With data from its closest approach to the Sun to date, the ESA/NASA Solar Orbiter mission has uncovered convincing evidence regarding the origin of magnetic switchbacks and how their physical production mechanism may contribute to the acceleration of the solar wind.
Solar Orbiter has made the very first remote sensing observation matching with a magnetic phenomenon known as a solar switchback – rapid and significant deflections in the magnetic field of the solar wind. The latest observation gives a complete picture of the structure, proving that it is, in fact, S-shaped, as was predicted. Additionally, the global view offered by the data from the Solar Orbiter suggests that these quickly varying magnetic fields may have their origins close to the Sun.
While many spacecraft have flown over these locations, in situ data only allows for a single measurement. Therefore, plasma and magnetic field parameters recorded at a single site must be used to infer the structure and geometry of the switchback.
In the middle of the 1970s, the US-German Helios 1 and 2 spacecraft saw unexpected reversals of the Sun’s magnetic field. These strange reversals were always brief and sudden, lasting somewhere from a few seconds and several hours until the magnetic field reversed course.
In the late 1990s, the Ulysses spacecraft also looked at these magnetic structures from a long way away from the Sun. Ulysses operated primarily outside of the Earth’s orbit, as opposed to a third of the Earth’s orbital radius from the Sun, where the Helios missions made their closest pass.
With the arrival of NASA’s Parker Solar Probe in 2018, their number increased sharply. As a result, it was hypothesized that S-shaped kinks in the magnetic field are to blame for the increased frequency of abrupt magnetic field reversals in solar-proximity regions. Switchbacks is the name given to the phenomenon as a result of its perplexing behavior. Several hypotheses were put forth as to possible patterns in their development.
On 25 March 2022, Solar Orbiter was using its Metis instrument to gather information as it approached a near flyby of the Sun, bringing it into Mercury’s orbit. Metis is able to capture photographs of the Sun’s outer atmosphere, often known as the corona, by shielding itself from the intense glare of light coming from the surface of the Sun. The corona’s electrically charged particles travel into space along the magnetic field lines of the Sun. Plasma is the name given to the electrically charged particles themselves.
At about 20:39 UT, Metis took a picture of the sun’s corona that showed a kink in the plasma that looked like a S. Daniele Telloni, who works at the Astrophysical Observatory of the National Institute of Astrophysics in Torino, Italy, thought it looked suspiciously like a solar switchback.
When he compared a concurrent image captured by the Solar Orbiter’s Extreme Ultraviolet Imager (EUI) instrument with the visible light image captured by Metis, he could see that the candidate switchback was occurring above an active region designated as AR 12972. Magnetic activity and sunspots are linked to active zones. Further examination of the Metis data revealed that, as would be expected from an active zone that has not yet released its energy reserves, the speed of the plasma above this region was extremely slow.
Daniele immediately recognized this as a generating mechanism for the switchbacks proposed by Prof. Gary Zank of the University of Alabama in Huntsville, USA, which examined how distinct magnetic zones near the Sun’s surface interact with one another.
There are both open and closed magnetic field lines close to and above the Sun in places where there is a lot of activity. The closed lines are magnetic loops that curve and disappear back into the Sun after arching up into the solar atmosphere. Above these field lines, very little plasma can escape into space, hence the solar wind typically moves slowly. Open field lines, on the other hand, come from the Sun and connect to the magnetic field between the planets in the Solar System. The fast solar wind is created by these magnetic highways, down which plasma can travel freely.
Daniele and Gary established that switchbacks happen when a region of open field lines and a region of closed field lines interact. Field lines may link into more stable structures as they congregate. This releases energy and causes an S-shaped disruption to fly into space, which a passing spacecraft would record as a switchback. It works something like cracking a whip.
“The first image from Metis that Daniele showed suggested to me almost immediately the cartoons that we had drawn in developing the mathematical model for a switchback,” says Gary Zank, who proposed one of the theories for the origin of switchbacks.
“Of course, the first image,” according to Gary Zank, “was just a snapshot and we had to temper our enthusiasm until we had used the excellent Metis coverage to extract temporal information and do a more detailed spectral analysis of the images themselves. The results proved to be absolutely spectacular!”
Together with a group of other researchers, they built a computer model of the behavior. When they added calculations for how the structure would get longer as it moved through the solar corona, they found that their results were strikingly similar to the Metis image.
Daniele, whose results are published in a paper in The Astrophysical Journal Letters, says, “I would say that this first image of a magnetic switchback in the solar corona has revealed the mystery of their origin.”
The study of switchbacks may help solar physicists learn more about the processes that speed up and heat the solar wind as it travels away from the Sun. This is due to the fact that as a spacecraft travels through a series of switchbacks, it frequently records a localized acceleration of the solar wind.
“The next step,” according to Daniele, is to try to statistically link switchbacks observed in situ with their source regions on the Sun.”
To put it another way, it would be possible to observe what had happened on the surface of the sun by having a spacecraft fly through the magnetic reversal. This is precisely the type of connection science for which Solar Orbiter was created, but it does not necessarily mean the spacecraft must fly through the switchback. It could be something else in space, like the Parker Solar Probe.
“This is exactly the kind of result we were hoping for with Solar Orbiter,” adds Daniel Müller, ESA Project Scientist for Solar Orbiter.
“With every orbit,” adds Daniel Müller, “we obtain more data from our suite of ten instruments. Based on results like this one, we will fine-tune the observations planned for Solar Orbiter’s next solar encounter to understand the way in which the Sun connects to the wider magnetic environment of the Solar System. This was Solar Orbiter’s very first close pass to the Sun, so we expect many more exciting results to come.”
On October 13, Solar Orbiter will make its subsequent near pass of the Sun, this time from within the orbit of Mercury at a distance of 0.29 times the Earth-Sun distance.
In order to correct its orbit around the Sun, Solar Orbiter made a gravity-assist flyby of Venus on September 4. Subsequent Venus flybys will begin inclining the spacecraft’s orbit so that it can explore higher latitudes and more polar parts of the Sun.
Image Credit: Getty
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