Ionospheric Modification Experiments Research Articles

The Apparent Behavior of Electron Density during an Alternating O/X-Mode Heating Experiment

We present the observations of the artificial ionospheric modification experiment of EISCAT on 18 October 2012 in Tromsø, Norway. When the pump of alternating O mode and X mode is switched on, the UHF radar observation shows some strong enhancements in electron density, ion lines and plasma lines. Based on some existing theories, we find the following: First, during the experiment, the frequency of plasma line (fL), ion line (fia) and pump (fh) matches fL = fh − 3fia and = fh − 5fia occasionally demonstrated that the cascade process occurred. Second, through quantitative calculation, we found that the O-mode component mixed in X-mode wave satisfies the thresholds of the parametric decay instability and the oscillation two-stream instability, from which we infer that the HF-induced plasma lines (HFPLs) and HF-enhanced ion lines (HFILs) observed in X-mode pulse could have been caused by the O-mode component mixed in X-mode wave. Third, the UHF radar observation shows some apparent enhancements over a wide altitude range (from approximately the reflection altitude to ~670 km) in electron density during X-mode pulse, which also does not, in fact, correspond to a true increase in electron density, but due to the enhancement in ion line or the enhancement in radar backscatter induced by some unknown mechanism.

Power threshold and saturation of parametric decay instabilities near the upper hybrid resonance in plasmas

Parametric decay instabilities (PDIs) occur for large-amplitude waves in quadratically nonlinear media, where they provide a limit of validity of linear theories and allow efficient coupling between different, well-defined wave modes. We investigate PDIs near the upper hybrid resonance in plasmas by injection of high-power electron cyclotron (EC) waves at the ASDEX Upgrade tokamak. Our measurements of PDIs have an unprecedented frequency resolution, far below the ion cyclotron frequency, allowing the first observations of secondary and tertiary PDIs during the saturation phase in a controlled laboratory setting. Furthermore, we are for the first time able to systematically compare theoretical predictions of the EC wave power thresholds, which must be exceeded to excite such PDIs, with experimental observations, validating the theory. Our findings are relevant for EC wave heating and current drive in tokamaks and stellarators, including future fusion power plants, as well as in low-temperature laboratory and industrial plasmas, inertial confinement fusion, and ionospheric modification experiments.

NSEE Yielding Electron Temperature Measurements at the Arecibo Observatory

AbstractNarrowband stimulated electromagnetic emissions (NSEE), a component of radio emissions created during high‐frequency (HF) radiowave ionospheric modification experiments, occur within 1 kHz of the HF pump frequency. NSEE was observed and studied for the first time at the HAARP (High Frequency Active Auroral Research Program) facility in Alaska (Norin et al., 2009, https://doi.org/10.1103/PhysRevLett.102.065003; Bernhardt et al., 2010, https://doi.org/10.1103/PhysRevLett.104.165004). Magnetized stimulated Brillouin scatter (MSBS) is a component of NSEE, which was also first observed at HAARP (Norin et al., 2009, https://doi.org/10.1103/PhysRevLett.102.065003; Bernhardt et al., 2010, https://doi.org/10.1103/PhysRevLett.104.165004) and later at European Incoherent Scatter Scientific Association (Fu et al., 2015, https://doi.org/10.5194/angeo-33-983-2015). Ion‐acoustic and electrostatic ion cyclotron modes are the daughter products of MSBS and can be used for the determination of the electron temperature and of the presence of minor ion species, respectively, in the HF‐modified ionosphere. Here we present the first observations of the MSBS process at magnetic midlatitudes, excited during radio wave ionospheric modification experiments at the Arecibo Observatory. The NSEE observations, in combination with a theoretical model and the wave matching conditions, are used to estimate background ionospheric parameters. A qualitative comparison of the MSBS component of the NSEE spectrum with the thermal ion line measured by incoherent scatter radar is presented.

Investigating Transport and Dissipation in the Subauroral E Region With Ionospheric Modification Experiments and Very High Frequency Radar Backscatter

AbstractIonospheric modification experiments have been performed at the High‐Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska, using a Very High Frequency (VHF) coherent scatter radar in Homer, Alaska, for experimental diagnostics. The experiments were intended to determine the threshold pump electric field required to initiate thermal parametric instability in the E region. The pump power level was ramped systematically to determine the threshold, and the experiment was repeated at four closely spaced pump frequencies. This provided threshold estimates at four E region altitudes. The theory for thermal parametric instability based on the work of Dysthe et al. (1983, https://doi.org/10.1063/1.863993) has been modified for application in the E region. The theory considers magneto‐ionic effects on the pump mode, linear mode conversion theory for upper hybrid wave generation, wave heating, and the effects of transport and dissipation based on fluid theory. The theory amounts to an eigenvalue problem where the eigenvalue is the threshold pump electric field for instability. The theory shows how the threshold depends on ionospheric transport coefficients and on the fractional cooling rate for inelastic electron‐neutral collisions. The theoretical predictions for threshold are roughly consistent with experimental values although the latter are probably affected by excess ionospheric absorption.

