Whistler Waves and Electron Deficit in the Solar Wind: Insights from Particle-in-Cell Simulations

In-situ observations of the solar wind reveal that the electron velocity distribution function (VDF) is composed of a quasi-Maxwellian core, which constitutes the majority of the electron population, along with two more sparse components: the halo, consisting of suprathermal and quasi-isotropic electrons, and the strahl, an escaping beam population. Recent measurements by the Parker Solar Probe (PSP) and Solar Orbiter (SO) have identified an additional feature in the non-thermal VDF structure: the deficit—a depletion in the sunward region of the VDF, long predicted by exospheric models but only recently extensively observed.  
Using Particle-in-Cell simulations, we analyze electron VDFs that reproduce those typically observed in the inner heliosphere and explore the potential role of the electron deficit in triggering kinetic instabilities. Prior studies and in-situ data indicate that strahl electrons can drive oblique whistler waves unstable, leading to their scattering. This process enables suprathermal electrons to access phase-space regions that satisfy resonance conditions with parallel-propagating whistler waves.  

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 results are validated against current PSP and SO observations. Specifically, the study provides insights into the origins of the frequently observed parallel anti-sunward whistler waves in the heliosphere, their correlation with electron-deficit distributions, and a non-collisional process regulating heat flux.