The Damping and Instability of Ion-acoustic Waves in the Solar Wind: Solar Orbiter Observations

Abstract

Observations of solar wind velocity distribution functions (VDFs) commonly reveal fine-scale structures. These features strongly influence kinetic processes such as wave damping and instability, yet their role remains poorly understood. We use a Gaussian Mixture Model (GMM) to separate proton and $\alpha$-particle (fully ionized helium) VDFs from Solar Orbiter Proton and Alpha-particle Sensor (PAS) measurements, and assess how measured VDFs affect the damping of compressive fluctuations with the Arbitrary Linear Plasma Solver (ALPS). We analyze the dispersion relation and polarization properties of ion-acoustic (IA) waves in the solar wind. Protons and $\alpha$-particles are represented by the measured VDFs derived from PAS observations. For comparison, we also perform calculations using the bi-Maxwellian assumption for the VDFs. Fine-scale structures of the measured proton VDFs reduce the damping rate of IA waves, even when $T_e \simeq T_i$. In some cases, we find that the measured VDFs drive the IA mode unstable, while the corresponding bi-Maxwellian representations predict strong damping. These results demonstrate that resolving the fine-scale structures of VDFs is essential for accurately capturing the kinetic physics of the solar wind.