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Browsing by Author "Bernhardt, P. A."

Now showing 1 - 6 of 6
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    Early time evolution of a chemically produced electron depletion
    Scales, Wayne A.; Bernhardt, P. A.; Ganguli, Gurudas (American Geophysical Union, 1995-01-01)
    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.
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    Early time evolution of negative-ion clouds and electron-density depletions produced during electron-attachment chemical-release experiments
    Scales, Wayne A.; Bernhardt, P. A.; Ganguli, Gurudas (American Geophysical Union, 1994-01-01)
    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 X B and del N x 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.
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    Ion gyro-harmonic structuring in the stimulated radiation spectrum and optical emissions during electron gyro-harmonic heating
    Mahmoudian, A.; Scales, Wayne A.; Bernhardt, P. A.; Samimi, A.; Kendall, E.; Ruohoniemi, J. Michael; Isham, B.; Vega-Cancel, O.; Bordikar, M. (American Geophysical Union, 2013-03-01)
    Stimulated electromagnetic emissions (SEEs) are secondary radiation produced during active space experiments in which the ionosphere is actively heated with high power high frequency (HF) ground-based radio transmitters. Recently, there has been significant interest in ion gyro-harmonic structuring the SEE spectrum due to the potential for new diagnostic information available such as electron acceleration and creation of artificial ionization layers. These relatively recently discovered gyro-harmonic spectral features have almost exclusively been studied when the transmitting frequency is near the second electron gyro-harmonic frequency. The first extensive systematic experimental investigations of the possibility of these spectral features for third electron gyro-harmonic heating are provided here. Discrete spectral features shifted from the transmit frequency ordered by harmonics of the ion gyro-frequency were observed for third electron gyro-harmonic heating for the first time at a recent campaign at the High Frequency Active Auroral Research Program (HAARP) facility. These features were also closely correlated with a broader band feature at a larger frequency shift from the transmit frequency known as the downshifted peak (DP). The power threshold of these spectral features was measured, as well as their behavior with heater beam angle, and proximity of the transmit frequency to the third electron gyro-harmonic frequency. Comparisons were also made with similar spectral features observed during second electron gyro-harmonic heating during the same campaign. A theoretical model is provided that interprets these spectral features as resulting from parametric decay instabilities in which the pump field ultimately decays into high frequency upper hybrid/electron Bernstein and low frequency neutralized ion Bernstein IB and/or obliquely propagating ion acoustic waves at the upper hybrid interaction altitude. Coordinated optical and SEE observations were carried out in order to provide a better understanding of electron acceleration and precipitation processes. Optical emissions were observed associated with SEE gyro-harmonic features for pump heating near the second electron gyro-harmonic during the campaign. The observations affirm strong correlation between the gyro-structures and the pump-induced optical emissions.
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    Ion gyroharmonic structures in stimulated radiation during second electron gyroharmonic heating: 1. Theory
    Samimi, A.; Scales, Wayne A.; Fu, H.; Bernhardt, P. A.; Briczinski, S. J.; McCarrick, M. J. (American Geophysical Union, 2013-01-01)
    Stimulated electromagnetic emissions (SEEs) may provide important diagnostic information about space plasma composition, energetics, and dynamics during active experiments in which ground-based high-powered radio waves are transmitted into the ionosphere. The nonlinear plasma processes producing this secondary radiation are not well understood particularly for some recent observations where the transmitter (pump) frequency is near the second harmonic of the electron gyrofrequency. New, more comprehensive, experimental observations of spectral features within 1 kHz of the pump wave frequency are reported here to begin more careful comparisons of the experimental observations and a possible theoretical underpinning, which is also provided. The experimental observations typically show two distinct types of secondary radiation spectra, which are (a) discrete narrowband harmonic spectral structures ordered by the ion gyrofrequency and (b) broadband spectral structure with center frequency near 500 Hz and similar spectral bandwidth. A theoretical model is provided that interprets these spectral features as resulting from parametric decay instabilities in which the pump field ultimately decays into high-frequency upper hybrid/electron Bernstein and low-frequency neutralized ion Bernstein and/or obliquely propagating ion acoustic waves at the upper hybrid interaction altitude. Detailed calculations of the threshold level, growth rate, unstable wave number, and frequency bandwidth of the instabilities are provided for comparisons with experimental observations. An assessment of the effect of the critical instability parameters are provided including pump electric field strength, proximity of the pump frequency to the electron gyrofrequency and pump electric field geometry. The model shows quite reasonable agreement with the experimental observations. Further discussions are provided of connections with past observed SEE spectral features and potential new diagnostic information provided by these newly categorized spectra. Citation: Samimi, A., W. A. Scales, H. Fu, P. A. Bernhardt, S. J. Briczinski, and M. J. McCarrick (2013), Ion gyroharmonic structures in stimulated radiation during second electron gyroharmonic heating: 1. Theory, J. Geophys. Res. Space Physics, 118, 502-514, doi:10.1029/2012JA018146.
