Plasma is constantly streaming away from the Sun, forming the solar wind. A likely source of this plasma is coronal holes, regions of the Sun’s corona with magnetic field lines that open outward. Chitta et al. observed a coronal hole in the extreme ultraviolet using the ESA/NASA Solar Orbiter spacecraft and identified several types of small-scale jets within it. Large numbers of jets occurred during the observation, but each one lasted only a few dozen seconds. The authors calculated that the jets provide enough energy and plasma to supply a large fraction of the solar wind, at least during quiet periods. Image credit: ESA / NASA / Solar Orbiter / EUI Team / Lakshmi Pradeep Chitta, Max Planck Institute for Solar System Research / CC BY-SA 3.0 IGO.
“We could only detect these tiny jets because of the unprecedented high-resolution, high-cadence images produced by EUI,” said Dr. Lakshmi Pradeep Chitta, an astronomer at the Max Planck Institute for Solar System Research.
“In particular, the images were taken in the extreme ultraviolet channel of EUI’s high resolution imager, which observes million-degree solar plasma at a wavelength of 17.4 nm.”
“Of particular importance is the fact that the analysis shows that these features are caused by the expulsion of plasma from the solar atmosphere.”
Solar astronomers have known for decades that a significant fraction of the solar wind is associated with magnetic structures called coronal holes — regions where the Sun’s magnetic field does not turn back down into the Sun. Instead, the magnetic field stretches deep into the Solar System.
Plasma can flow along these ‘open’ magnetic field lines, heading into the Solar System, creating the solar wind.
But the question was: how did the plasma get launched?
The traditional assumption was that because the corona is hot, it will naturally expand and a portion of it will escape along the field lines.
But the new results look into the coronal hole that was situated at the Sun’s south pole, and the individual jets that were revealed challenge the assumption that the solar wind is produced only in a steady continuous flow.
“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 Dr. Andrei Zhukov, an astronomer at Royal Observatory of Belgium.
The energy associated with each individual jet is small. At the top end of coronal phenomena are the X-class solar flares, and at the lower end are the so-called nanoflares.
There is a billion times more energy in an X-flare than in a nanoflare.
The tiny jets discovered by Solar Orbiter are even less energetic than that, manifesting around a thousand times less energy than a nanoflare, and channeling most of that energy into the expulsion of the plasma.
The ubiquity of them implied by the new observations suggests that they are expelling a substantial fraction of the material we see in the solar wind.
And there could be even smaller, more frequent events providing yet more.
“I think it’s a significant step to find something on the disc that certainly is contributing to the solar wind,” said Dr. David Berghmans, also from Royal Observatory of Belgium.
At present Solar Orbiter is still circling the Sun close to its equator. So in these observations, EUI is looking across the south pole at a grazing 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 Dr. Daniel Müller, a researcher at ESA.
Source: Sci News