Polytropic Analysis of Large-scale Compressive Fluctuations in the Solar Wind: Fluid and Kinetic Behavior

Large-scale compressive plasma fluctuations in the solar wind 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 behavior 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 identify two 1–2 hr intervals of compressive fluctuations with a low probability of streamline crossings, determine the effective polytropic index of protons and electrons for both events, and compare them with the theoretical expectations of MHD, CGL, and kinetic slow modes. One interval exhibits characteristics of the MHD slow mode having a clear isotropic closure for the protons with γp ≈ 1.7, while the other interval is more consistent with the kinetic slow mode. The calculated electron polytropic index is γe ≈ 0.7 for both events. We decompose the signals into three different frequency bands and repeat our analysis, finding a similar difference in the proton polytropic behavior between the two events. A scale dependence is also observed, suggesting that kinetic effects become more prominent at smaller scales.