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    On ion gyro-harmonic structuring in the stimulated electromagnetic emission spectrum during second electron gyro-harmonic heating
    Samimi, A.; Scales, Wayne A.; Bernhardt, P. A.; Briczinski, S. J.; Selcher, C. A.; McCarrick, M. J. (Copernicus Publications, 2012)
    Recent observations show that, during ionospheric heating experiments at frequencies near the second electron gyro-harmonic, discrete spectral lines separated by harmonics of the ion-gyro frequency appear in the stimulated electromagnetic emission (SEE) spectrum within 1 kHz of the pump frequency. In addition to the ion gyro-harmonic structures, on occasion, a broadband downshifted emission is observed simultaneously with these spectral lines. Parametric decay of the pump field into upper hybrid/electron Bernstein (UH/EB) and low-frequency ion Bernstein (IB) and oblique ion acoustic (IA) modes is considered responsible for generation of these spectral features. Guided by predictions of an analytical model, a two-dimensional particle-in-cell (PIC) computational model is employed to study the nonlinear processes during such heating experiments. The critical parameters that affect the spectrum, such as whether discrete gyroharmonic on broadband structures is observed, include angle of the pump field relative to the background magnetic field, pump field strength, and proximity of the pump frequency to the gyro-harmonic. Significant electron heating along the magnetic field is observed in the parameter regimes considered.
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    Stimulated Brillouin scattering during electron gyro-harmonic heating at EISCAT
    Fu, H. Y.; Scales, Wayne A.; Bernhardt, P. A.; Briczinski, S. J.; Kosch, M. J.; Senior, A.; Rietveld, Michael T.; Yeoman, T. K.; Ruohoniemi, J. Michael (European Geosciences Union, 2015)
    Observations of secondary radiation, stimulated electromagnetic emission (SEE), produced during ionospheric modification experiments using ground-based, highpower, high-frequency (HF) radio waves are considered. The High Frequency Active Auroral Research Program (HAARP) facility is capable of generating narrowband SEE in the form of stimulated Brillouin scatter (SBS) and stimulated ion Bernstein scatter (SIBS) in the SEE spectrum. Such narrowband SEE spectral lines have not been reported using the European Incoherent Scatter (EISCAT) heater facility before. This work reports the first EISCAT results of narrowband SEE spectra and compares them to SEE previously observed at HAARP during electron gyro-harmonic heating. An analysis of experimental SEE data shows observations of emission lines within 100 Hz of the pump frequency, interpreted as SBS, during the 2012 July EISCAT campaign. Experimental results indicate that SBS strengthens as the pump frequency approaches the third electron gyro-harmonic. Also, for different heater antenna beam angles, the CUTLASS radar backscatter induced by HF radio pumping is suppressed near electron gyro-harmonics, whereas electron temperature enhancement weakens as measured by EISCAT/UHF radar. The main features of these new narrowband EISCAT observations are generally consistent with previous SBS measurements at HAARP.