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

Abstract: Context. In a previous paper (Afify et al. 2024), we have investigated the stability threshold of the ion-ion acoustic instability (IIAI) in parameter regimes compatible with recent Parker Solar Probe (PSP, (Fox et al. 2016)) observations, in the presence of a Maxwellian electron distribution. We found that observed parameters are close to the instability threshold, but IIAI requires a higher electron temperature than observed. Aims. As electron distributions in the solar wind present clear non-Maxwellian features, we investigate here if deviations from the Maxwellian distribution could explain the observed IIAI. We address specifically the kappa ( $\kappa$ ) and core-strahl distributions for the electrons. Methods. We perform analytical studies and kinetic simulations using a Vlasov-Poisson code in a parameter regime relevant to PSP observations. The simulated growth rates are validated against kinetic theory. Results. We show that the IIAI threshold changes in the presence of $\kappa$ or core-strahl electron distributions, but not significantly. In the latter case, the expression of an effective temperature for an equivalent Maxwellian electron distribution given in Jones et al. (1975) is confirmed by simulations. Such an effective temperature could simplify stability assessment of future observations.