A. Mahmoudian

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.

First modulation of high‐frequency polar mesospheric summer echoes by radio heating of the ionosphere

The first high-frequency (HF, 8 MHz) observations of the modulation of polar mesospheric summer echoes (PMSE) by artificial radio heating of the ionosphere are presented and compared to observations at 224 MHz and model predictions. The experiments were performed at the European Incoherent Scatter facility in northern Norway. It is shown that model results are in qualitative and partial quantitative agreement with the observations, supporting the prediction that with certain ranges of ice particle radii and concentration, PMSE at HF radar wavelengths can be enhanced by heating due to the dominance of dust charging over plasma diffusion.

Irregularities Associated With Creation of Dusty Plasmas in the Near-Earth Space Environment

Plasma irregularities associated with the creation of an artificial dust layer in the Earths ionosphere are investigated with a 2-D plasma simulation model. The scenario considered in this investigation is the production of electron irregularities due to growth of a plasma instability driven by inhomogeneities in the boundary between the background plasma and the expanding charged dust layer. The mechanism is shown to effectively generate irregularities for early times after a localized release of dust for the parameter regimes of upcoming space experiments.

Investigation of the temperature gradient instability as the source of midlatitude quiet time decameter‐scale ionospheric irregularities: 1. Observations

Super Dual Auroral Radar Network (SuperDARN) radars regularly observe decameter-scale ionospheric irregularities at midlatitudes during quiet geomagnetic conditions. The mechanism responsible for the growth of such irregularities is still unknown. Previous results based on data from the Wallops SuperDARN HF radar and Incoherent Scatter Radar have suggested that the Temperature Gradient Instability (TGI) could be responsible for only part of the observed irregularities. This conclusion was reached based on the relative orientation of horizontal electron temperature and density gradients. However, the TGI theory requires driving gradients to be perpendicular to perpendicular to the geomagnetic field B. Since midlatitude field lines are approximately 20° off vertical, we have reexamined the original data and computed gradients along the meridional direction perpendicular to B. Distinctions have to be made between the topside and bottomside F region due to the strong influence of vertical gradients. We find that the TGI growth is possible in the topside F region for the duration of the experiment, even before irregularities were observed. We show that the absence of observed irregularities during favorable TGI growth conditions is not a consequence of HF propagation but of higher E region electron irregularity growth. We conclude that the TGI is a valid mechanism to explain the generation of all irregularities observed during the experiment.

Impact of active geomagnetic conditions on stimulated radiation during ionospheric second electron gyroharmonic heating

Recently, narrowband emissions ordered near the H+ (proton) gyrofrequency (fcH) were reported in the stimulated electromagnetic emission (SEE) spectrum during active geomagnetic conditions. This work presents new observations and theoretical analysis of these recently discovered emissions. These emission lines are observed in the stimulated electromagnetic emission (SEE) spectrum when the transmitter is tuned near the second electron gyroharmonic frequency (2fce) during recent ionospheric modification experiments at the High Frequency Active Auroral Research (HAARP) facility near Gakona, Alaska. The spectral lines are typically shifted below and above the pump wave frequency by harmonics of a frequency roughly 10% less than fcH (≈ 800 Hz) with a narrow emission bandwidth less than the O+ gyrofrequency (≈ 50 Hz). However, new observations and analysis of emission lines ordered by a frequency approximately 10% greater than fcH are presented here for the first time as well. The interaction altitude for the heating for all the observations is in the range of 160 km up to 200 km. As described previously, proton precipitation due to active geomagnetic conditions is considered as the reason for the presence of H+ ions known to be a minor background constituent in this altitude region. DMSP satellite observations over HAARP during the heating experiments and ground-based magnetometer and riometer data validate active geomagnetic conditions. The theory of parametric decay instability in multi-ion component plasma including H+ ions as a minority species described in previous work is expanded in light of simultaneously observed preexisting SEE features to interpret the newly reported observations. Impact of active geomagnetic conditions on the SEE spectrum as a diagnostic tool for proton precipitation event characterization is discussed.

