Beyond bi-Maxwellians: The Influence of Non-Equilibrium Features on the Development of Microinstabilities in Solar Wind Plasma Waves

The hot and low-density solar wind plasma is a weakly collisional system, and thus a variety of non-equilibrium features can develop in velocity distribution functions (VDFs) and are observed in the solar wind. When examining plasma waves in the solar wind, typical treatment of proton VDFs involves modeling them with simplified bi-Maxwellian fits. While this simplification makes the calculation of plasma response straight-forward, it may also cause us to neglect microinstabilities triggered by non-equilibrium features or their impact on suppression or enhancement of waves. As microinstabilities are important to the processes that transfer energy at large MHD scales and dissipate them at smaller kinetic scales in collisionless plasmas, we wish to accurately model them using observed solar wind proton VDFs. In this work, we investigate how deviations from a two-component bi-Maxwellian VDF affect the onset and evolution of parallel-propagating microinstabilities associated with solar wind protons. We use the Arbitrary Linear Plasma Solver (ALPS) numerical dispersion solver to find the real frequencies, growth/damping rates, and wave eigenfunctions of the Alfvén and fast modes using proton VDFs extracted from Wind spacecraft observations. We compare this wave behavior to that obtained by applying the same procedure to core-and-beam bi-Maxwellian fits of the Wind proton VDFs. We find several significant differences in the plasma waves obtained for the data and bi-Maxwellian fits, including both the driving and suppression of instabilities in the data compared to the model. By application of the quasi-linear diffusion operator to our VDFs, we pinpoint resonantly interacting regions in velocity space where differences between the model and data VDF are seen to significantly affect the plasma wave behavior.