Direct Measurements Map Fast Solar-Wind Acceleration

Fast solar wind from polar coronal holes is a standard reference case for heliospheric models, but its early acceleration has rarely been sampled directly. Badman et al. follow one unusually favorable coronal-hole-like stream across three distances in March 2025: Parker Solar Probe at 9.86 solar radii, Solar Orbiter near 60 solar radii, and Wind near Earth’s L1 point. The alignment lets the authors compare a common source stream through the interval where its velocity, thermal state, and Alfvénic fluctuations change most rapidly.

Core Contribution

The paper’s contribution is a radial, in-situ measurement of a fast stream while it is still accelerating below the Alfvén surface. Parker observed the flow at roughly 10 solar radii with a speed only about 60% of its asymptotic value, while the Solar Orbiter measurements indicate that acceleration is largely complete by 60 solar radii. The stream measured at 1 au has properties consistent with the polar-coronal-hole wind characterized by Ulysses, despite originating from a large equatorial coronal hole during this observing geometry.

That comparison matters because it ties low-coronal inferences to a single evolving plasma stream rather than to statistical populations collected at different times and source regions. The authors interpret the agreement with historical Doppler-dimming constraints as evidence that the low-coronal acceleration of this coronal-hole-like stream resembles polar-coronal-hole outflow.

Technical Approach

The analysis ballistically maps the Parker Solar Probe, Solar Orbiter, and Wind trajectories back to 2.5 solar radii, then traces potential-field source-surface magnetic footpoints to the photosphere. Figure 1 places those mapped paths on an EUV image from 24 March 2025. The three footpoint sets closely overlap, supporting the premise that the instruments sampled the same broad fast-wind structure rather than unrelated streams.

The authors then compare radial magnetic field, density, proton speed, Alfvén speed, and proton and electron temperatures as functions of heliographic longitude. Their stream selection emphasizes the velocity peak, with median-filtered baselines used to separate the large-scale profile from short-timescale variability. They also estimate the Alfvén surface from each spacecraft’s measurements and use velocity-space distributions to characterize the stream’s fluctuations.

At Parker, the proton temperature exceeds 2 MK and the flow remains substantially sub-Alfvénic. By Solar Orbiter’s distance, the velocity profile has approached its 1-au value. The paper compares those measurements with distance-binned Parker and Solar Orbiter statistics, coronal UVCS constraints, and externally forced iso-polytropic profiles. This is not a new acceleration model; it is a measurement-led constraint on which modeled radial profiles are compatible with the event.

Results and Analysis

Figure 2 shows the coordinated plasma measurements across the three radii. The near-Sun observation is the decisive result: the stream has reached only about 60% of its eventual speed at approximately 10 solar radii, yet Solar Orbiter suggests that the remaining acceleration is mostly over by 60 solar radii. The authors therefore argue for gradual, steady acceleration through the first few tens of solar radii, rather than an impulse concentrated entirely in the low corona.

The energy accounting sharpens the physical implication. The paper reports that observed thermal pressure gradients cannot provide the acceleration required by the inferred profile. Parker instead sees abundant Alfvénic fluctuation energy, which the authors identify as a likely major contributor. This supports a wave-mediated contribution, but the measurements do not directly establish the dissipation or momentum-transfer mechanism responsible for each increment of acceleration.

The fluctuation evolution is also informative. Near Parker, the velocity perturbations are near-transverse and occupy small angular excursions; at Solar Orbiter they develop into full-reversal switchbacks. Figure 3 visualizes this progression in velocity space and relates it to the local Alfvénic fluctuation scale. The observation constrains models that treat switchbacks as either fully formed at the source or generated only far from the Sun: in this event, their large-amplitude reversal character develops between the two sampling radii.

Figure 4 puts the peak stream measurements against historical spacecraft statistics and low-coronal ultraviolet observations. The agreement is persuasive for this event, especially because the three in-situ points are source-connected. Still, the result is a detailed case study, not a survey of coronal holes across solar conditions. Its value is as a stringent radial benchmark for solar-wind acceleration and fluctuation-evolution models.