W. A. Scales

Low Frequency Oscillations in A Plasma with Spatially Variable Field-Aligned Flow

The effects of a transverse gradient in the plasma flow velocity parallel to the ambient magnetic field are analyzed. A transverse velocity gradient in the parallel ion flow, even in small magnitude, can increase the parallel phase speed of the ion-acoustic waves sufficiently to reduce ion Landau damping. This results in a significantly lower threshold current for the current driven ion acoustic instability. Ion flow gradients can also give rise to a new class of ion cyclotron waves via inverse cyclotron damping. A broadband wave spectrum with multiple cyclotron harmonics is possible. A combination of the multiple cyclotron harmonic waves can result in spiky electric field structures with their peaks separated by an ion cyclotron period. A spatial gradient in the parallel electron flow is also considered but it is found to play a minimal role in the low frequency regime. Relevance of these to natural plasma environments is discussed.

Three dimensional character of whistler turbulence

It is shown that the dominant nonlinear effect makes the evolution of whistler turbulence essentially three dimensional in character. Induced nonlinear scattering due to slow density perturbation resulting from ponderomotive force triggers energy flux toward lower frequency. Anisotropic wave vector spectrum is generated by large angle scatterings from thermal plasma particles, in which the wave propagation angle is substantially altered but the frequency spectrum changes a little. As a consequence, the wave vector spectrum does not indicate the trajectory of the energy flux. There can be conversion of quasielectrostatic waves into electromagnetic waves with large group velocity, enabling convection of energy away from the region. We use a two-dimensional electromagnetic particle-in-cell model with the ambient magnetic field out of the simulation plane to generate the essential three-dimensional nonlinear effects.

Electron temperature effects on small-scale plasma irregularities associated with charged dust in the Earth's mesosphere

Polar mesospheric summer echoes (PMSEs) are strong radar echoes in the 50-MHz to 1.3-GHz frequency range produced by scattering from electron irregularities in the earths mesosphere. The electron irregularities believed to produce PMSEs result from electron charging on subvisible dust that exists in the mesosphere. The study of PMSEs is a forefront issue in near earth space science because of the tremendous remote sensing diagnostic possibilities that exist for studying the earths middle atmosphere. Recently, experimental results have shown that PMSEs may be modulated by radio wave heating the irregularity source region with a ground-based ionospheric heating facility. The strength of the PMSEs is reduced within second time scales upon turning on the radio wave heating, and there is a similar time scale for recovery of the PMSEs upon turning the radio wave heating off. The numerical model is used to investigate the time evolution of the effects of radio wave heating on the irregularities believed to produce PMSEs. The effects of dust charging and chemistry as well as diffusion is included in the model. The results indicate that diffusion of electrons and ions are of primary importance at early times after turn on or turn off of the radio wave heating while dust charging may have important effects at later times. Also, the scale size of the irregularities has important effects on the reduction and recovery behavior of the irregularities.

Early time evolution of negative ion clouds and electron density depletions produced during electron attachment chemical release experiments

Two-dimensional electrostatic particle-in-cell simulations are used to study the early time evolution of electron depletions and negative ion clouds produced during electron attachment chemical releases in the ionosphere. The simulation model considers the evolution in the plane perpendicular to the magnetic field and a three-species plasma that contains electrons, positive ions, and also heavy negative ions that result as a by-product of the electron attachment reaction. The early time evolution (less than the negative ion cyclotron period) of the system shows that a negative charge surplus initially develops outside of the depletion boundary as the heavy negative ions move across the boundary. The electrons are initially restricted from moving into the depletion due to the magnetic field. An inhomogenous electric field develops across the boundary layer due to this charge separation. A highly sheared electron flow velocity develops in the depletion boundary due to E × B and ▽N × B drifts that result from electron density gradients and this inhomogenous electric field. Structure eventually develops in the depletion boundary layer due to low-frequency electrostatic waves that have growth times shorter than the negative ion cyclotron period. It is proposed that these waves are most likely produced by the electron-ion hybrid instability that results from sufficiently large shears in the electron flow velocity.

