With data from its closest pass by the sun yet, the ESA/NASA Solar Orbiter spacecraft has found fascinating clues about the origin of magnetic reversals and shows how their natural formation mechanism can help accelerate the solar wind. Solar Orbiter has made the first remote-sensing observation consistent with a magnetic phenomenon called a solar reversal—sudden and large deflections of the solar wind’s magnetic field. The new observation provides a complete picture of the structure, in this case confirming that it has an S-shaped character, as predicted. Furthermore, the global perspective provided by the Solar Orbiter data shows that these rapidly changing magnetic fields can have their origin near the sun’s surface. While many spacecraft have previously flown through these enigmatic regions, in situ data only allow a measurement at a single point and time. Consequently, the structure and shape of the return must be inferred from the plasma and magnetic field properties measured at a point. When the German-American Helios 1 and 2 spacecraft flew close to the sun in the mid-1970s, both probes recorded sudden reversals of the sun’s magnetic field. These mysterious reversals were always abrupt and always temporary, lasting from a few seconds to several hours before the magnetic field returned to its original direction. Credit: ESA & NASA/Solar Orbiter/Metis Teams; D. Telloni et al. (2022) These magnetic structures were also examined 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 the Helios missions made their closest passes, Ulysses operated mostly beyond Earth orbit. Their number increased dramatically with the arrival of NASA’s Parker Solar Probe in 2018. This clearly showed that sudden magnetic field reversals are more numerous near the sun and led to the idea that they were caused by S-shaped twists in the magnetic field. This puzzling behavior earned the phenomenon the name of return. Various ideas were proposed as to how they might be formed. On March 25, 2022, the Solar Orbiter was just one day away from a close pass of the sun — bringing it into the orbit of the planet Mercury — and its Metis instrument was taking data. Metis blocks out the bright glow of light from the sun’s surface and takes pictures of the sun’s outer atmosphere, known as the corona. Particles in the corona are electrically charged and follow the sun’s magnetic field lines into space. The electrically charged particles themselves are called plasma. Around 20:39 UT, Metis captured an image of the solar corona showing a distorted S-shaped bend in the coronal plasma. To Daniele Telloni, National Institute of Astrophysics-Astrophysical Observatory of Turin, Italy, it looked suspiciously like a solar change. The sun as seen by the ESA/NASA Solar Orbiter spacecraft on March 25, 2022, one day before its closest approach of about 0.32 au, which brought it into the orbit of the planet Mercury. The central image was taken by the Extreme Ultraviolet Imager (EUI) instrument. The outer image was taken by the Metis coronagraph, an instrument that blocks the bright light of the sun’s surface to view the sun’s faint outer atmosphere, known as the corona. The Metis image has been edited to show structures in the crown. This revealed the return (the prominent white/blue feature at about the 8 o’clock position in the lower left). It appears to trace back to the active region on the sun’s surface, where loops of magnetism have permeated the sun’s surface. Credit: ESA & NASA/Solar Orbiter/EUI & Metis Teams and D. Telloni et al. (2022) By comparing the Metis image, taken in visible light, with a simultaneous image taken by the Solar Orbiter’s Extreme Ultraviolet Imager (EUI) instrument, he saw that the candidate exchange was taking place above an active region recorded as AR 12972 .The 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 would be expected from an active region that has not yet released its stored energy. Daniele immediately thought that this looked like a production mechanism for the inversions proposed by Professor Gary Zank, 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 over active regions, there are open and closed magnetic field lines. The closed lines are loops of magnetism that flow through the solar atmosphere before curling up and disappearing back into the sun. Very little plasma can escape into space above these field lines and so the speed of the solar wind tends to be slow here. Open field lines are the opposite, emanating from the sun and associated with the interplanetary magnetic field of the Solar System. They are magnetic highways along which plasma can flow freely and cause the fast solar wind. Daniele and Gary demonstrated that switching occurs when there is an interaction between a region of open field lines and a region of closed field lines. As the field lines become crowded, they can be reconnected into more stable configurations. Rather like the cracking of a whip, this releases energy and creates an S-shaped disturbance that travels through space, which a passing spacecraft will record as a return. The observation of Metis switching is consistent with the sound theoretical mechanism for producing solar magnetic reversals proposed in 2020 by Professor Gary Zank. The key observation was that the switching could be seen to originate from above a solar active region. This sequence shows the chain of events that researchers believe took place. (a) Active regions on the sun can have open and closed magnetic field lines. The closed lines climb into the solar atmosphere before curving back into the sun. Open field lines are associated with the interplanetary magnetic field of the Solar System. (b) When an open magnetic region interacts with a closed region, the magnetic field lines can reconnect, creating a roughly S-shaped field line and producing a burst of energy. (c) As the field line responds to reconnection and release of energy, a bend is created that propagates outward. This is the return. Similar switching is also sent in the opposite direction, down the field line and into the sun. Credit: Zank et al. (2020) According to Gary Zank, who proposed one of the theories for the origin of the return change, “The first image from Metis that Daniele showed me almost immediately suggested the animations we had drawn to develop the mathematical model for a change return. Of course, the first image was just a snapshot, and we had to temper our excitement until we used the excellent Metis coverage to extract temporal information and do a more detailed spectral analysis of the images themselves. The results turned out to be absolutely spectacular. “ Along 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 picture, especially after including calculations of how the structure would elongate as it propagated outward through the solar corona. . “I would say that this first image of a magnetic change in the solar corona has revealed the mystery of their origin,” says Daniele, whose results are published in a paper in The Astrophysical Journal Letters. In understanding the spins, solar physicists may also take a step toward understanding the details of how the solar wind is accelerated and heated away from the sun. This is because when spacecraft fly through transits, they often record a local acceleration of the solar wind. ESA’s Solar Orbiter has solved the mystery of a magnetic phenomenon in the solar wind. It has captured the first image of a ‘switchback’ in the solar corona, confirming the predicted ‘S’ shape. Switching is defined by rapid reversals in the direction of the magnetic field. The observed switching is associated with an active region associated with sunspots and magnetic activity where there is an interaction between open and closed magnetic field lines. The interaction releases energy and sends the S-shaped disturbance into space. The new data suggest that the reversals could originate near the solar surface and may be important for understanding the acceleration and heating of the solar wind. Credit: European Space Agency “The next step is to try to statistically link the fluctuations observed in situ with their source regions on the sun,” says Daniele. In other words, to fly a spacecraft through the magnetic reversal and be able to see what has happened on the solar surface. That’s exactly the kind of docking science the Solar Orbiter was designed to do, but it doesn’t necessarily mean the Solar Orbiter has to fly through the switch. It could be another spacecraft, like the Parker Solar Probe. Since the in situ data and the remote sensing data are simultaneous, Daniele can perform the correlation. “This is exactly the result we expected with Solar Orbiter,” says Daniel Müller, ESA’s Solar Orbiter Project Scientist.
