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.
The solar wind, the continuous plasma outflow from the Sun's corona into interplanetary space, is largely permeated by low-frequency Alfvénic turbulence characterized by strong correlations between plasma velocity and magnetic field variations across a broad range of temporal and spatial scales (J. Belcher & L. Davis Jr 1971;C.-Y. Tu & E. Marsch 1995;R. Bruno & V. Carbone 2013). These Alfvénic fluctuations, found in wind streams originating from coronal holes, are primarily propagating away from the Sun and they contain enough energy to heat the plasma, although the turbulent dissipation and heating mechanisms are not yet fully understood. A smaller population of sunward-propagating fluctuations is also present, and the imbalance between outward and inward wave populations tends to decrease with radial distance (W. H. Matthaeus & M. L. Goldstein 1982;B. Bavassano et al. 2000). The relative proportion of counter-propagating fluctuations determines the strength of nonlinear interactions and, therefore, the overall turbulent state (B. D. Chandran 2008;A. A. Schekochihin 2022a).
The Alfvénic solar wind, characterized by imbalanced turbulence, also contains a smaller fraction of compressible fluctuations at inertial and kinetic scales (B. Bavassano & R. Bruno 1989;C.-Y. Tu & E. Marsch 1994). These fluctuations generally involve variations in magnetic and thermal pressure. The latter, acting through density fluctuations, generates Alfvén velocity gradients that can enhance wave reflection and dissipation in plasmas. Indeed, incompressible reflection-driven turbulence alone may be insufficient to heat the corona and accelerate the solar wind (S. R. Cranmer & M. E. Molnar 2023; A. Verdini et al. 2019). Compressible effects, on the other hand, could provide the additional reflection required to enhance the turbulent cascade and the associated heating rate (M. Asgari-Targhi et al. 2021;A. Verdini et al. 2019). They can also trigger other dynamical processes such as Alfvén wave instabilities (N. F. Derby Jr 1978) or nonlinear wave steepening (R. H. Cohen & R. M. Kulsrud 1974), that contribute further to heating (A. González et al. 2021;C. González et al. 2023C. González et al. , 2024)).
The compressible component of the solar wind has been characterized in past work by analyzing the correlation between changes in thermal and magnetic pressures, typically represented as fluctuations in plasma density and the magnetic field magnitude. The mode polarization is identified by its correlation, being positive for fast modes and negative for slow magnetosonic modes, respectively. Observations at 1 au and at larger heliocentric distances have shown that compressible fluctuations typically display negative correlation (U. Villante & M. Vellante 1982;D. Roberts et al. 1987;B. Bavassano et al. 2004), which has been interpreted as a signature of non-propagating pressure-balanced structures (PBS) or MHD/kinetic slow modes (ion-acoustic and mirror modes) (S. Yao et al. 2011;G. Howes et al. 2012; K. Klein et al. 2012; Y. Narita & E. Marsch 2015;D. Verscharen et al. 2017).
On the other hand, Helios measurements between 0.3 and 1 au revealed a mixture of fast and slow-mode waves (C.-Y. Tu & E. Marsch 1994). Observations during the first PSP encounters detected the presence both modes, with a greater contribution from the slow mode (C. Chaston et al. 2020). In contrast, S. Zhao et al. (2021) showed that most of the power is associated with the fast mode, while a smaller fraction is attributed to slow modes. Additionally, C. Chen et al. (2020) reported a reduction in the slow-mode component in the inner heliosphere. On the other hand, recent measurements in the inner heliosphere have revealed enhanced levels of density fluctuations which have been associated with slow modes (L.-L. Zhao et al. 2025). Remote sensing observations of the outer corona also indicate increased relative density fluctuations (M. Hahn et al. 2018;M. Miyamoto et al. 2014). Thus, to date, no conclusive observations have been established on which wave mode dominates closer to the Sun. Furthermore, 3D simulations of these regions demonstrate both the development of density fluctuations and the coexistence of fast and slow waves (S. Chiba et al. 2025).
Among the various mechanisms that can generate compressible fluctuations in plasmas, those associated with Alfvénic fluctuations are particularly relevant in the context of Alfvénic solar wind. As Alfvén waves reach large amplitude, nonlinearities become stronger, enabling different parametric instabilities (R. Z. Sagdeev & A. A. Galeev 1969;N. F. Derby Jr 1978;M. L. Goldstein 1978;J. V. Hollweg 1994;B. Buti et al. 2000;L. Matteini et al. 2010). Moreover, density fluctuations can also be driven by envelope-modulated Alfvén waves that arise from the plasma’s response to variations in wave magnetic pressure (R. H. Cohen & R. M. Kulsrud 1974). These mechanisms produce fast and/or slow modes depending on different plasma
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