Direct Measurement of Polar Coronal Hole-like Solar Wind in Its Acceleration Phase
Year: 2026
Type: article
Abstract: The early evolution of fast polar coronal hole (PCH) solar wind remains largely unconstrained by in situ measurements. In 2025 March, Parker Solar Probe (Parker) at its closest approach of 9.86 solar radii ( R ⊙ ) measured outflow from a large equatorial coronal hole (ECH), which was also measured at 1 au and at intermediate distances by Solar Orbiter (also near its perihelion). At 1 au the stream properties are consistent with PCH properties established by Ulysses. The stream was measured by Parker substantially below the Alfvén surface, with proton temperatures in excess of 2 MK and a speed at ∼10 R ⊙ which was only ∼60% of its asymptotic value. The Solar Orbiter data indicate that the acceleration is largely complete by 60 R ⊙ . Spherically polarized fluctuations in the stream are observed to develop from near-transverse and small-angle at Parker to full reversal “switchbacks” at Solar Orbiter. Comparison of the implied acceleration profile to historical Doppler-dimming measurements suggests that the stream’s low coronal acceleration is similar to that of PCH flows. Consistent with previous work, this acceleration requires significantly more energy than can be provided by the observed thermal pressure gradients, with a significant contribution likely from the abundant Alfvénic fluctuation energy observed at Parker. These observations provide unique constraints on models of the radial evolution of the fastest solar wind, and indicate that these wind streams experience gradual, steady acceleration over their first few tens of solar radii of evolution.
Authors: Samuel T. Badman, Yeimy J. Rivera, Michael Louis Stevens, Mari Paz Miralles, Steven R. Cranmer, Andrea Verdini, Lynn B. Wilson, Xiangyu Wu, Georgios Nicolaou, Jean-Baptiste Dakeyo, Kai Jaffarove, Tamar Ervin, Dominic S. Payne, Michael Terres, Kristopher G. Klein, Jasper S. Halekas, Sujan Prasad Gautam, William H Matthaeus, Riddhi Bandyopadhyay, Srijan Bharati Das, Tatiana Niembro, Christopher J. Owen, Rungployphan Kieokaew, Ali Rahmati, Orlando M. Romeo, Federico Fraschetti, Marco Velli, Lorenzo Matteini, Kristoff W. Paulson, Lidiya Ahmed, Stuart D. Bale, Phyllis Whittlesey, Marc Pulupa, Nour E. Raouafi, Timothy S. Horbury, Jean Morris, Helen O’Brien, Roberto Livi, Davin E. Larson, Adam J. Finley, Philippe Louarn
Institutions: Center for Astrophysics Harvard & Smithsonian, University of Florence, Goddard Space Flight Center, University College London, University of California System, University of Michigan, University of Arizona, University of Iowa, University of Delaware, Université de Toulouse, Imperial College London, Harvard University, Johns Hopkins University Applied Physics Laboratory, European Space Agency
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $2,400
Funders: National Science Foundation, National Aeronautics and Space Administration, University of Delaware, Princeton University, Johns Hopkins University, UK Space Agency, UK Research and Innovation, European Space Agency, Centre National d’Etudes Spatiales, Science and Technology Facilities Council
OpenAlex ID: W7206172083