Compressive Waves in the Solar Wind

Large-scale compressive plasma fluctuations are a minor component of solar wind turbulence but still significantly shape the turbulent cascade. They perturb the pressure and internal energy of the plasma, and thus influence the evolution of the solar wind’s bulk properties (i.e., density, temperature) and can subject the plasma to various large-scale temperature anisotropy and beam instabilities. Observations of the solar wind show that these fluctuations are typically characterized by an anticorrelation between the plasma density and the magnitude of the magnetic field, and thus share polarization properties with slow waves. The nature of the slow modes in the solar wind with respect to the polarization properties of the plasma has been found to be in better agreement with the magnetohydrodynamic (MHD) slow mode predictions compared to that of the kinetic slow mode.

The polytropic behaviour of the plasma in compressive fluctuations may provide further insight into the nature of the slow mode, since the MHD, Chew–Goldberger–Low (CGL), and kinetic slow modes predict different proton polytropic indices (γ). Using Solar Orbiter observations, we determine the effective polytropic index of protons and electrons for two compressive fluctuations events, and compare them with the theoretical expectations of MHD, CGL, and kinetic slow modes. The first event exhibits characteristics of the MHD slow mode (γp ≈ 1.7) while the second event is more consistent with the kinetic slow mode (γp ≈ 3). We show that the Coulomb collisionality of the first event is stronger than the second event which may explain the different behaviour between the two events. Additionally, multiscale analysis shows that nature of the two events does not change significantly with scale. However, a scale dependence is observed for both events that suggests that kinetic effects become more prominent at smaller scales.

Figure 1. Polytropic index results for protons and electrons in the first (left) and second (right) compressive fluctuations events.  is the Pearson correlation coefficient. Panels (a)–(c) show the proton results and panels (d)–(f) show the electron results. Panels (a) and (d) show the parallel, panels (b) and (e) the perpendicular, and panels (c) and (f) the isotropic polytropic index results. The colour of the data points represents the time instance of the corresponding measurement in the interval. The first event shares characteristics with the MHD slow mode with an isotropic proton polytropic index of  ≈ 5/3, while the second event shares characteristics with an Ion Acoustic wave with a parallel proton polytropic index of  ≈ 3

See publication for details:
Ioannou, C. et al. (2025) ‘Polytropic Analysis of Large-scale Compressive Fluctuations in the Solar Wind: Fluid and Kinetic Behavior’, The Astrophysical Journal, 988(2), p. 253. Available at: https://doi.org/10.3847/1538-4357/adeb7b