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ABSTRACT Close to the Sun, Parker Solar Probe (PSP) traverses the sub-Alfvénic solar wind, a magnetically controlled plasma environment. Farther from the Sun, the magnetic field amplitude and plasma density weaken enough to establish a super-Alfvénic environment where kinetic energy and turbulence become more prominent. Switchbacks (i.e. large directional deviations from the mean field) are shown to occur almost exclusively in the super-Alfvénic regime. In this study, we analyse magnetic fluctuations from PSP encounters 8 through 19 to study their relationship to Alfvén Mach number ($$M_\mathrm{ A}$$) and switchback parameter (Z). We find that the fluctuation of the magnetic field magnitude normalized to the mean magnetic field and of the radial velocity normalized to local Alfvén speed increase with both Alfvén Mach number and switchback parameter. However, there is distinct saturation in the increments of the normalized radial velocity fluctuations in the Alfvén Mach number range of $$\sim$$4–6. Results are interpreted in terms of the Chandrasekhar criterion for Kelvin–Helmholtz activity. Overall, these findings are in agreement with earlier studies suggesting switchback generation through non-linear shear flow dynamics.
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This content will become publicly available on January 28, 2027
Characterization of the Alfvén Transition in the Young Solar Wind using Parker Solar Probe Observations Approaching Solar Maximum
Abstract Close to Earth, the solar wind is usually super-Alfvénic, i.e., the speed of the solar wind is much larger than the Alfvén speed. However, in the lower coronal regions, the solar wind is mostly sub-Alfvénic. With the Parker Solar Probe (PSP) crossing the boundary between the sub- and super-Alfvénic flow, R. Bandyopadhyay et al. performed a turbulence characterization of the sub-Alfvénic solar wind with initial data from encounters 8 and 9. In this study, we reexamine the turbulence properties such as turbulence amplitude, anisotropy of the magnetic field variance, intermittency, and switchback strength using PSP data from encounters 8–19. The later orbits probe lower altitudes and experience sub-Alfvénic conditions more frequently, providing a greater statistical coverage to contrast sub- and super-Alfvénic solar wind. These later orbits also extend the observations from near solar minimum at launch to near solar maximum conditions. Also, by isolating the intervals where the solar wind speed is approximately equal to the Alfvén speed, we explore the transition in more detail. We show that the amplitude of the normalized magnetic field fluctuation is smaller for the sub-Alfvénic samples. While solar wind turbulence in general is shown to be anisotropic, the sub-Alfvénic samples are more anisotropic than the super-Alfvénic samples, in general. Further, we show that the sub- and super-Alfvénic samples do not show much distinction in terms of intermittency strength. Finally, consistent with prior results, we find no evidence for polarity reversing >90° switchbacks in the sub-Alfvénic solar wind.
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- Award ID(s):
- 2347952
- PAR ID:
- 10696481
- Publisher / Repository:
- The American Astronomical Society
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 997
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 259
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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