Long-lived Magnetic Switchbacks Tracked across 0.32 au through BepiColombo–Solar Orbiter Radial Alignment

Abstract Magnetic switchbacks, often observed in the near-Sun solar wind, have received increased interest in recent years due to their potential role in mediating the heating and acceleration of the solar wind, but their origin remains debated. In this work, we present a coordinated observation of a switchback cluster by BepiColombo (0.35 au) and Solar Orbiter (0.67 au), obtained during the alignment between 2021 October 6 and 8, which enabled the first direct investigation of switchback evolution across heliocentric distances. In particular, the stream observed by the spacecraft can be tracked back to the boundary of an equatorial coronal hole. Large-scale switchbacks exhibit strong sublinear expansion, thus appearing almost unevolved in morphology during the propagation when the spacecraft cutting-angle effect is taken into account. The stable magnetic configuration of the analyzed switchbacks suggests that they can be identified as small-scale flux ropes. Indeed, for shear-driven instabilities triggered by stream interaction with the background slow wind, short-living (one eddy turnover time, τ ∼ 1 hr) switchbacks would be expected compared to the travel time from BepiColombo to Solar Orbiter (∼38 hr). These findings provide critical insights on switchback origin and evolution, potentially constraining future phenomenologies on their formation. A useful consequence of our observations is that statistical analyses on switchback evolution should always account for the cutting-angle effect.