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Alfvénic velocity spikes and rotational flows in the near-Sun solar wind

Abstract

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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Fig. 1: Overview of the first encounter of PSP with the Sun.
Fig. 2: Solar-wind fluctuations near the closest approach.
Fig. 3: A closer look at a velocity spike.
Fig. 4: Large circulation of solar wind observed near the Sun.

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Acknowledgements

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.

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Authors and Affiliations

Authors

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J.C.K. is the SWEAP Principal Investigator (PI) and led the data analysis and writing of this Article. S.D.B. is the FIELDS PI and a SWEAP Co-Investigator and provided the magnetic-field observations. J.W.B. leads the US group that developed the solar-wind Faraday cup, and provided guidance on identifying Alfvénic fluctuations. M.B. provided a pre-amplifier ASIC used within the SPAN electron instruments. A.W.C. is the SPC instrument scientist and ensured that the instrument met its performance requirements and was calibrated. B.D.G.C. contributed to the theoretical calculations and the writing of the manuscript. D.W.C. managed the effort at UCB. D.G. was the institutional lead at NASA MSFC, responsible for materials testing and calibration of SPC. S.P.G. provided recommendations on measurement requirements to detect instabilities. L.G. provided related solar observations and results. J.S.H. contributed to the analysis of the electron observations and to the writing of the manuscript. G.C.H. provided a time-of-flight ASIC to reduce the size and power of the SPAN ion instrument. T.S.H. participated in the analysis of the Alfvénic spikes. Q.H. identified magnetic flux ropes. K.G.K. contributed to the writing of the manuscript and provided warm-plasma growth rate calculations. K.E.K. led the SWEAP Science Operations Center and coordinated observing plans between the instruments and the project. M.V. contributed to the writing of the manuscript and the discussion on the relationship between Alfvénic fluctuations and angular momentum. D.E.L. is the institutional lead at Berkeley, responsible for the implementation of the SPAN instruments and the SWEAP Electronics Module suite-wide computer. R.L. is the SPAN ion instrument scientist. B.L. identified flux ropes and other signatures of coronal mass ejections in the data. P.L. coordinated solar furnace testing of the SPC materials before launch. M. Maksimovic peformed the absolute calibration of the density measurements. M. Martinovic evaluated the quality of the velocity distribution functions. N.V.P. carried out numerical simulations. J.D.R. contributed to the design of the Faraday cup. R.M.S. helped to interpret the electron pitch-angle distributions. J.T.S. identified potential field rotation causes. M.L.S. provided the overall data pipeline for SWEAP and SPC high-level data products. A.S. estimated the location of the heliospheric current sheet. P.L.W. set up the SPC calibration at MSFC and then became SPAN electron instrument scientist at Berkeley. K.H.W. arranged the SPC calibration at MSFC. G.P.Z. leads the SWEAP theory team. R.J.M. leads the FIELDS fluxgate magnetometer. D.J.M. is the ISIS PI and provided the energetic-particle data. R.L.M. leads the EPI-Lo energetic-particle instrument. M.P. is the FIELDS SOC lead. N.E.R. is the PSP Project Scientist and reviewed jets and similar coronal transients. N.A.S. runs the ISIS Science Operations Center. All authors participated in planning the observations and data collection, reviewed and discussed the observations, and read, provided feedback and approved the contents of the manuscript.

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Correspondence to J. C. Kasper.

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Extended data figures and tables

Extended Data Fig. 1 Overview of the second PSP encounter with the Sun.

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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