Narrowband stimulated electromagnetic emissions (SEE) spectra: A new ionospheric diagnostic technique

Use of high frequency (HF) heating experiments has been extended in recent years as a useful methodology for plasma physicists wishing to remotely study the properties and behavior of the ionosphere as well as nonlinear plasma processes. Our recent work using high latitude heating experiments has lead to several important discoveries that have enabled assessment of active geomagnetic conditions, determination of minor ion species and their densities, ion mass spectrometry, electron temperature measurements in the heating ionosphere, as well a deeper understanding of physical processes associated with electron acceleration and formation of field aligned irregularities. All of these diagnostic capabilities have been made possible for the first time by utilizing the narrowband Stimulated Electromagnetic Emission SEE spectrum. Narrowband SEE spectra from the High Frequency Active Auroral Research Program (HAARP) facility are presented with theoretical models to demonstrate these new capabilities along with other supporting diagnostic measurements including, range-time-intensity of FAIs (field aligned irregularities) using SuperDARN HF radar, enhanced ion lines detected by MUIR radar, DMSP satellite observations over HAARP, and coordinated observations of pump induced optical emissions.

Investigation of magnetized dusty plasmas in the laboratory and near-earth space environment

The field of dusty plasmas has become a vigorous and established area of research for a number of decades now. In this work, two computational models are mainly developed to study possible plasma turbulence during the charged aerosol release experiments in space plasmas. Meanwhile, its applications for magnetized dusty plasmas for complex plasma research with upcoming experiment facilities at Auburn University is considered. Two new hybrid and full fluid two-dimensional computational models have been developed to investigate instabilities in nonuniform magnetized dusty plasmas. The magnetic fields have been applied to study the effect on plasma wave excitation.

Investigation of temperature gradient instability as the source of mid-latitude decameter-scale quiet-time ionospheric irregularities

Summary form only given. SuperDARN HF radars regularly observe decameter-scale ionospheric irregularities at mid-latitudes during quiet geomagnetic conditions. The mechanism responsible for the growth of such common irregularities is still unknown. Previous joint measurements by Millstone Hill Incoherent Scatter Radar (ISR) and SuperDARN HF radar located at Wallops Island, Virginia have identified the presence of opposed meridional electron density and temperature gradients in the region of decameter-scale electron density irregularities period. These gradients have been proposed to be responsible for low velocity Sub-Auroral Ionospheric Scatter (SAIS) observed by SuperDARN radars. Temperature gradient instability (TGI) is investigated as the potential source of irregularities associated with these SuperDARN echoes. The electrostatic dispersion relation for TGI has been extended into the kinetic regime appropriate for SuperDARN radar frequencies by including Landau damping, finite gyro-radius effects, and temperature anisotropy. This dispersion relation has been compared with the fluid model of the TGI proposed by Hudson and Kelley [1976]. The variations of TGI growth rate with electron collision frequency, temperature gradients, density gradients, and the angle between wave vector and magnetic field have been studied. Since temperature and density gradients are a persistent feature in the mid-latitude ionosphere near the plasmapause, the drift mode growth rate at short wavelengths may explain the observed mid-latitude ionospheric irregularities. The calculations of electron temperature and density gradients in the direction perpendicular to the geomagnetic field have shown that the TGI growth is possible in the top-side F-region for the duration of the experiment. A time series for the growth rate has been developed for mid-latitude ionospheric irregularities observed by SuperDARN in the top-side F-region [Greenwald et al., 2006]. This time series is computed for both perpendicular and meridional density and temperature gradients. These observations show the role of TGI is dominant over the gradient drift instability (GDI) in this case. Nonlinear evolution of the TGI has been studied utilizing gyro-kinetic “Particle In Cell” (PIC) simulations with Monte Carlo collisions. This allows detailed study of saturation amplitude, particle flux, heat flux, diffusion coefficient, and thermal diffusivity of the resistive drift wave turbulence. The simulation results have been compared with the linear theory for the short and long wavelength regime. A critical comparison of computational modeling results and experimental observations is discussed.

Ion gyro-harmonic structuring in the stimulated radiation spectrum during third electron gyro-harmonic heating

Recently, there has been significant interest in ion gyro-harmonic structuring the Stimulated Electromagnetic Emission SEE spectrum due to the potential for new diagnostic information available about the heated volume and ancillary processes such as creation of artificial ionization layers. These relatively recently discovered emission lines have almost exclusively been studied for second electron gyro-harmonic heating. The first extensive systematic investigations of the possibility of these spectral features for third electron gyro-harmonic heating are provided here. Objectives include the consideration of the variation of the spectral behavior under pump power, proximity to the gyro-harmonic frequency, and beam angle. Also, the relationship between such spectral features and electron acceleration and creation of plasma irregularities was an important focus.