Characterization of compressible fluctuations in solar wind streams dominated by balanced and imbalanced turbulence: Parker Solar Probe, Solar Orbiter and Wind observations

TL;DR

This work addresses how compressible fluctuations arise and evolve in solar wind streams that are either balanced or imbalanced in their turbulence by analyzing multi-spacecraft data (Wind, Parker Solar Probe, Solar Orbiter) across 0.1–1 au. It classifies intervals using cross-helicity $\sigma_c$ and employs wavelet coherence between density and magnetic-pressure fluctuations to identify mode content, highlighting the role of local plasma conditions via the beta parameter $\beta$. The key finding is that slow magnetosonic modes largely dominate compressible fluctuations in both turbulence regimes, with near-Sun density fluctuations being elevated and magnetic compressibility increasing with heliocentric distance, while fast-mode–like fluctuations in Alfvénic wind do not conform to standard MHD predictions, indicating the need for extended theory that includes expansion and kinetic effects. The results constrain solar wind heating/acceleration mechanisms and point to slow-mode dynamics as a significant contributor near the Sun, motivating further theoretical and numerical work on compressible turbulence in expanding plasmas.

Abstract

Characterizing compressible fluctuations in the solar wind is essential for understanding their role in solar wind acceleration and heating, yet their origin and evolution across different turbulence regimes remain poorly understood. In this study, we carry out a statistical analysis of the properties of compressible fluctuations in solar wind dominated by balanced and imbalanced turbulence. Using in-situ measurements from Wind, Solar Orbiter, and Parker Solar Probe, we investigate the scale dependence of density and magnetic field fluctuations and their correlations with plasma beta and radial distance. Our results indicate that solar wind compressibility is likely affected by both expansion effects and compressible dynamics governed by local plasma conditions. The non-Alfvenic wind is dominated by anti-correlated fluctuations, whereas the Alfvenic wind contains a mixture of correlated and anti-correlated fluctuations, though the latter remain prevalent. While the anti-correlated component is consistent with MHD slow magnetosonic modes, the correlated (fast mode-like) component is not reproduced by predictions from either linear MHD theory or nonlinear models of forced compressible fluctuations. Nevertheless, the dominant slow mode component explains the observed dependence on beta and the enhanced density fluctuations measured by Parker Solar Probe. This further suggests that slow mode waves contribute significantly to the compressible energy budget near the Sun and may play an important role in solar wind heating and acceleration close to the Sun.

Figures (8)

• Figure 1: Values of normalized rms of the density (top panel) and magnetic pressure fluctuations (middle panel) and proton beta (bottom panel) averaged over each short sub-interval as a function of radial distance. Green and blue color correspond to PSP and SolO datasets, respectively. Alfvénic sub-intervals are marked with circles and non-Alfvénic intervals with crosses. The power law coefficients from the best-fit curves are reported in the legend. The corresponding linear regression lines are plotted in black for the Alfvénic intervals and in gray for the non-Alfvénic intervals.
• Figure 2: Scatter plots showing the ensemble-averaged distribution of the normalized rms of magnetic field fluctuations amplitude vs the ensemble-averaged normalized rms of density (top panel) and magnetic field magnitude (bottom panel) fluctuations obtained from the long-interval datasets. Circles and crosses denote Alfvénic and non‑Alfvénic solar wind streams, respectively. The time scale $\tau$ is color coded and distinct colormaps are used to indicate PSP (green), SolO (blue) and Wind (orange) datasets.
• Figure 3: Correlation between compressible fluctuations and plasma beta. The left column shows results for Alfvénic intervals, while the right column corresponds to non-Alfvénic intervals. Magnetic field magnitude fluctuations are shown in the top panels, density fluctuations in the middle panels, and their ratio in the bottom panels. The Pearson correlation for each dataset is reported in the legends. Thick lines represent the linear regression fit for each mission, reported here as a reference for general trends.
• Figure 4: Ratios of the parallel to perpendicular component of magnetic field (top panels) and proton velocity (bottom panels) rms fluctuations as a function of radial distance (left) and plasma beta (right). Data points are obtained after averaging over each short sub-interval and binned, with bars indicating the variations of the data in each bin. Different colors are used to represent datasets from PSP (green), SolO (blue) and Wind (orange).
• Figure 5: Distribution of correlation measures of compressible fluctuations for Alfvénic (top) and non-Alfvenic intervals (bottom). The panels show the PDFs of coherence (left), Pearson correlation (middle), and phase angle between $\delta n/\langle n\rangle$ and $\delta (|\mathbf{B}|^2)/\langle|\mathbf{B}|^2\rangle$ (right). Results are shown for Wind (orange), SolO (blue), and for PSP (green). The legends show the mean and the standard deviation for each distribution.