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Direct Measurement of Polar Coronal Hole-like Solar Wind in its Acceleration Phase
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Direct Measurement of Polar Coronal Hole-like Solar Wind in its Acceleration Phase

Samuel T Badman, Yeimy J Rivera, Michael L Stevens, Mari Paz Miralles, Steven R Cranmer, Andrea Verdini, Lynn B Wilson III, Xiangyu Wu, Georgios Nicolaou, Jean-Baptiste Dakeyo, …
arXiv
arXiv
08/31/2026
DOI: 10.48550/arxiv.2609.00318
url
https://doi.org/10.48550/arxiv.2609.00318View
Preprint (Author's original) This preprint has not been evaluated by subject experts through peer review. Preprints may undergo extensive changes and/or become peer-reviewed journal articles. Open Access

Abstract

The early evolution of fast polar coronal hole (PCH) solar wind remains largely unconstrained by in situ measurements. In March 2025, 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 indicates 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.
 Physics - Solar and Stellar Astrophysics Physics - Space Physics

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