Theory and fluid simulations of ion and electron acoustic instabilities in Parker Solar Probe observations close to the Sun.

Multiple electron and ion beams have been observed by the Parker Solar Probe (PSP) in the low solar atmosphere (Sun et al. 2021; Liu et al. 2023). In the presence of two resonant counter-steaming ion and electron populations, we expect the development of ion and electron acoustic instabilities, respectively (Mozer et al. 2020; Chen et al. 2020; Verscharen et al. 2022). Ion acoustic waves have indeed been observed by PSP (Mozer et al. 2021 a,b, 2023a) with characteristics that differ from previous observations. The latter is a coupled pair of high and low frequencies. Moreover, they have an electrostatic nature and a long duration of several hours. Their importance comes from the absence of whistler waves very close to the Sun, which seem to play a major role in heat flux regulation further away from the Sun (Halekas et al. 2021; Micera et al. 2021) and the recent observations that ensure the heating of core electrons and ions during the existence of such electrostatic waves (Kellogg 2020; Cattell et al. 2022; Mozer et al. 2022, 2023b). Employing the theory and multi-fluid simulations for both ion and electron acoustic instabilities (Kakad et al. 2013; Kakad & Kakad 2019; Afify et al. 2023) in plasma regimes compatible with PSP observations gives reasonable results. However, this study will be complemented by kinetic simulations with a fully kinetic code that implements solar wind plasma expansion self-consistently, EB-iPic3D (Innocenti et al. 2019), since the fluid analysis is unable to address the contribution of resonant electrons when the wave phase velocity is close to the electron thermal velocity. Indeed, the fluid simulations can capture decently the linear stage of these instabilities while becoming less accurate in the nonlinear stage (Kakad et al. 2014). This study highlights many phenomena, such as the mechanism behind the onset and propagation of different time domain structures such as electron and ion acoustic waves, how they modify the electron-ion velocity distribution functions, and the heating of the core electrons and ions.