Characterization of the Trans-Alfvénic Region Using Observations from Parker Solar Probe

TL;DR

The paper extends the analysis of the trans-Alfvénic transition by using Parker Solar Probe data from Encounters 8–19 to classify solar wind intervals into sub-, trans-, and super-Alfvénic via the Alfvén Mach number $M_A$ and to quantify turbulence amplitude $\delta B/B$, variance anisotropy $A_b$, intermittency via partial variance of increment (PVI), and switchbacks with the parameter $z$. It finds that sub-Alfvénic wind has smaller normalized turbulence, greater variance anisotropy, similar intermittency to super-Alfvénic wind, and weaker switchbacks, consistent with coronal-like turbulence and RMHD-dominated dynamics near the Sun. The results support a picture where, closer to the Sun, the solar wind transitions from a more compressible, three-dimensional MHD regime to a highly anisotropic, low-$\beta$ RMHD-like regime, with implications for coronal heating and solar-wind acceleration models.

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, Bandyopadhyay et al. (2022) performed a turbulence characterization of the sub-Alfvénic solar wind with initial data from encounters 8 and 9. In this study, we re-examine the turbulence properties such as turbulence amplitude, anisotropy of the magnetic field variance, intermittency and switchback strength extending with PSP data for 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. 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 degrees switchbacks in the sub-Alfvénic solar wind

Figures (6)

• Figure 1: Comparison of distributions of the Mach number $M_A$ for the analyzed dataset shown in Fig. \ref{['fig:PSP_data']} with the total datasets in encounters $8-19$. The data set is divided into sub-Alfvénic ($M_A<0.85$), trans-Alfvénic ($0.85\leq M_A\leq 1.15$) and super-Alfvénic ($M_A>1.15$) samples by the red vertical dashed lines. Embedded: Comparision of the number of datasets in each category (sub-Alfvénic, trans-Alfvénic and super-Alfvénic).
• Figure 2: Probability distribution function (PDF) of (a) the magnetic-field turbulence amplitude (in nT) and (b) the normalized magnetic-field turbulence amplitude in the sub-Alfvénic and super-Alfvénic solar wind intervals as observed by the PSP in encounters $8-19$.
• Figure 3: Probability distribution function of variance anisotropy in the sub-Alfvénic and super-Alfvénic solar wind intervals observed by PSP in encounters $8-19$. The vertical solid (black) line indicates the value $A_b = 2$, which corresponds to isotropic distribution.
• Figure 4: Probability distribution function of PVI in the sub-Alfvénic, and super-Alfvénic solar wind intervals observed by PSP in encounters $8-19$. Values of PVI$>2.5$ represent the non-Gaussian and coherent structures such as current sheets.
• Figure 5: Probability distribution function of switchback parameter ($z$) in the sub-, and super-Alfvénic solar wind intervals observed by PSP in encounters $8-19$. The vertical solid (black) line indicates the value $z = 1/2$, which corresponds to a marginal reversal of polarity of the magnetic field. Higher z values represent stronger switchback.