Solar Orbiter traces a magnetic switchback in the solar wind back to its source on the Sun

The European Space Agency says its Solar Orbiter spacecraft has followed a magnetic switchback, an S-shaped fold in the magnetic field carried by the solar wind, back to where it began on the Sun. The result comes from a paper led by Jesse Coburn of CNRS/LPP in France, titled “On the Coronal Origin of Magnetic Switchbacks in the Solar Wind” and now published in Nature Astronomy. According to ESA, the team identified the source by analysing the particles the spacecraft found inside the structure.

The solar wind is a constant outflow of hot, electrically charged gas, or plasma, that drags the Sun’s magnetic field out into the Solar System. Along the way, the field lines can twist, break or bend back on themselves. Switchbacks are one such feature: they have often been detected close to the Sun, but how they form is still debated. ESA recalls that in 2022 Solar Orbiter observed one from a distance and confirmed the S-shape that researchers had predicted but never seen directly.

This time the spacecraft passed straight through a switchback. “Solar Orbiter flew through a very large switchback,” Coburn says, explaining that this let the team measure rarely observed particles carrying clues to their origin. In measurements from the Solar Wind Analyser (SWA), taken while Solar Orbiter was about halfway between Earth and the Sun, the researchers found a combination of charged oxygen and carbon particles. ESA says this mix can only have formed inside hot magnetic loops at the solar surface.

Coburn describes two leading explanations for how switchbacks, and with them the solar wind, come about. The particle mix supports the first, interchange reconnection: open field lines, along which material can escape into space, meet closed loops that arc back to the Sun, and the lines break and reconnect so that plasma once trapped in a loop gets out. The competing explanation relies on waves and turbulence. Co-author Stephanie Yardley of Northumbria University, UK, says the team sees signs of those too, but likely only once the switchback has travelled away from the Sun. In her view both processes are involved and “they simply operate at different stages in a switchback’s lifetime”.

Besides the in situ particle data, the team examined images of the solar disc and modelled the magnetic fields of the Sun and the surrounding space. ESA says the researchers built a new model to work out where the plasma came from, linking Solar Orbiter’s measurements with data from NASA’s Solar Dynamics Observatory to locate the source region. According to ESA, the work also shows that the Sun’s atmosphere leaves its signature on solar wind particles, which could let scientists reconstruct the history of plasma measured far from the Sun.