The Breakdown of the Fluid Approximation for Electrons in a Plasma Wake

Abstract

A comparative study using a fluid-based analytic solution, hybrid particle-ion fluid-electron particle-in-cell (PIC), and fully kinetic PIC is carried out to examine a collisionless, mesothermal plasma flow over a large, unbiased plate. We find that the plasma wake may be characterized into two regions based on the electron characteristics: a fluid electron expansion region and a kinetic electron expansion region. In the fluid electron expansion region, the electrons may be considered to be an equilibrium fluid and all the three approaches lead to a similar result. In the kinetic electron expansion region, the local electron velocity distributions are strongly non-Maxwellian and the commonly used fluid approximation for electrons breaks down. The results also show that the error of the hybrid PIC simulation strongly correlates with a nonequilibrium parameter based on the weighted deviation of the electron velocity distribution from the equilibrium. Plain Language SummaryThe formation of a plasma wake behind a large moving object is applicable to many problems in space science and engineering, ranging from the ionospheric plasma flow around spacecrafts to the solar wind flow around asteroids. A common approach used in almost all previous studies of plasma wakes is to assume that electrons can be modeled as a fluid, in order to simplify analyses and save on computational time in simulations. While the assumption of fluid electrons has been widely adopted, its validity has never been carefully examined. This paper presents the first study on the validity of the fluid approximation of electrons when applied to a collisionless plasma wake. The results show that these wakes consist of two regions: a fluid electron expansion region and a kinetic electron expansion region. The electrons in the kinetic expansion region are in a nonequilibrium state, and their microscopic characteristics lead to anisotropic and complex macroscopic properties. Thus, the commonly used fluid approximation for electrons breaks down in this region. The conclusions of this study will have an immediate impact on all ongoing studies of space plasma interactions with spacecraft and solar wind interactions with small asteroids. Key Points:• The validity of the commonly used fluid electron model in a collisionless plasma wake is investigated • The plasma wake consists of a fluid electron expansion region and a kinetic electron expansion region • The fluid electron model breaks down in the kinetic expansion region due to nonequilibrium electrons

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