Solar Orbiter traces a magnetic switchback to hot loops on the Sun's surface

The measurement worked because the switchback was big. "Solar Orbiter flew through a very large switchback," Coburn said. "Because of this, we were able to sample rarely observed particles there that have tell-tale fingerprints of their origin." ^[3] In 2022 the same spacecraft confirmed from a distance that a switchback is S-shaped. Scientists had predicted that shape but had never seen it directly ^[12]. This time the spacecraft measured what was inside one.

The fingerprint is the plasma's composition. The instrument found a mix of charged oxygen and carbon that, according to the team, could only have formed in one way: inside hot magnetic loops at the Sun's surface ^[5]. The ions show what kind of region the plasma came from. Images of the solar disc and models of the magnetic field, from the Sun out into space, put that region at a location ^[11]. The release does not give the measured ion abundances or an uncertainty on the source location.

"There are two main competing theories for how a switchback, and by extension the solar wind, forms," Coburn said ^[6]. In interchange reconnection, a region of open field lines, which stretch out into space, meets a region of closed loops. The lines reconnect, and plasma that had been trapped in a loop escapes ^[7]. The rival account centres on waves and turbulence. Solar Orbiter has already found those processes heating and accelerating the wind ^[8]. The team saw signs of them in this event too, "but likely only after the switchback heads out into space," said co-author Stephanie Yardley of Northumbria University ^[9]. "Our finding reconciles the two, showing that they simply operate at different stages in a switchback's lifetime," she said ^[10].

The thing this doesn't tell you is how often it happens. Switchbacks are seen often near the Sun, and how they form is still debated ^[13]. This result comes from one of them ^[1]. I think the single crossing is enough to show that interchange reconnection can launch a switchback full of plasma from a surface loop. Showing that it launches most of them, or that none start in turbulence, will take the same composition signature turning up in other events.

The constraint the measurement supplies is on how switchbacks form ^[6]. ESA's release connects it to space weather through solar magnetism in general, which it calls "the unruly instigator of dangerous solar storms" ^[15].