The prediction of a supersonic solar wind1 was first confirmed by spacecraft near Earth2,3 and later by spacecraft at heliocentric distances as small as 62 solar radii4. These missions showed that plasma accelerates as it emerges from the corona, aided by unidentified processes that transport energy outwards from the Sun before depositing it in the wind. Alfvénic fluctuations are a promising candidate for such a process because they are seen in the corona and solar wind and contain considerable energy5,6,7. Magnetic tension forces the corona to co-rotate with the Sun, but any residual rotation far from the Sun reported until now has been much smaller than the amplitude of waves and deflections from interacting wind streams8. Here we report observations of solar-wind plasma at heliocentric distances of about 35 solar radii9,10,11, well within the distance at which stream interactions become important. We find that Alfvén waves organize into structured velocity spikes with duration of up to minutes, which are associated with propagating S-like bends in the magnetic-field lines. We detect an increasing rotational component to the flow velocity of the solar wind around the Sun, peaking at 35 to 50 kilometres per second—considerably above the amplitude of the waves. These flows exceed classical velocity predictions of a few kilometres per second, challenging models of circulation in the corona and calling into question our understanding of how stars lose angular momentum and spin down as they age12,13,14.
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The SWEAP Investigation and this study are supported by the PSP mission under NASA contract NNN06AA01C. The SWEAP team expresses its gratitude to the scientists, engineers and administrators who have made this project a success, both within the SWEAP institutions and from NASA and the project team at JHU/APL. J.C.K. acknowledges support from the 2019 Summer School at the Center for Computational Astrophysics, Flatiron Institute. The Flatiron Institute is supported by the Simons Foundation. S.D.B. acknowledges the support of the Leverhulme Trust Visiting Professorship programme. T.S.H. was supported by UK STFC ST/S0003641/1.
The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
The figure is in the same format as Fig. 1. Spikes in the velocity are again seen to be coincident with the magnetic-field reversals, but the jump in the speed is smaller, probably because the Alfvén speed was lower in E2 than E1. The density at perihelion is substantially lower.
Extended Data Fig. 2 Schematic of an S-shaped magnetic structure creating a field reversal, heat-flux reversal and a spike in velocity.
This figure illustrates the possible geometry of an S-shaped propagating Alfvénic disturbance (grey box) and how it would appear to the spacecraft (black square) as it flew through the spike on the green trajectory. The pink lines with arrows indicate the configuration of the magnetic field, with all field lines ultimately pointing back to the Sun. Arrows at each black square indicate the vector velocity (blue), electron strahl (orange) and magnetic field (red) seen by the spacecraft. If this was a purely Alfvénic structure, then the spike would move away from the Sun in an antiparallel direction to B at the local Alfvén speed, CA. In the frame of the spike, the shape of the structure would be static, with plasma flowing in along field lines on the upper left and through the spike and emerging at the lower right, always flowing at CA. In the frame of the spacecraft, the constant flow along field lines in the propagating spike frame would translate into a radial increase of V by CA when B is perpendicular to the R direction, and a maximum jump of 2CA when B is completely inverted. Because the heat flux flows away from the Sun along magnetic field lines, it would rotate so as to always be antiparallel to B and appear locally to be flowing back to the Sun at the centre of this disturbance.
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Kasper, J.C., Bale, S.D., Belcher, J.W. et al. Alfvénic velocity spikes and rotational flows in the near-Sun solar wind. Nature 576, 228–231 (2019). https://doi.org/10.1038/s41586-019-1813-z
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