Solar Orbiter solved the mystery of the magnetic field reversal on the Sun: what scientists have learned

Back in the mid-1970s, when the German-American spacecraft Helios 1 and 2 first approached

Sun, scientists first detected a sudden reversal of the star's magnetic field. Further observations with the help of various research missions showed that this phenomenon is not uncommon.

Such shifts were always temporary andalways sudden. They could last from several minutes to several hours, but always ended with the magnetic field returning to its original direction. The secret of magnetic switching remained a mystery to astrophysicists for a long time, until the Solar Orbiter probe flew as close as possible to our star.

What happened?

At the end of March 2022, the Solar Orbiter probemade the first closest approach and passed at a distance of only 0.32 AU. from a star. Recall that 1 a.u. is the approximate average distance between the Sun and the Earth. It is 149.6 million km. At the time of closest approach, the device was inside the orbit of Mercury, and its instruments and sensors collected data and took pictures of the surface of the star.

One such instrument is the Metis coronograph.This device blocks the bright glare of light from the surface of the Sun and takes pictures of the corona - the outer atmosphere of a star. The corona is an area that is crucial for connecting external observations with processes occurring inside the heliosphere. Metis simultaneously captures visible and far ultraviolet radiation from the solar corona. This helps scientists obtain coronal dynamics at maximum resolution.

Around midnight Moscow time on the night of 25On March 26, Metis took an image of the solar corona showing a distorted S-shaped kink in the coronal plasma. This structure reminded scientists of one of the theories that explains the origin of magnetic switching in the Sun.

The researchers analyzed the data obtainedat the same time with other instruments, and found that an unusual phenomenon was observed over the active region of AR 12972. This is a set of changing structural formations in a certain limited area of \u200b\u200bthe solar atmosphere in which a change in the magnetic field is observed. As a rule, outwardly this is manifested by several dark spots.

Further analysis showed that the plasma velocity(charged particles) over this area was very low, which is typical for active zones that have not yet released the stored energy. All observations are in good agreement with the reverse switching theory, which explains magnetic switching in the Sun.

Formation of magnetic switching on the Sun and S-shaped ejection in the corona. Image: ESA & NASA/Solar Orbiter/EUI & Metis Teams and D. Telloni et al. (2022); Zank et al. (2020)

How it works?

Near the Sun and especially over active regionsthere are closed and open magnetic field lines. The former are magnetic loops that curve into a corona and return to the inside of the star, while the latter radiate from the Sun and connect with the solar system's interplanetary magnetic field.

Because charged particles move along linesmagnetic field, only a small amount of plasma can leave the surface of the star when moving along closed lines, but open ones generate a fast flow of the solar wind.

Researchers have shown that magneticswitching occurs during the interaction between areas with open and closed lines of force. As the lines of force come together, they connect into more stable configurations. Like whiplash, this releases energy and creates an S-shaped perturbation that goes into space.

The researchers note that even a simple snapshotof the solar corona, obtained by Solar Orbiter, completely coincided with the mathematical models that astrophysicists drew when they tried to theoretically explain the unusual phenomenon.

To confirm the relationship between observations andtheoretical explanation, the scientists built a computer model that used the theoretical model to describe the processes on the surface of the Sun. The study showed that all Solar Orbiter observational data are exactly the same as predicted. Moreover, the elongation of the S-shaped perturbation through the solar corona that the model predicted could be observed in the images.

In a paper published in The AstrophysicalJournal Letters, scientists noted that the Solar Orbiter probe obtained the first image of magnetic switching in the solar corona, which explained this phenomenon.

Formation of magnetic switching on the Sun. Image: Zank et al. (2020)

Why is it important?

Explanation of magnetic switching opens the wayto the study of sudden changes in the solar wind. Spacecraft that pass through a magnetic field reversal often observe a local acceleration of the solar wind.

In future observations, scientists planto statistically relate the reverse switching observed by the spacecraft to their original regions on the Sun. This will help to better predict the behavior of the solar wind, the evolution and internal processes on the Sun, and ultimately space weather and its impact on satellite missions and weather on Earth.

Scientists plan to tune the Solar Orbiter instruments for the next approach in such a way as to understand how the Sun connects with the wider magnetic environment of the solar system.

What's next?

Despite impressive discoveries, the probe's missionSolar Orbiter is just getting started. The orbit of this spacecraft is designed in such a way as to take the probe beyond the plane of the ecliptic of the solar system and observe the polar regions of the star.

Its orbit was chosen to bein close resonance with Venus, meaning that it returns to the planet's vicinity every few revolutions around the Sun to use its gravity to change or tilt its orbit.

September 4, 2022 Solar Orbiter for the third timeapproached Venus and is now moving in the direction of the Sun. According to calculations, on October 13, 2022, it will set a new record by approaching the Sun at a distance of 0.29 AU. (44.08 million km). Perhaps this time, scientists will be able to collect new data that will tell more about the magnetic field of our star.

The orbit of the Solar Orbiter probe from its location on September 14 to closest approach on October 14, 2022. Image: ESA

But even if it's not, there's a probe aheadmany possibilities. The ship of the mission, calculated until 2030, will approach the Sun about twice a year. At the same time, from 2025, Solar Orbiter will use flybys near Venus to rise first by 17°, and later up to 33° above the plane of the Sun's equator and look into little-studied regions of the star.

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