Meers Maxwell Oppenheim

Nonlinear two‐stream instabilities as an explanation for auroral bipolar wave structures

The evolution of two counter-streaming electron beams is shown by means of 2-D kinetic simulations to lead to electron distributions and coherent localized bipolar plasma wave structures with features similar to those measured by the FAST satellite in the auroral ionosphere. Electrostatic whistler waves are generated at later times when the bipolar structures begin to lose coherence and break up in the dimension transverse to the geomagnetic field.

Evolution of electron phase-space holes in 3D

Electron phase-space holes are regions of de- pletedelectrondensitycommonlygeneratedduringthenon- linear stage of the two-stream instability. Recently, bipolar electric eld structures | a signature of electron holes | havebeenidentiedintheacceleration regionof theauroral ionosphere. This paper compares the evolution of electron holes in 2-D and 3-D using massively-parallel PIC simula- tions. In 2-D, the holes decay after hundreds of plasma periods while emitting electrostatic whistler waves. In the 3-Dsimulations,electronholesalsogounstableandgenerate whistlers but, due to physical processes not present in 2-D, energy flows out of the whistlers and into highly perpen- dicular lower hybrid modes. As a result of this dierence, 3-D holes do not decay as far as 2-D holes. The dierences between 2-D and 3-D evolution may have important impli- cationsforholelongevityandwavegenerationintheauroral ionosphere.

The anomalous diffusion of meteor trails

Radars frequently detect meteor trails created by the ablation of micro-meteoroids between 70 and 120 km altitude in the atmosphere. Plasma simulations show that density gradients at the edges of meteor trails drive gradient-drift instabilities which develop into waves with perturbed electric fields often exceeding hundreds of mV/m. These waves create an anomalous cross-field diffusion that can exceed the cross-field (⟂ B) ambipolar diffusion by an order of magnitude. The characteristics of the instabilities and anomalous diffusion depend on the trail altitude, latitude, and density gradient. A simple relation defines the minimum altitude at which meteor trail density gradients drive plasma instabilities and anomalous diffusion. These results impact a number of meteor radar studies, including those that use diffusion rates to determine trail altitude, and atmospheric temperature.

Electrodynamics of meteor trail evolution in the equatorial E-region ionosphere

Using analytical models and kinetic simulations, this paper shows that weakly ionized meteor trails near the geomagnetic equator evolve through three distinct stages. First, a large electric field is generated perpendicular to both the geomagnetic field and the trail. Second, plasma density waves grow asymmetrically across the trail. Third, turbulence develops in the trails. Throughout this process, the electron E × B-drift velocity plays an essential role in controlling the motion of the trail. These plasma dynamics have important implications for the interpretation of meteor radar echoes.

Saturation of the Farley-Buneman instability via nonlinear electron E×B drifts

The Farley-Buneman instability is a collisional two-stream instability observed in the E region ionosphere at altitudes in the range 90–120 km. While linear theory predicts the dominant wavelengths, it cannot fully describe the behavior of this nonlinearly saturated instability, as observed by radar and rocket measurements. This paper explores the nonlinear behavior of this phenomenon and the resulting waves through simulations and theory. Our two-dimensional simulations model wave behavior in the plane perpendicular to the Earths magnetic field, applying a fluid model to describe the electron dynamics and either a particle or a fluid model to describe ion behavior. The results show the growth, saturation, and nonlinear behavior of the instability for a much longer period of time than was possible with the pure particle codes used in previous studies. These simulations show (1) growth of Farley-Buneman waves, (2) the development of secondary waves which propagate along the extrema and perpendicular to the Farley-Buneman waves, (3) turning of the primary waves away from the electron drift direction, (4) a saturated wave phase velocity below the one predicted by linear theory but above the acoustic speed and (5) nonlinear electron E×Bo drifting dominates the behavior of the saturated waves. This paper describes both the simulation techniques and fundamental results. Additionally, this paper outlines a theory explaining the dominant nonlinear process seen in this instability.

A saturation mechanism for the Farley-Buneman instability

Studies with a reduced-mode two-fluid model have revealed a promising candidate for the saturation mech- anism of the Farley-Buneman instability in the daytime equatorial electrojet. The mechanism operates by redis- tributing the zero-order electron E B flow eld. Sec- ondary waves generated nonlinearly by the instability are responsible for the flow eld modication. Saturation oc- curs because the modied flow reduces the principal charge transport responsible for the growth of the primary wave. Two-dimensional particle simulations of the instability ex- hibit saturation via the same mechanism.

