Particle-in-Cell simulations of the electron-deficit instability in the solar wind

Abstract

<p>An ambipolar electric field is expected to develop in the solar wind, to maintain quasineutrality between the ions and electrons, which, due to their smaller mass, have a higher mobility than ions (Pierrard and Lemaire, 1996; Maksimovic et al., 2001). An electron deficit in the sunwards direction associated with this electric potential has recently been observed in the near-Sun solar wind by Parker Solar Probe (Berčič et al., 2021a; Halekas et al., 2021). <br>In this work, we use fully kinetic Particle-in-Cell (PIC) simulations, performed using the Energy Conserving Semi-Implicit Method (ECSIM, Lapenta 2017) to investigate whether this electron deficit may act as a source of whistler waves. <br>We start from an electron velocity distribution function (VDF) modelled after those observed in the inner heliosphere, and prove that the sunward electron deficit drives a kinetic instability and generates whistler waves that propagate quasi-parallel to the background magnetic field. These waves interact with the electron VDF and are able to scatter, through resonant interactions, the electrons near the cut-off towards smaller values of perpendicular velocities and larger absolute values of parallel velocities. Consequently, the sunward electron deficit is filled as the instability evolves. As this initial deviation from thermodynamic equilibrium is reduced a decrease in the electron heat flux occurs. The properties of the instability are closely related to the position of the cut-off in the distribution. <br>The results of our PIC simulations confirm recent Solar Orbiter (SO) observations (Berčič et al., 2021b) that showed a consistent correlation between parallel whistler wave detection and the presence of the sunward deficit in the electron VDF.</p>