Quasi-parallel anti-sunward propagating whistler waves associated to the electron-deficit in the near-Sun solar wind: Particle-in-Cell simulation

TL;DR

This study uses Particle-in-Cell simulations to explore how electron deficits in the solar wind can trigger anti-sunward whistler waves, revealing a wave-particle interaction mechanism consistent with recent spacecraft observations.

Contribution

It introduces a new simulation-based explanation for the origin of anti-sunward whistler waves linked to electron deficits in the solar wind.

Findings

Electron deficits can initiate whistler wave instabilities.

Wave-particle interactions fill the depleted electron regions.

The mechanism aligns with Parker Solar Probe and Solar Orbiter data.

Abstract

In-situ observations of the solar wind have shown that the electron velocity distribution function (VDF) consists of a quasi-Maxwellian core, comprising most of the electron population, and two sparser components: the halo, which are suprathermal and quasi-isotropic electrons, and an escaping beam population, the strahl. Recent Parker Solar Probe (PSP) and Solar Orbiter (SO) observations have added one more ingredient to the known non-thermal features, the deficit-a depletion in the sunward region of the VDF, already predicted by exospheric models but never so extensively observed. By employing Particle-in-Cell simulations, we study electron VDFs that reproduce those typically observed in the inner heliosphere and investigate whether the electron deficit may contribute to the onset of kinetic instabilities. Previous studies and in-situ observations show that strahl electrons drive oblique whistler waves unstable, which in turn scatter them. As a result, suprathermal electrons can occupy regions of phase space where they fulfil resonance conditions with the parallel-propagating 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 gradually filled, as this wave-particle interaction process, triggered by the depletion itself, takes place. Our findings are compared and validated against current PSP and SO observations: among others, our study provides a mechanism explaining the presence in the heliosphere of regularly observed parallel anti-sunward whistler waves; suggests why these waves are frequently observed in concomitant with distributions presenting an electron deficit; describes a non-collisional heat flux regulating process.