Spatiotemporal Characteristics of HF‐Induced Ionospheric Turbulence Revealed by Diagnostic Stimulated Electromagnetic Emission and Test Radio Waves at HAARP

AbstractThis paper describes the dynamic characteristics of ionospheric plasma turbulences excited by high‐power high‐frequency radio waves during ionospheric modification experiments at the High‐frequency Active Auroral Research Program heating facility, Gakona, Alaska. The data come from measurements of high‐frequency‐induced stimulated electromagnetic emission (SEE) and anomalous absorption of test radiowaves. In addition to quasi‐continuous pump (QCP) waves that excite the “standard” SEE, short pulses of diagnostic waves at frequencies shifted from the QCP frequency have been injected to generate diagnostic SEE. This experimental setup allowed us to study the spectra and damping rates of the plasma waves related to both diagnostic and QCP waves, as well as the QCP‐related small‐scale magnetic field‐aligned irregularities at different altitudes, ±25 km around the center of the heated volume. The results of numerical simulations of the upper hybrid‐related SEE and diagnostic SEE under a “double‐transformation” scheme with the use of an empirical model of the irregularities' spatial spectrum and its dynamics are in good agreement with the observations.

FDTD Analysis of Propagation and Absorption in Nonuniform Anisotropic Magnetized Plasma Slab

In this paper, a current density convolution finite-difference time-domain method is extended to study the electromagnetic wave propagation and absorption in inhomogeneous magnetized plasmas. First of all, we analyze the wave propagation in homogeneous magnetized plasmas for arbitrary magnetic inclination angle. Then, we studied three types of plasma absorption, including plasma resonance, electron cyclotron resonance, and upper hybrid resonance in homogeneous and inhomogeneous magnetized plasmas. This model is applied to simulate the heating mechanism during ionospheric modification experiments by high-power high-frequency radio waves. These results have important implications for the interpretation of physical mechanisms for the interaction of high power radio waves with magnetized ionospheric plasmas and design of future experiments.

Electron density inversed by plasma lines induced by suprathermal electron in the ionospheric modification experiment

Incoherent scatter radar (ISR) is the most powerful ground-based measurement facility to study the ionosphere. The plasma lines are not routinely detected by the incoherent scatter radar due to the low intensity, which falls below the measured spectral noise level of the incoherent scatter radar. The plasma lines are occasionally enhanced by suprathermal electrons through the Landau damping process and detectable to the incoherent scatter radar. In this study, by using the European Incoherent Scatter Association (EISCAT) UHF incoherent scatter radar, the experiment observation presents that the enhanced plasma lines were observed. These plasma lines were considered as manifest of the suprathermal electrons generated by the high-frequency heating wave during the ionospheric modification. The electron density profile is also obtained from the enhanced plasma lines. This study can be a promising technique for obtaining the accurate electron density during ionospheric modification experiment.

Two‐Dimensional Vlasov Simulations of Fast Stochastic Electron Heating in Ionospheric Modification Experiments

AbstractIonospheric heating experiments using high‐frequency ordinary (O)‐mode electromagnetic waves have shown the induced formation of magnetic field‐aligned density striations in the ionospheric F region, in association with lower hybrid (LH) and upper hybrid (UH) turbulence. In recent experiments using high‐power transmitters, the creation of new plasma regions and the formation of descending artificial ionospheric layers (DAILs) have been observed. These are attributed to suprathermal electrons ionizing the neutral gas, so that the O‐mode reflection point and associated turbulence is moving to a progressively lower altitude. We present the results of two‐dimensional (2‐D) Vlasov simulations used to study the mode conversion of an O‐mode pump wave to trapped UH waves in a small‐scale density striation of circular cross section. Subsequent multiwave parametric decays lead to UH and LH turbulence and to the excitation of electron Bernstein (EB) waves. Large‐amplitude EB waves result in rapid stochastic electron heating when the wave amplitude exceeds a threshold value. For typical experimental parameters, the electron temperature is observed to rise from 1,500 K to about 8,000 K in a fraction of a millisecond, much faster than Ohmic heating due to collisions which occurs on a timescale of an order of a second. This initial heating could then lead to further acceleration due to Langmuir turbulence near the critical layer. Stochastic electron heating therefore represents an important potential mechanism for the formation of DAILs.

A theoretical investigation on the parametric instability excited by X‐mode polarized electromagnetic wave at Tromsø

AbstractRecent ionospheric modification experiments performed at Tromsø, Norway, have indicated that X‐mode pump wave is capable of stimulating high‐frequency enhanced plasma lines, which manifests the excitation of parametric instability. This paper investigates theoretically how the observation can be explained by the excitation of parametric instability driven by X‐mode pump wave. The threshold of the parametric instability has been calculated for several recent experimental observations at Tromsø, illustrating that our derived equations for the excitation of parametric instability for X‐mode heating can explain the experimental observations. According to our theoretical calculation, a minimum fraction of pump wave electric field needs to be directed along the geomagnetic field direction in order for the parametric instability threshold to be met. A full‐wave finite difference time domain simulation has been performed to demonstrate that a small parallel component of pump wave electric field can be achieved during X‐mode heating in the presence of inhomogeneous plasma.

