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Gakona, AK, United States

Hysell D.L.,Cornell University | Nossa E.,Cornell University | McCarrick M.,Marsh Creek LLC
Radio Science | Year: 2011

Ionospheric modification experiments have been performed at the High frequency Active Auroral Research Program involving the creation and suppression of artificial field-aligned density irregularities (AFAIs) using O-mode and X-mode continuous wave emissions, respectively. The emission frequencies were offset so that the O-mode upper hybrid interaction height nearly matched the X-mode reflection height in the ionospheric E region. AFAIs created by O-mode heating were observed using a 30 MHz coherent scatter radar imager. Simultaneous X-mode heating was found to suppress the AFAI intensity and increase the threshold power for excitation by approximately a factor of 5 in our experiments. The effects are attributed in part to the broadening of the upper hybrid interaction region and in part to increased O-mode absorption, which reduces the amplitude of the standing wave pattern in that region. Preliminary estimates based on local calculations suggest that the electron temperature in the E region was increased by a factor of about 1.6 in these experiments. Copyright 2011 by the American Geophysical Union. Source


Hysell D.L.,Cornell University | Nossa E.,Cornell University | McCarrick M.,Marsh Creek LLC
Radio Science | Year: 2010

Ionospheric modification experiments have been carried out using the HAARP facility along with a 30 MHz coherent scatter radar imager in Alaska to examine properties of artificial E region field-aligned plasma density irregularities (FAIs). In one set of experiments, the RF emission power was varied gradually in order to determine the threshold electric field for irregularity generation. A threshold O mode peak electric field amplitude of 170-195 mV/m at an altitude of 99 km and a heating frequency of 2.7 MHz was identified based on the full-wave formalism of Thidé and Lundborg (1986). In another, the pump frequency was varied gradually to investigate the suppression of the FAIs at frequencies near the second electron gyroharmonic frequency (2Ωe). Coherent echoes were found to be suppressed for pump frequencies in an asymmetric band 40-50 kHz wide around 2Ωe but only for irregularities driven marginally above threshold. Theoretical context for these results is provided. Copyright 2010 by the American Geophysical Union. Source


Mahmoudian A.,Virginia Polytechnic Institute and State University | Scales W.A.,Virginia Polytechnic Institute and State University | Bernhardt P.A.,U.S. Navy | Fu H.,Virginia Polytechnic Institute and State University | And 2 more authors.
Radio Science | Year: 2013

Stimulated Electromagnetic Emissions (SEEs), secondary electromagnetic waves excited by high power electromagnetic waves transmitted into the ionosphere, produced by the Magnetized Stimulated Brillouin Scatter (MSBS) process are investigated. Data from four recent research campaigns at the High Frequency Active Auroral Research Program (HAARP) facility is presented in this work. These experiments have provided additional quantitative interpretation of the SEE spectrum produced by MSBS to yield diagnostic measurements of the electron temperature and ion composition in the heated ionosphere. SEE spectral emission lines corresponding to ion acoustic (IA) and electrostatic ion cyclotron (EIC) mode excitation were observed with a shift in frequency up to a few tens of Hz from the pump frequency for heating near the third harmonic of the electron gyrofrequency 3fce. The threshold of each emission line has been measured by changing the pump wave power. The excitation threshold of IA and EIC emission lines originating at the reflection and upper hybrid altitudes is measured for various beam angles relative to the magnetic field. Variation of strength of MSBS emission lines with pump frequency relative to 3fce and 4fce is also studied. A full wave solution has been used to estimate the amplitude of the electric field at the interaction altitude. The estimated instability threshold using the theoretical model is compared with the threshold of MSBS lines in the experiment and possible diagnostic information for the background ionospheric plasma is discussed. Simultaneous formation of artificial field-aligned irregularities (FAIs) and suppression of the MSBS process is investigated. This technique can be used to estimate the growth time of artificial FAIs which may result in determination of plasma waves and physical process involved in the formation of FAIs. Key Points excitation threshold of MSBS lines originated at UH and reflection altitudes Simultaneous observation of formation of FAIs and suppression of MSBS process variation of MSBS emission lines with pump frequency and heater beam angle © 2013. American Geophysical Union. All Rights Reserved. Source


Bordikar M.R.,Virginia Polytechnic Institute and State University | Scales W.A.,Virginia Polytechnic Institute and State University | Samimi A.R.,Virginia Polytechnic Institute and State University | Bernhardt P.A.,U.S. Navy | And 2 more authors.
Geophysical Research Letters | Year: 2013

This work presents the first observations of unique narrowband emissions ordered near the hydrogen ion (H+) gyrofrequency (fcH) in the stimulated electromagnetic emission spectrum when the transmitter is tuned near the second electron gyroharmonic frequency (2fce) during ionospheric modification experiments. The frequency structuring of these newly discovered emission lines is quite unexpected since H+ is known to be a minor constituent in the interaction region which is near 160 km altitude. The spectral lines are typically shifted from the pump wave frequency by harmonics of a frequency about 10% less than fcH (≈ 800 Hz) and have a bandwidth of less than 50 Hz which is near the O+ gyrofrequency fcO. A theory is proposed to explain these emissions in terms of a parametric decay instability in a multi-ion species plasma due to possible proton precipitation associated with the disturbed conditions during the heating experiment. The observations can be explained by including several percent H+ ions into the background plasma. The implications are new possibilities for characterizing proton precipitation events during ionospheric heating experiments. Key Points First observations of H+ ion structuring in SEE during heating experiment Disturbed magnetic conditions imply proton precipitation likely plays a key role Theory is provided to explain emissions by PDI in multi-ion plasma. ©2013. American Geophysical Union. All Rights Reserved. Source


Samimi A.,Virginia Polytechnic Institute and State University | Scales W.A.,Virginia Polytechnic Institute and State University | Fu H.,Virginia Polytechnic Institute and State University | Bernhardt P.A.,U.S. Navy | And 2 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

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. © 2012. American Geophysical Union. All Rights Reserved. Source

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