The Parker Solar Probe and Solar Orbiter spacecraft fly close to the Sun and measure the solar wind — the stream of charged particles that constantly blows from our star. In this wind, you encounter sharp kinks in the magnetic field, shaped like the letter Z. At first, it seemed such kinks exist only where the wind is fast, and in the slow zone they disappear. New research shows the kinks never vanished; they just went unnoticed because of how the data were processed.
Previously, scientists checked for kinks by comparing the magnetic field to a short average taken over a few minutes. But the kinks themselves are large fluctuations that throw off that average, so the kinks kind of 'ate' themselves in the calculations. Also, they looked at the instantaneous wind speed: if it temporarily spiked near a kink, the whole point was attributed to the fast wind, even though the wind was actually slow on average. When the authors recalculated the data with a longer average (several hours) and compared the field to the direction of the Parker spiral (the stable trajectory of the solar wind), the kinks were found in the slow zone too.
The result changes the picture of where kinks originate. Previously, it was thought they are born only in the fast wind, and the slow wind has none. Now it's clear that kinks most likely arise in the quiet solar corona and are amplified as the wind expands in the slow phase. Then, in the fast wind, their larger versions continue to grow, while the small ones are destroyed by turbulence. So kinks are a common signature of the solar wind, not just a feature of fast streams.
as written by the authors arXiv:2602.03724
We analyze \emph{Parker Solar Probe} and \emph{Solar Orbiter} measurements of magnetic-field reversals (``switchbacks'') across the Alfv\'en surface ($$M_a\simeq 1$$), where $$M_a$$ is the Alfv\'en Mach number. The reported ``sub-Alfv\'enic switchback dropout'' follows from two diagnostic biases: conditioning on an instantaneous $$M_a$$, which is transiently elevated above unity by radial-velocity enhancements during large-amplitude Alfv\'enic rotations, and short-window local-mean backgrounds that partially track these rotations and suppress deflection angles. Treating $$M_a$$ as a bulk-stream property via rolling medians and referencing deflections to event-independent backgrounds -- a Parker-spiral direction or a sufficiently long rolling median -- recovers sub-Alfv\'enic switchbacks systematically. The mean deflection $$\langle \theta \rangle$$ separates into two regimes with $$M_a$$. For $$M_a \lesssim 1$$, $$\langle \theta \rangle$$ rises rapidly with weak dependence on the background window, consistent with expansion-driven amplification of Alfv\'enic fluctuations. For $$M_a \gtrsim 1$$, the evolution becomes scale dependent: large-scale $$\langle \theta \rangle$$ continues to grow with $$M_a$$ at reduced rate, while small-scale growth saturates, consistent with turbulent decay and dissipation. Collectively, these results indicate that switchbacks need not originate only in the super-Alfv\'enic solar wind. Instead, they are consistent with a formation pathway in which coronal fluctuations are amplified by large-scale expansion through the sub-Alfv\'enic regime, with subsequent propagation into the super-Alfv\'enic wind where turbulent decay modifies their scale-dependent properties.
The solar wind blows at speeds from 300 to 800 kilometers per second, but even in its fastest part, it is thousands of times more rarefied than the air in a room.