Impact of Two-Population $\alpha$-particle Distributions on Plasma Stability

Abstract

The stability of weakly collisional plasmas is well represented by linear theory, and the generated waves play an essential role in the thermodynamics of these systems. The velocity distribution functions (VDF) characterizing kinetic particle behavior are commonly represented as a sum of anisotropic bi-Maxwellians. For the majority of in situ observations of solar wind plasmas enabled by heliospheric missions, a three bi-Maxwellian model is commonly applied for the ions, assuming that the VDF consists of a proton core, proton beam, and a single He ($\alpha$) particle population, each with their own density, bulk velocity, and anisotropic temperature. Resolving an $\alpha$-beam component was generally not possible due to instrumental limitations. The Solar Orbiter Solar Wind Analyser Proton and Alpha Sensor (SWA PAS) resolves velocity space with sufficient coverage and accuracy to routinely characterize secondary $\alpha$ populations consistently. This design makes the SWA PAS dataset ideal for examining effects of the $\alpha$-particle beam on the plasma's kinetic stability. We test the wave signatures observed in the magnetic field power spectrum at ion scales and compare them to the predictions from linear plasma theory, Doppler-shifted into the spacecraft reference frame. We find that taking into account the $\alpha$-particle beam component is necessary to predict the coherent wave signatures in the observed power spectra, emphasizing the importance of separating the $\alpha$-particle populations as is traditionally done for protons. Moreover, we demonstrate that the drifts of beam components are responsible for the majority of the modes that propagate in oblique direction to the magnetic field, while their temperature anisotropies are the primary source of parallel Fast Magnetosonic Modes in the solar wind.