Overview of Ion-Driven Instabilities in the Inner Heliosphere

Linear theory is a well-established theoretical framework proven to accurately characterize instabilities in the solar wind weakly collisional plasma. We aim to describe the statistical properties of linear ion-driven instabilities between 0.3 and 1 au. We analyzed ∼1.5M proton and alpha particle Velocity Distribution Functions (VDFs) observed by Helios I & II, and ~5M VDFs observed by Wind, using Plasma in a Linear Uniform Magnetized Environment (PLUME) dispersion solver to calculate growth rate, frequency, wavevector, and the power emitted or absorbed by each VDF component. The descriptive statistical analysis shows that the stability of the solar wind is primarily determined by the collisional processing, rather than the distance from the Sun. We use this data set to train Stability Analysis Vitalizing Instability Classification (SAVIC) Machine Learning algorithm capable to classify the predicted unstable modes into physically meaningful “textbook” types. This method enables us to map the instability properties in multi-dimensional phase space. The proton-core-induced Ion Cyclotron (IC) mode dominates the collisionally young solar wind, while the alpha population plays more important role in the wave energy dynamics of the older wind. We also demonstrate that the proton beam population is not affected by the collisions and has the core-beam drift as the main source of free energy that determines its overall behavior in the solar wind. SAVIC code used here is publicly available for the community.