Highly structured slow solar wind emerging from an equatorial coronal hole

S. D. Bale (), S. T. Badman, J. W. Bonnell, T. A. Bowen, D. Burgess, A. W. Case, C. A. Cattell, B. D. G. Chandran, C. C. Chaston, C. H. K. Chen, J. F. Drake, T. Dudok Wit, J. P. Eastwood, R. E. Ergun, W. M. Farrell, C. Fong, K. Goetz, M. Goldstein, K. A. Goodrich, P. R. Harvey, T. S. Horbury, G. G. Howes, J. C. Kasper, P. J. Kellogg, J. A. Klimchuk, K. E. Korreck, V. V. Krasnoselskikh, S. Krucker, R. Laker, D. E. Larson, R. J. MacDowall, M. Maksimovic, D. M. Malaspina, J. Martinez-Oliveros, D. J. McComas, N. Meyer-Vernet, M. Moncuquet, F. S. Mozer, T. D. Phan, M. Pulupa, N. E. Raouafi, C. Salem, D. Stansby, M. Stevens, A. Szabo, M. Velli, T. Woolley and J. R. Wygant
Additional contact information
S. D. Bale: University of California
S. T. Badman: University of California
J. W. Bonnell: University of California
T. A. Bowen: University of California
D. Burgess: Queen Mary University of London
A. W. Case: Smithsonian Astrophysical Observatory
C. A. Cattell: University of Minnesota
B. D. G. Chandran: University of New Hampshire
C. C. Chaston: University of California
C. H. K. Chen: Queen Mary University of London
J. F. Drake: University of Maryland
T. Dudok Wit: University of Orléans, CNRS
J. P. Eastwood: Imperial College London
R. E. Ergun: University of Colorado
W. M. Farrell: Code 695, NASA Goddard Space Flight Center
C. Fong: University of California
K. Goetz: University of Minnesota
M. Goldstein: University of Maryland Baltimore County
K. A. Goodrich: University of California
P. R. Harvey: University of California
T. S. Horbury: Imperial College London
G. G. Howes: University of Iowa
J. C. Kasper: Smithsonian Astrophysical Observatory
P. J. Kellogg: University of Minnesota
J. A. Klimchuk: NASA Goddard Space Flight Center
K. E. Korreck: Smithsonian Astrophysical Observatory
V. V. Krasnoselskikh: University of Orléans, CNRS
S. Krucker: University of California
R. Laker: Imperial College London
D. E. Larson: University of California
R. J. MacDowall: Code 695, NASA Goddard Space Flight Center
M. Maksimovic: Université PSL, Sorbonne Université, CNRS
D. M. Malaspina: University of Colorado
J. Martinez-Oliveros: University of California
D. J. McComas: Princeton University
N. Meyer-Vernet: Université PSL, Sorbonne Université, CNRS
M. Moncuquet: Université PSL, Sorbonne Université, CNRS
F. S. Mozer: University of California
T. D. Phan: University of California
M. Pulupa: University of California
N. E. Raouafi: Johns Hopkins University Applied Physics Laboratory
C. Salem: University of California
D. Stansby: Imperial College London
M. Stevens: Smithsonian Astrophysical Observatory
A. Szabo: NASA Goddard Space Flight Center
M. Velli: University of California
T. Woolley: Imperial College London
J. R. Wygant: University of Minnesota

Nature, 2019, vol. 576, issue 7786, 237-242

Abstract: Abstract 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.

Date: 2019
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DOI: 10.1038/s41586-019-1818-7

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