Modelling near-Sun solar wind electron distribution functions

We provide a picture of the global dynamics of electrons in the inner heliosphere through the study of non-linear interactions affecting the non-thermal features of the solar wind electron velocity distribution function (VDF).
More than 50 years of in-situ observations of the solar wind have shown that the electron VDF consists of a quasi-Maxwellian core, comprising most of the electrons, and two sparser components, the halo, which is formed by suprathermal and quasi-isotropic electrons, and an escaping beam population, the strahl (Marsch 2006; Halekas et al. 2020).
Recent Parker Solar Probe and Solar Orbiter (SO) observations have confirmed the existence of an additional non-thermal feature: the deficit, i.e., a depletion in the sunward region of the VDF (Berčič et al., 2021a; Halekas et al., 2021). This feature had already been predicted by exospheric models (Lemaire and Scherer, 1971; Maksimovic et al., 2001).
By employing Particle-in-Cell (PIC) simulations, we study electron VDFs that reproduce those typically observed in the inner heliosphere and investigate how the electron deficit contributes to the onset of kinetic instabilities and to heat flux regulation in the solar wind.
The strahl electrons drive oblique whistler waves unstable which scatter them in turn (Verscharen et al., 2019, Micera et al., 2021). Our simulation results show that, as a consequence of these scattering processes, the suprathemral electrons occupy regions of phase space where they fulfil resonance conditions with the parallel-propagating fast-magnetosonic/whistler wave.
The suprathermal electrons lose kinetic energy, resulting in the generation of unstable waves. The sunward side of the VDF, initially depleted of electrons, is thus gradually filled by electrons that are resonant with the triggered whistler waves.
As this initial deviation from thermodynamic equilibrium is reduced, a decrease in the electron heat flux occurs.
Our study provides a mechanism that explains the presence of the regularly observed parallel sunward whistler waves in the heliosphere (Tong et al., 2019), whose origin remains uncertain (Vasko et al., 2020). It also suggests an explanation for recent SO observations of whistler waves, concomitant with distributions in which the three above-described non-thermal features are observed (Berčič et al., 2021b, Coburn et al., 2023).