly discovered coronal network helps to accelerate the charged particles of the solar wind. “We assume that the architecture of the magnetic field carries over to the slow-moving solar wind and plays an important role in accelerating solar wind particles into space,” says Chitta.
The discovery of the coronal web and magnetic reconnections in the mean solar corona fits data from the Parker Solar Probe and Solar Orbiter solar probes, which recently made their debut magnetic “switchbacks” observed in the solar wind. These transient switchbacks in the coronal magnetic field lines could also result from the dynamic restructuring of the coronal network.
“Our results support reconnection-based slow solar wind models,” write Chitta and his team. The researchers hope to gain detailed insights into the processes in the middle solar corona from further data from the currently active solar probes, but also from future missions. Because some of the planned probes should have instruments on board that specifically target the middle corona. (Nature Astronomy, 2022; doi: 10.1038/s41550-022-01834-5)
Source: Nature Astronomy, Max Planck Institute for Solar System Research