Impact of nonthermal electron distributions on the triggering of the ion-ion acoustic instability near the Sun: Kinetic simulations

Context. We previously investigated the stability threshold of the ion-ion acoustic instability (IIAI) in parameter regimes compatible with recent Parker Solar Probe (PSP) observations, in the presence of a Maxwellian electron distribution. We find that the observed parameters are close to the instability threshold, but IIAI requires a higher electron temperature than what is observed. Aims. As electron distributions in the solar wind present clear non-Maxwellian features, we investigated if deviations from the Maxwellian distribution could explain the observed IIAI. We address specifically the kappa (κ) and core-strahl distributions for the electrons. Methods. We performed analytical studies and kinetic simulations using a Vlasov-Poisson code in a parameter regime relevant to PSP observations. The simulated growth rates were validated against kinetic theory. Results. We show that the IIAI threshold changes in the presence of κ or core-strahl electron distributions, but not significantly. In the latter case, simulations confirm the expression of an effective temperature for an equivalent Maxwellian electron distribution. Such an effective temperature could simplify stability assessments of future observations.