Early time evolution of a chemically produced electron depletion

The early time evolution of an ionospheric electron depletion produced by a radially expanding electron attachment chemical release is studied with a two-dimensional simulation model. The model includes electron attachment chemistry, incorporates fluid electrons, particle ions and neutrals, and considers the evolution in a plane perpendicular to the geomagnetic field for a low beta plasma. Timescales considered are of the order of or less than the cyclotron period of the negative ions that result as a by-product of the electron attachment reaction. This corresponds to time periods of tenths of seconds during recent experiments. Simulation results show that a highly sheared azimuthal electron flow velocity develops in the radially expanding depletion boundary. This sheared electron flow velocity and the steep density gradients in the boundary give rise to small-scale irregularities in the form of electron density cavities and spikes. The nonlinear evolution of these irregularities results in trapping and ultimately turbulent heating of the negative ions. 27 refs.

Theoretical and simulation studies of broad up‐shifted sideband generation in ionospheric stimulated radiation

A second order four-wave parametric process proposed to be responsible for the generation of the broad up-shifted sideband in ionospheric stimulated radiation is studied by using a theoretical model and one dimensional electrostatic particle-in-cell simulations. The results indicate that (1) the initial evolution in nonlinear simulations is well described by the proposed theoretical model, (2) the simulation frequency power spectrum in a saturated nonlinear state exhibits important characteristics of the experimental observations and (3) the full nonlinear evolution of this four-wave process is crucial in interpreting the asymmetric nature of the wave frequency spectrum observed during experiments.

Small‐scale plasma irregularities produced during electron attachment chemical releases

In situ measurements of small-scale plasma density irregularities made during sounding rocket experiments that released electron attachment materials into the ionosphere are presented. A 2D electrostatic simulation model that includes attachment chemistry is used to study the source and evolution of these irregularities. The simulation shows (1) that large electron flow velocity shears develop on the boundary of the electron depletion and (2) these shears drive a plasma instability that is the likely source of the irregularities.

Electron gyroharmonic effects on ionospheric stimulated Brillouin scatter

Stimulated Brillouin scattering (SBS) and resonant phenomena are well known in the context of laser fusion, fiber optics, and piezoelectric semiconductor plasmas, as well as in various biological applications. Due to recent advances, active space experiments using high-power high-frequency (HF) radio waves may now produce stimulated Brillouin scattering (SBS) in the ionospheric plasma. The sensitivity of the narrowband SBS emission lines to pump frequency stepping across electron gyroharmonics is reported here for the first time. Experimental observations show that SBS emission sidebands are suppressed as the HF pump frequency is stepped across the second and third electron gyroharmonics. A correlation of artificially enhanced airglow and SBS emission lines excited at the upper hybrid altitude is observed and studied for second gyroharmonic heating. The SBS behavior near electron gyroharmonics is shown to have important diagnostic applications for multilayered, multi-ion component plasmas such as the ionosphere.

Observations and theory of ion gyro-harmonic structures in the stimulated radiation spectrum during second electron gyro-harmonic heating

Recent observations of Stimulated Electromagnetic Emissions SEE for ionospheric heating near the second electron gyro-harmonic frequency at the HAARP facility are provided. These observations show previously unobserved structures ordered by harmonics of the ion gyro-frequency. An analytical model is presented for three-wave coupling between the pump wave, upper hybrid/electron Bernstein waves, and electrostatic ion cyclotron harmonic waves. It is shown that for pump wave frequencies near the second electron gyro-harmonic, a band of upper hybrid/electron Bernstein waves separated by harmonics of the ion gyro-frequency can be destabilized. A new 2-D computational model using the Particle-In-Cell PIC method is used to more thoroughly investigate the nonlinear processes involved in producing these spectral features and provide results reasonably in line with predictions of the simplified analytical model.

Determination of aircraft wake vortex radar cross section due to coherent Bragg scatter from mixed atmospheric water vapor

Remote detection and tracking of wing-tip-generated wake vortices are important for hazard avoidance, especially near airports. Aircraft that fly through these hazardous vortices experience sudden induced roll. Experiments have demonstrated that there is sufficient radar cross section for remote detection at frequencies ranging from VHF to C band (100 MHz to 5 GHz). The mechanism that yields this radar cross section is Bragg scattering from the index of refraction variations due to the atmospheric water vapor being mixed by the wake vortex system. Refractive index variations of the size that correspond to half the operating radar wavelength produce the observed radar return. Previous analysis has postulated turbulence within the wake vortex to be the generator of the index of refraction variations. In this work, a new mechanism is identified that does not assume turbulence within the wake vortex system. This “laminar flow mechanism” mixes the stratified atmosphere as the wake vortex system swirls and descends, which causes refractive index structure that stretches into successively smaller spirals over time. The results are quantitatively consistent with experimental data.