Chuteng Zhou

Kinetic electron and ion instability of the lunar wake simulated at physical mass ratio

The solar wind wake behind the moon is studied with 1D electrostatic particle-in-cell (PIC) simulations using a physical ion to electron mass ratio (unlike prior investigations); the simulations also apply more generally to supersonic flow of dense magnetized plasma past non-magnetic objects. A hybrid electrostatic Boltzmann electron treatment is first used to investigate the ion stability in the absence of kinetic electron effects, showing that the ions are two-stream unstable for downstream wake distances (in lunar radii) greater than about three times the solar wind Mach number. Simulations with PIC electrons are then used to show that kinetic electron effects can lead to disruption of the ion beams at least three times closer to the moon than in the hybrid simulations. This disruption occurs as the result of a novel wake phenomenon: the non-linear growth of electron holes spawned from a narrow dimple in the electron velocity distribution. Most of the holes arising from the dimple are small and quickly...

Non-linear plasma wake growth of electron holes

An objects wake in a plasma with small Debye length that drifts across the magnetic field is subject to electrostatic electron instabilities. Such situations include, for example, the moon in the solar wind and probes in magnetized laboratory plasmas. The instability drive mechanism can equivalently be considered drift down the potential-energy gradient or drift up the density-gradient. The gradients arise because the plasma wake has a region of depressed density and electrostatic potential into which ions are attracted along the field. The non-linear consequences of the instability are analysed in this paper. At physical ratios of electron to ion mass, neither linear nor quasilinear treatment can explain the observation of large-amplitude perturbations that disrupt the ion streams well before they become ion-ion unstable. We show here, however, that electron holes, once formed, continue to grow, driven by the drift mechanism, and if they remain in the wake may reach a maximum non-linearly stable size, beyond which their uncontrolled growth disrupts the ions. The hole growth calculations provide a quantitative prediction of hole profile and size evolution. Hole growth appears to explain the observations of recent particle-in-cell simulations.

Plasma electron hole kinematics. I. Momentum conservation

We analyse the kinematic properties of a plasma electron hole: a non-linear self-sustained localized positive electric potential perturbation, trapping electrons, which behaves as a coherent entity. When a hole accelerates or grows in depth, ion and electron plasma momentum is changed both within the hole and outside, by an energization process we call jetting. We present a comprehensive analytic calculation of the momentum changes of an isolated general one-dimensional hole. The conservation of the total momentum gives the holes kinematics, determining its velocity evolution. Our results explain many features of the behavior of hole speed observed in numerical simulations, including self-acceleration at formation, and hole pushing and trapping by ion streams.

Plasma electron hole kinematics. II. Hole tracking Particle-In-Cell simulation

The kinematics of a 1-D electron hole is studied using a novel Particle-In-Cell simulation code. A hole tracking technique enables us to follow the trajectory of a fast-moving solitary hole and study quantitatively hole acceleration and coupling to ions. We observe a transient at the initial stage of hole formation when the hole accelerates to several times the cold-ion sound speed. Artificially imposing slow ion speed changes on a fully formed hole causes its velocity to change even when the ion stream speed in the hole frame greatly exceeds the ion thermal speed, so there are no reflected ions. The behavior that we observe in numerical simulations agrees very well with our analytic theory of hole momentum conservation and the effects of “jetting.”

Plasma electron hole oscillatory velocity instability

In this paper, we report a new type of instability of electron holes (EHs) interacting with passing ions. The nonlinear interaction of EHs and ions is investigated using a new theory of hole kinematics. It is shown that the oscillation in the velocity of the EH parallel to the magnetic field direction becomes unstable when the hole velocity in the ion frame is slower than a few times the cold ion sound speed. This instability leads to the emission of ion-acoustic waves from the solitary hole and decay in its magnitude. The instability mechanism can drive significant perturbations in the ion density. The instability threshold, oscillation frequency and instability growth rate derived from the theory yield quantitative agreement with the observations from a novel high-fidelity hole-tracking Particle-In-Cell (PIC) code.

Dynamics of a slow electron hole coupled to an ion-acoustic soliton

This paper demonstrates stable embedding of an electron phase-space hole into an ion-acoustic soliton simulated using one-dimensional Particle-In-Cell simulation, forming a stable Coupled Hole-Soliton pair, which is a coupled state of a fluid soliton and a Bernstein-Green-Kruskal mode electron phase-space hole. Collision tests reveal that its collisional dynamics are a hybrid of soliton collision and electron hole merging. This hybrid state is separated from the classical free electron hole branch by a gap in their ion-frame velocities. Transition is possible from the coupled state to the free state by ion Landau damping and in the opposite direction by hole growth. Buneman instability simulation is performed, showing generation of both types of electron holes depending on the ion temperature. The results from our work can be readily applied to better understand the electrostatic solitary wave observations in space plasmas.This paper demonstrates stable embedding of an electron phase-space hole into an ion-acoustic soliton simulated using one-dimensional Particle-In-Cell simulation, forming a stable Coupled Hole-Soliton pair, which is a coupled state of a fluid soliton and a Bernstein-Green-Kruskal mode electron phase-space hole. Collision tests reveal that its collisional dynamics are a hybrid of soliton collision and electron hole merging. This hybrid state is separated from the classical free electron hole branch by a gap in their ion-frame velocities. Transition is possible from the coupled state to the free state by ion Landau damping and in the opposite direction by hole growth. Buneman instability simulation is performed, showing generation of both types of electron holes depending on the ion temperature. The results from our work can be readily applied to better understand the electrostatic solitary wave observations in space plasmas.