The Sun’s magnetic field lines are far from straight. Sometimes they can even bend backward, toward the star. This phenomenon is known as a “switchback.” Recently, the Solar Orbiter spacecraft investigated one of these structures and discovered how they form.
A solar switchback. Source: phys.org
Switchbacks in the Sun’s Magnetic Field
According to phys.org, the European Space Agency-led Solar Orbiter spacecraft flew through an S-shaped bend, known as a “switchback,” in the solar wind’s magnetic field. By studying the distinctive particles inside the switchback, Solar Orbiter traced its origin back to the Sun’s surface and revealed more about the solar magnetism responsible for dangerous solar storms.
The Sun has an intense and restless magnetic field. This field not only governs the star itself but also extends outward, carried by the solar wind—a stream of hot, charged particles, or plasma, that constantly flows from the Sun into space. These field lines can twist, break, or bend backward as they travel through the Solar System. Together, the solar wind and its dynamic magnetic field lines produce all kinds of activity that we can study to understand what is happening on and around our star.
In 2022, ESA reported that Solar Orbiter had detected a bend in the solar wind’s magnetic field known as a switchback. Although switchbacks have frequently been observed near the Sun, scientists still debate how they form. By observing from a distance, Solar Orbiter helped solve part of this puzzle, confirming that a switchback has an S shape—something scientists had predicted but had not yet seen directly.
The spacecraft has now added another important piece to the puzzle by tracing the bend back to its source on the Sun. “Solar Orbiter flew through a very large switchback,” says Jesse Coburn of CNRS/LPP, France, lead author of a new paper in Nature Astronomy. “This allowed us to sample rarely observed particles there that carry distinctive fingerprints of their origins.”
How Does a Switchback Form, and What Determines Its Motion?
To establish this connection, Coburn and his colleagues used Solar Orbiter’s Solar Wind Analyser (SWA) instrument to sample the plasma making up the switchback. At the time, the spacecraft was approximately halfway between Earth and the Sun. They detected a mixture of charged oxygen and carbon atoms that could have formed in only one way: inside hot magnetic loops on the Sun’s surface.
“There are two main competing theories about how switchbacks—and, along with them, the solar wind—form,” Coburn adds. “The particular mixture of particles detected by Solar Orbiter provides compelling evidence for a formation process known as interchange reconnection.”
This type of reconnection occurs when regions of the Sun with different magnetic properties interact. In the Sun’s atmosphere, open regions have field lines that stretch into the distance like highways, allowing material to travel rapidly along them into space. Closed regions have lines that initially extend into space and then bend back toward the Sun, forming closed loops. When an open region interacts with a closed one, the lines can come together, abruptly break, and reconnect in a different configuration, allowing plasma previously trapped inside a loop to escape into space.
That is what happened with this switchback—but there is more to the story. An alternative theory of switchback formation involves processes associated with waves and turbulence—the same kind of waves that Solar Orbiter identified as crucial to heating and accelerating the solar wind.
The scientists note that once a switchback leaves the Sun, waves and turbulence take over the physical processes and determine how the structure moves.
Overall, it appears that both processes—reconnection, and waves and turbulence—play a role in how switchbacks form and move through space. The researchers’ discovery brings these two approaches together, showing that they simply operate at different stages of a structure’s lifetime.
From Switchbacks to Solar Storms
To make this discovery, the researchers studied direct observations of switchback particles from Solar Orbiter’s SWA instrument, analyzed images of the solar disk, and modeled the magnetic fields of both the Sun and the surrounding space. They developed a new model to determine where the plasma originated. This model combined Solar Orbiter’s measurements with data from NASA’s Solar Dynamics Observatory to reveal the switchback’s solar source in exceptional detail.
Beyond switchbacks, the discovery sheds light on how the Sun heats its atmosphere and accelerates solar wind particles into space. It also shows that the Sun’s atmosphere leaves its signature on the particles making up this wind, allowing us to read the history of solar plasma even far from the Sun.
“The solar wind connects Earth to the Sun, and understanding its dynamics is crucial to how we can protect our planet from extreme space conditions,” says Daniel Müller, ESA’s Solar Orbiter project scientist. “The more we know, the better we can prepare for solar storms to protect our space infrastructure and technology.”
This discovery would simply not have been possible without Solar Orbiter—no other spacecraft combines the proximity to the Sun and the instruments needed to establish this connection. It is an excellent example of how the mission is delivering exactly the science we hoped for, connecting the Sun to its surroundings and revealing more details about our star.
