The Effects of Non-Equilibrium Velocity Distributions on Ion-Scale Waves in the Solar Wind

While typical treatments of plasma waves in the solar wind assume a combination of hot, drifting bi-Maxwellian distribution functions, velocity distribution functions (VDFs) observed in the solar wind often contain significant departures from this idealized model. By representing these VDFs with simplified bi-Maxwellian fits, we may be ignoring microinstabilities triggered by these non-equilibrium features or neglecting their impact on suppressing or enhancing waves predicted by simpler models. Such departures from the idealized models could be important to the processes that transfer energy at large MHD scales and dissipate them at smaller kinetic scales in collisionless plasmas. In this work, we investigate how deviations from a bi-Maxwellian VDF affect the onset and evolution of microinstabilities associated with solar wind protons. We use the Arbitrary Linear Plasma Solver (ALPS) to find the growth rates, wave eigenfunctions, and power of the expected waves using solar wind proton distributions measured by the Wind spacecraft. These parameters are then compared to the results for a bi-Maxwellian fit of the same proton distributions. This procedure allows us to quantify how important these non-equilibrium features are to the evolution of the plasma. We find that in resonant interaction regions that contain structure in the spacecraft distribution that is smoothed over by a bi-Maxwellian fit, there are significant differences in the growth rates between the spacecraft data and bi-Maxwellian model. This type of analysis can also be effectively applied to the non-Maxwellian proton distributions observed by Parker Solar Probe, the Magnetospheric Multiscale mission, and Solar Orbiter, allowing us to study these non-equilibrium structures in a variety of solar wind and magnetospheric contexts.