Direct Measurement of Polar Coronal Hole-like Solar Wind in Its Acceleration Phase
Abstract
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.
// Source
Authors: Samuel T. Badman, Yeimy J. Rivera, Michael L. Stevens, M. P. Miralles, Steven R. Cranmer, Andrea Verdini, Lynn B. Wilson III, Xiangyu Wu, Georgios Nicolaou, Jean-Baptiste Dakeyo, Kai Jaffarove, Tamar Ervin, D. Payne, Michael Terres, Kristopher G. Klein, Jasper S Halekas, Sujan Prasad Gautam, William H. Matthaeus, Riddhi Bandyopadhyay, Srijan Bharati Das, Tatiana Niembro, Christopher John Owen, Rungployphan Kieokaew, Ali Rahmati, Orlando Romeo, Federico Fraschetti, Marco Velli, Lorenzo Matteini, K. W. Paulson, Lidiya Ahmed, Stuart D. Bale, Phyllis L. Whittlesey, M. Pulupa, Nour Eddine Raouafi, T. S. Horbury, Jean Morris, Helen O’Brien, R. Livi, Davin Larson, Adam J. Finley, P. Louarn
Institutions: Imperial College London, Harvard University, University of Michigan, University of Iowa, University College London, University of Arizona, Goddard Space Flight Center, University of Florence, University of Delaware, University of California System, Johns Hopkins University Applied Physics Laboratory, European Space Agency, Center for Astrophysics Harvard & Smithsonian, Université de Toulouse