During the solar minimum, when the Sun is at its least active, the solar wind1,2 is observed at high latitudes as a predominantly fast (more than 500 kilometres per second), highly Alfvénic rarefied stream of plasma originating from deep within coronal holes. Closer to the ecliptic plane, the solar wind is interspersed with a more variable slow wind3 of less than 500 kilometres per second. The precise origins of the slow wind streams are less certain4; theories and observations suggest that they may originate at the tips of helmet streamers5,6, from interchange reconnection near coronal hole boundaries7,8, or within coronal holes with highly diverging magnetic fields9,10. The heating mechanism required to drive the solar wind is also unresolved, although candidate mechanisms include Alfvén-wave turbulence11,12, heating by reconnection in nanoflares13, ion cyclotron wave heating14 and acceleration by thermal gradients1. At a distance of one astronomical unit, the wind is mixed and evolved, and therefore much of the diagnostic structure of these sources and processes has been lost. Here we present observations from the Parker Solar Probe15 at 36 to 54 solar radii that show evidence of slow Alfvénic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals that are associated with jets of plasma and enhanced Poynting flux and that are interspersed in a smoother and less turbulent flow with a near-radial magnetic field. Furthermore, plasma-wave measurements suggest the existence of electron and ion velocity-space micro-instabilities10,16 that are associated with plasma heating and thermalization processes. Our measurements suggest that there is an impulsive mechanism associated with solar-wind energization and that micro-instabilities play a part in heating, and we provide evidence that low-latitude coronal holes are a key source of the slow solar wind.
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The data used in this study are available at the NASA Space Physics Data Facility (SPDF), https://spdf.gsfc.nasa.gov/index.html.
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The FIELDS experiment on the Parker Solar Probe spacecraft was designed and developed under NASA contract NNN06AA01C. The FIELDS team acknowledges the contributions of the Parker Solar Probe mission operations and spacecraft engineering teams at the Johns Hopkins University Applied Physics Laboratory. S.D.B. acknowledges the support of the Leverhulme Trust Visiting Professorship programme. Contributions from S.T.B. were supported by NASA Headquarters under the NASA Earth and Space Science Fellowship Program grant 80NSSC18K1201. This work uses data obtained by the Global Oscillation Network Group (GONG) programme, managed by the National Solar Observatory, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. The data were acquired by instruments operated by the Big Bear Solar Observatory, High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de Astrofísica de Canarias and Cerro Tololo Interamerican Observatory. D.B. was supported by UK STFC grant ST/P000622/1. J.P.E. and T.S.H. were supported by UK STFC grant ST/S000364/1. D.S. was supported by UK STFC grant ST/N000692/1. C.H.K.C. is supported by STFC Ernest Rutherford Fellowship number ST/N003748/2. T.D.d.W. and V.V.K. are supported by CNES.
The authors declare no competing interests.
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Extended data figures and tables
Extended Data Fig. 1 Variation of PFSS neutral line topology with time and magnetogram choice at RSS = 2.5R⊙.
Colour maps of BR at the source surface from PFSS extractions with source-surface radius RSS = 2.5R⊙. Red indicates positive polarity and blue indicates negative polarity. The black line shows the PIL (the contour of BR = 0). Superposed is the ballistically projected PSP trajectory coloured by the measured polarity. Perihelion occurred around 330° longitude. Left to right, the columns show extractions from the NSO/GONG, SDO/HMI and DeRosa LMSAL models. From top to bottom, the models are evaluated at a weekly cadence spanning six weeks about perihelion, with input magnetograms from each source taken as close in time as possible. The grey shading shows the region ±60° about the central meridian on the date of the model evaluation, indicating the portion of the Sun that could be observed at the time of observation.
Extended Data Fig. 2 Variation of PFSS neutral line topology with time and magnetogram choice at RSS = 2.0R⊙.
Colour maps of BR at the source surface from PFSS extractions with RSS = 2.0R⊙. Other features are as described in Extended Data Fig. 1.
Extended Data Fig. 3 Variation of PFSS Neutral Line topology with time and magnetogram choice at RSS = 1.2R⊙.
Colour maps of BR at the source surface from PFSS extractions with RSS = 1.2R⊙. Other features are as described in Extended Data Fig. 1.
Extended Data Fig. 4 Synoptic maps of extreme-ultraviolet coronal emission from Carrington rotation 2,210, assembled from the STEREO-A/EUVI and SDO/AIA instruments.
Top, 171-Å data showing coronal Fe ix emission at around 600,000 K. This is the background of Fig. 1c, d. Bottom, 193-Å (AIA) and 195-Å (EUVI) data showing emission from coronal Fe xii emission at around 1,000,000 K. The brightness is positively correlated with the integrated plasma density squared along the line of sight. The dark regions in both images are probable locations of coronal holes, which are threaded by open magnetic field lines that allow plasma to evacuate into interplanetary space, resulting in under-dense regions. Carrington rotation 2,210 occurred from 20:51 26 October 2018 ut to 04:11 23 November 2018 ut.
Extended Data Fig. 5 During encounter 1, PSP connected magnetically to a small negative-polarity equatorial coronal hole.
This schematic shows a potential field extrapolation of the solar magnetic field at the time of the first perihelion pass of PSP. The solar surface is shown, coloured by AIA 211-Å extreme-ultraviolet emission (see Extended Data Fig. 4 for other wavelengths). Coronal holes appear as a lighter shade. Superposed are various field lines initialized at the solar disk. Black lines indicate closed loops, blue and red illustrate open field lines with negative and positive polarities, respectively. As depicted here and in Fig. 1c, d, at perihelion PSP connected to a negative equatorial coronal hole. The ‘switchbacks’ (the jets) observed by PSP (Fig. 1a) are illustrated as kinks in the open field lines emerging from the coronal hole that connect to PSP. (Note that neither the radial distance to the spacecraft nor the scale or amplitude of the jets or switchbacks are to scale.) Spacecraft graphic is courtesy of NASA/Johns Hopkins APL.
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Bale, S.D., Badman, S.T., Bonnell, J.W. et al. Highly structured slow solar wind emerging from an equatorial coronal hole. Nature 576, 237–242 (2019) doi:10.1038/s41586-019-1818-7