Turbulence driven by two-stream instability in a magnetized plasma

Although the nonlinear evolution of the two-stream instability has been a subject of numerical and theoretical studies for many years, recent spacecraft measurements of nonlinear electrostatic bipolar wave structures in the auroral ionosphere have prompted new studies. Using parallel two-dimensional (2D) particle-in-cell electrostatic simulations with initially counterstreaming cold electrons in a magnetized plasma, the evolution of wave turbulence and particle distributions has been followed for over 10 000 inverse plasma frequencies. Ions are introduced in the frame of one of the electron beams (motivated by measurements). Wave turbulence evolves in at least five separate stages. After tens of ωe−1, trapping produces bipolar wave structures with long range order across B. After hundreds of ωe−1, the structures break up and whistlers appear. After thousands of ωe−1, ion cyclotron waves driven by a bump-on-tail in the evolved electron distribution become prominent. After 6000 ωe−1, the ion cyclotron waves...

Plasma Instabilities in Meteor Trails: Linear Theory

Ablation of micrometeoroids between 70 and 130 km altitude in the atmosphere creates plasma columns with densities exceeding the ambient ionospheric electron density by many orders of magnitude. Density gradients at the edges of these trails can create ambipo- lar electric fields with amplitudes in excess of 100 mV/m. These fields combine with diamag- netic drifts to drive electrons at speeds exceeding 2 km/s. The fields and gradients also ini- tiate Farley-Buneman and gradient-drift (FBGD) instabilities. These create field-aligned plasma density irregularities which evolve into turbulent structures detectable by radars with a large power-aperture product, such as those found at Jicamarca, Arecibo, and Kwajalein. This pa- per presents a theory of meteor trail instabilities using both fluid and kinetic methods. In par- ticular, it discusses the origin of the driving electric field, the resulting electron drifts, and the linear plasma instabilities of meteor trails. It shows that, though the ambipolar electric field changes amplitude and even direction as a function of altitude, the electrons always drift in the positive direction where is the density and the geomagnetic field. The lin- ear stability analysis predicts that instabilities develop within a limited range of altitudes with the following observational consequences: (1) non-specular meteor trail echoes will be field aligned; (2) non-specular echoes will return from a limited range of altitudes compared with the range over which the head echo reflection indicates the presence of plasma columns; and (3) anomalous cross-field diffusion will occur only within this limited altitude range with con- sequences for calculating diffusion rates and temperatures with both specular and non-specular radars.

Spectral characteristics of the Farley-Buneman instability: Simulations versus observations

The Farley-Buneman instability is a collisional two-stream instability observed in the E region ionosphere at altitudes in the range of 95–110 km. While linear theory predicts the dominant wavelengths, it cannot fully describe the behavior of this nonlinearly saturated instability as observed by radar and rocket measurements. We simulate the behavior of this instability in the plane perpendicular to the Earths magnetic field, using a two-dimensional hybrid code which models electron dynamics as a fluid and ion dynamics with a particle-in-cell approach. The results show the growth, saturation, and nonlinear behavior of the instability for a much longer period of time than was possible with the pure particle codes used in previous studies. This paper describes the spectra from these simulations and compares them to the observed spectra. Both the simulations and observations show that (1) type I spectra result from saturated two-stream waves for a broad range of elevation angles, (2) the phase velocity of these waves is below that predicted by linear theory, (3) mode coupling leads to type II-like spectra without the presence of a plasma density gradient as often thought necessary, (4) longer wavelengths due to mode coupling develop, and (5) spectral power decreases at a rate of 0.3 dB/degree of elevation angle.

Magnetosphere‐ionosphere coupling through E region turbulence: 2. Anomalous conductivities and frictional heating

Global magnetospheric MHD codes using ionospheric conductances based on laminar models systematically overestimate the cross-polar cap potential during storm time by up to a factor of two. At these times, strong DC electric fields penetrate to the E region and drive plasma instabilities that create turbulence. This plasma density turbulence induces non-linear currents, while associated electrostatic field fluctuations result in strong anomalous electron heating. These two effects will increase the global ionospheric conductance. Based on the theory of non-linear currents developed in the companion paper, this paper derives the correction factors describing turbulent conductivities and calculates turbulent frictional heating rates. Estimates show that during strong geomagnetic storms the inclusion of anomalous conductivity can double the total Pedersen conductance. This may help explain the overestimation of the cross-polar cap potentials by existing MHD codes. The turbulent conductivities and frictional heating presented in this paper should be included in global magnetospheric codes developed for predictive modeling of space weather.