Solar Orbiter solves the mystery of magnetic return

Solar Orbiter has made the first-ever remote sensing observation consistent with a magnetic phenomenon called solar tilt – sudden, large deviations in the solar wind’s magnetic field. The new observation provides a full view of the structure, in this case confirming that it has an S-shaped character, as expected. Additionally, the global perspective provided by the Solar Orbiter data indicates that these rapidly changing magnetic fields may have originated near the surface of the Sun.

While a number of spacecraft have already flown over these puzzling regions, in situ data only allows measurement at a single point and at a single time. Therefore, the structure and shape of the yaw must be inferred from the plasma and magnetic field properties measured at a point.

When German-American spacecraft Helios 1 and 2 flew close to the Sun in the mid-1970s, both probes recorded sudden reversals in the Sun’s magnetic field. These mysterious reversals were always abrupt and always temporary, lasting from seconds to hours before the magnetic field returned to its original direction.

These magnetic structures were also probed at much greater distances from the Sun by the Ulysses spacecraft in the late 1990s. Instead of one-third of Earth’s orbital radius from the Sun, where Helios missions performed their closest passage, Odysseus operated primarily beyond Earth’s orbit.

Their number increased dramatically with the arrival of NASA’s Parker Solar Probe in 2018. This clearly indicated that sudden magnetic field reversals are more numerous near the Sun, and led to the suggestion that they were caused by S-shaped folds in the magnetic field. . This puzzling behavior has earned the phenomenon the name of shoelaces. A number of ideas have been offered as to how these might form.

On March 25, 2022, Solar Orbiter was just one day away from a close pass of the Sun – bringing it back into orbit of the planet Mercury – and its Metis instrument was taking data. Metis blocks bright glare from the Sun’s surface and takes pictures of the Sun’s outer atmosphere, known as the corona. Corona particles are electrically charged and follow the Sun’s magnetic field lines in space. The electrically charged particles themselves are called a plasma.

Around 8:39 p.m. UT, Metis recorded an image of the solar corona that showed a distorted S-shaped bend in the coronal plasma. For Daniele Telloni, National Institute of Astrophysics – Astrophysical Observatory of Turin, Italy, it looked eerily like a solar flashback.

Comparing the Metis image, which had been taken in visible light, with a simultaneous image taken by Solar Orbiter’s Extreme Ultraviolet Imager (EUI) instrument, he saw that the candidate flashback took place above an active region cataloged as AR 12972. Active regions are associated with sunspots and magnetic activity. Further analysis of the Metis data showed that the plasma velocity above this region was very slow, as one would expect from an active region that has not yet released its stored energy.

Daniele immediately thought it sounded like a shoelace-generating mechanism proposed by Professor Gary Zank of the University of Alabama in Huntsville, USA. The theory looked at how different magnetic regions near the Sun’s surface interact with each other.

Near the Sun, and especially above the active regions, there are open and closed magnetic field lines. Closed lines are loops of magnetism that arc in the solar atmosphere before bending and disappearing into the Sun. Very little plasma can escape into space above these field lines, so solar wind speed tends to be slow here. Open field lines are the reverse, they emanate from the Sun and connect to the interplanetary magnetic field of the solar system. They are magnetic highways along which the plasma can flow freely and give rise to the fast solar wind.

Daniele and Gary proved that switchbacks occur when there is an interaction between a region of open field lines and a region of closed field lines. As the field lines come together, they can reconnect into more stable configurations. Much like cracking a whip, it releases energy and triggers an S-shaped disturbance moving through space, which a passing spacecraft would register as a flashback.

According to Gary Zank, who proposed one of the theories on the origin of the shoelaces, “The first image of Metis that Daniele showed me almost immediately suggested to me the caricatures we had drawn while developing the mathematical model of a shoelace. . Of course, the first image was just a snapshot and we had to temper our enthusiasm until we used the excellent cross-sectional coverage to extract temporal information and do more detailed spectral analysis of the images themselves. . The results were absolutely spectacular!

Together with a team of other researchers, they built a computer model of the behavior and found that their results looked strikingly similar to the Metis image, especially after including calculations on the elongation of the structure during its outward propagation through the solar corona. .

“I would say that this first image of a magnetic return in the solar corona revealed the mystery of their origin,” says Daniele, whose results are published in an article in The Astrophysical Journal Letters.

By understanding the switchbacks, solar physicists can also take a step toward understanding the details of how the solar wind is accelerated and heated away from the Sun. Indeed, when spacecraft cross switchbacks, they often register a localized acceleration of the solar wind.

“The next step is to try to statistically link the laces observed in situ with their source regions on the Sun,” explains Daniele. In other words, fly a spacecraft through the magnetic reversal and be able to see what happened on the solar surface. That’s exactly the kind of binding science Solar Orbiter was designed for, but that doesn’t necessarily mean Solar Orbiter has to fly through the yaw. It could be another spacecraft, such as Parker Solar Probe. As long as the in situ data and the remote sensing data are concurrent, Daniele can perform the correlation.

“This is exactly the kind of result we were hoping for with Solar Orbiter,” says Daniel Müller, ESA project scientist for Solar Orbiter. “With each orbit, we get more data from our suite of ten instruments. Based on results like this, we will refine the observations planned for Solar Orbiter’s next solar encounter to understand how the Sun connects to the wider magnetic environment of the Solar System. This was Solar Orbiter’s first-ever close pass of the Sun, so we expect many more exciting results.

Solar Orbiter’s next close pass of the Sun – still in Mercury’s orbit at a distance of 0.29 times the Earth-Sun distance – will take place on October 13. Earlier this month, on September 4, Solar Orbiter performed a gravity-assisted flyby of Venus to adjust its orbit around the Sun; future flybys of Venus will begin to increase the inclination of the spacecraft’s orbit to access higher latitude – more polar – regions of the Sun.

Notes for Editors

The observation of a magnetic reversal in the solar corona by D. Telloni et al is published in The Astrophysical Journal Letters. DOI 10.3847/2041-8213/ac8104

The research will be presented this week at the 8th Solar Orbiter Workshop in Belfast, Northern Ireland.

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