The Sun's magnetic field affects not only its surface, but also the behavior of the solar wind - the steady stream of charged particles that permeates the entire solar system. During a close flyby of the Sun, the European "Solar Orbiter" spacecraft has for the first time optically captured a phenomenon that had been detected by measuring instruments for some time: a solar switchback, a large, S-shaped plasma structure in which the magnetic field of the solar wind is temporarily located vice versa Comparing this recording with data from other measuring instruments enabled researchers to find out more about the cause of such switchbacks.
The solar wind shapes the entire solar system. Because the charged particles of the solar wind, which are accompanied by electromagnetic fields, interact with the gas envelopes of the planets and also with the surfaces of atmosphere-free celestial bodies such as the moon. Collisions with solar wind particles can excite gas particles and trigger auroras, but also initiate chemical reactions. The flow of the solar wind and the magnetic field it carries also form the heliosphere - a bubble that shields the solar system from the interstellar medium. But as steady as the solar wind appears on a large scale, it is turbulent and changeable locally. One of the phenomena occurring here are so-called switchbacks - sudden, locally occurring reversals of the solar wind magnetic field. The NASA space probe "Parker Solar Probe" has repeatedly detected the changes in the magnetic field and particle direction typical of such switchbacks in the solar wind, especially in the solar corona.
S-shaped plasma structure
However, the previous in-situ measurements of such switchbacks in the solar wind always only provided snapshots based on selective measurement data. It has therefore remained unclear how these magnetic turns arise and what they look like in their entirety. This is where ESA's Solar Orbiter spacecraft comes into play. Because it is equipped with telescopes and cameras, it is able to image the sun's surface as well as what is happening in the sun's corona in high resolution. On March 25, 2022, the spacecraft was about 48 million kilometers from the Sun when its coronagraph Metis captured a striking phenomenon: an S-shaped plasma structure, a few hundred thousand kilometers across, which appeared about 2.6 solar radii above the Sun's surface and moving away from her at high speed.
With these features, the kink in the plasma of the solar wind recorded by Metis closely matches those that characterize the solar switchbacks. The research team led by Daniele Telloni from the Astrophysical Observatory in Turin concludes that the Solar Orbiter space probe has thus provided a photo of this phenomenon for the first time. Because other instruments on the probe were also active during the recording, the scientists were able to directly trace the origin of this switchback. "The great strength of Solar Orbiter is that the space probe is equipped with instruments that can look into different layers of the sun at the same time," explains co-author Luca Teriaca from the Max Planck Institute for Solar System Research (MPS) in Göttingen. "In this way, we were able to trace the switchback to its origin for the first time." The decisive factor for this was primarily images from the Extreme Ultraviolet Imager, which captures the extremely short-wave ultraviolet radiation from the inner corona.
Conversion work in the solar magnetic field
Below the switchback, the EUV camera images showed a cluster of bright plasma arcs extending from the solar surface into the inner corona. Such coronal arcs often occur in association with active regions, areas of strong magnetic field strength on the Sun's surface. The plasma flows there along the curved, closed field lines of the sun's magnetic field. Using these observations and complementary model simulations, Telloni and his team were able to reconstruct the structure and processes that led to the formation of the switchback. "The calculations indicate that switchbacks form where the magnetic field reforms above an active region," explains co-author Regina Aznar Cuadrado from the MPS. Apparently this happens above all where closed loops of the magnetic field lines meet with areas of open field lines. These open magnetic field lines act like giant plasma lines, allowing material to flow from the Sun's surface into the corona.
Where both types of magnetic field lines interact, the magnetic field is restructured: the field lines come into contact, through which they connect differently than before. This reconnection of the field lines releases energy and, in extreme cases, can deflect the local magnetic field so severely that an S-shaped hairpin bend occurs - a switchback. The research team is now hoping that Solar Orbiter will be able to observe further switchbacks in the coming months. Ideally, the plasma disturbance then propagates towards and eventually reaches the spacecraft. In this way, not only the probe's telescopes and cameras, but also its measuring instruments could record data - and trace the phenomenon from its place of origin to the heliosphere for the first time.
Source: Daniele Telloni (Astrophysical Observatory of Torino, Turin) et al., Astrophysical Journal Letters, doi: 10.3847/2041-8213/ac8104