Fast solar wind was already turbulent while Parker Solar Probe was still deep in the Sun's outer atmosphere, a new analysis of the spacecraft's measurements finds. The study examines a stream described in an earlier paper published August 31. Its new contribution is a closer look at the motion and magnetic fluctuations within that stream.
The solar wind is a flow of charged particles from the Sun. During a March 22–23, 2025 pass, Parker sampled a fast stream about 10 solar radii from the Sun's center, where the flow was mostly above 400 kilometers per second. It was still in a region where magnetic disturbances can travel against the outflow. That made it possible to study the wind before it reached the more freely flowing conditions usually measured farther away.
The researchers combined measurements of the wind's speed and magnetic field to track fluctuations over different timescales. Most of the fluctuation energy traveled outward from the Sun, but a smaller inward-moving component was present. Magnetic changes ran mainly across the average magnetic field rather than along it: the parallel component accounted for less than 10% of the total fluctuation power for most of the selected interval.
The distribution of magnetic power across scales also followed a pattern associated with turbulence: it declined through a broad range of larger scales, then changed sharply near the scale at which proton motion matters. A separate calculation found that energy transfer generally ran from larger toward smaller scales. Together, these measurements support the authors' conclusion that a turbulent cascade had developed close to the Sun, rather than appearing only after the wind moved much farther out.
The earlier paper linked the fast stream to a large coronal hole, a region where magnetic field lines open into space, although the exact point where the wind left the Sun remains uncertain. The new study tests the stream's turbulence properties. Its proton-velocity measurements needed filtering because the instrument did not always capture the full particle distribution. The analysis covers one fast stream, so it cannot establish whether the same turbulence pattern is typical of fast wind or which process generated its inward-moving fluctuations.
Sources
- Bandyopadhyay and colleagues, turbulence analysis, posted September 22, 2026; especially Section II, Figures 2–3 and 6, and Section III.
- Badman and colleagues, earlier analysis of the same solar-wind stream, posted August 31, 2026.
- Lead image: Parker Solar Probe artist's rendering, NASA/Johns Hopkins APL/Steve Gribben; public domain in the United States. The illustration does not depict the measured 2025 stream.