Stimulated Brillouin scattering during electron gyro-harmonic heating at EISCAT

Abstract. Observations of secondary radiation, stimulated electromagnetic emission (SEE), produced during ionospheric modification experiments using ground-based, high-power, 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.

Generation of Artificial Acoustic-Gravity Waves and Traveling Ionospheric Disturbances in HF Heating Experiments

We report the results of our ionospheric HF heating experiments to generate artificial acoustic-gravity waves (AGW) and traveling ionospheric disturbances (TID), which were conducted at the High-frequency Active Auroral Research Program facility in Gakona, Alaska. Based on the data from UHF radar, GPS total electron content, and ionosonde measurements, we found that artificial AGW/TID can be generated in ionospheric modification experiments by sinusoidally modulating the power envelope of the transmitted O-mode HF heater waves. In this case, the modulation frequency needs to be set below the characteristic Brunt–Vaisala frequency at the relevant altitudes. We avoided potential contamination from naturally-occurring AGW/TID of auroral origin by conducting the experiments during geomagnetically quiet time period. We determine that these artificial AGW/TID propagate away from the edge of the heated region with a horizontal speed of approximately 160 m/s.

An Efficient 3-D FDTD Model of Electromagnetic Wave Propagation in Magnetized Plasma

Modeling electromagnetic wave propagation in the upper atmosphere is important for space weather effects, satellite communications ionospheric modification experiments, and many other applications. We propose a new methodology for solving and incorporating the current equation into the finite-difference time-domain (FDTD) form of Maxwell's equations for modeling electromagnetic wave propagation in magnetized plasma. This approach employs a version of Boris's algorithm applied to particle-in-cell plasma computational models. There are four primary advantages of this new method over previously developed three-dimensional FDTD models of electromagnetic wave propagation in magnetized plasma. Specifically, it: 1) requires less memory; 2) is more than 50% faster; 3) is easier to implement; and 4) permits the use of two different time step increments when solving the current equation versus Maxwell's equations that is useful for modeling high collisional regimes. The new algorithm is faster because it solves all the equations explicitly and there is no need to solve complicated matrix equations. Modeling of higher altitude ranges and higher frequency electromagnetic waves is much more feasible using this new method. Results of the new FDTD magnetized plasma model are provided and validated.

Heater‐induced ionization inferred from spectrometric airglow measurements

AbstractSpectrographic airglow measurements were made during an ionospheric modification experiment at High Frequency Active Auroral Research Program on 12 March 2013. Artificial airglow enhancements at 427.8, 557.7, 630.0, 777.4, and 844.6 nm were observed. On the basis of these emissions and using a methodology based on the method of Backus and Gilbert (1968, 1970), we estimate the suprathermal electron population and the subsequent equilibrium electron density profile, including contributions from electron impact ionization. We find that the airglow is consistent with heater‐induced ionization in view of the spatial intermittency of the airglow.

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.

Stimulated Brillouin scatter and stimulated ion Bernstein scatter during electron gyroharmonic heating experiments

Results of secondary radiation, Stimulated Electromagnetic Emission (SEE), produced during ionospheric modification experiments using ground‐based high‐power radio waves are reported. These results obtained at the High Frequency Active Auroral Research Program (HAARP) facility specifically considered the generation of Magnetized Stimulated Brillouin Scatter (MSBS) and Stimulated Ion Bernstein Scatter (SIBS) lines in the SEE spectrum when the transmitter frequency is near harmonics of the electron gyrofrequency. The heater antenna beam angle effect was investigated on MSBS in detail and shows a new spectral line postulated to be generated near the upper hybrid resonance region due to ion acoustic wave interaction. Frequency sweeping experiments near the electron gyroharmonics show for the first time the transition from MSBS to SIBS lines as the heater pump frequency approaches the gyroharmonic. Significantly far from the gyroharmonic, MSBS lines dominate, while close to the gyroharmonic, SIBS lines strengthen while MSBS lines weaken. New possibilities for diagnostic information are discussed in light of these new observations.

Reexamining X‐mode suppression and fine structure in artificial E region field‐aligned plasma density irregularities

Artificial field‐aligned plasma density irregularities (FAIs) were generated in the E region of the ionosphere above the High Frequency Active Auroral Research Program facility during campaigns in May and August of 2012 and observed using a 30 MHz coherent scatter radar imager in Homer, Alaska. The purpose of this ionospheric modification experiment was to measure the threshold pump power required to excite thermal parametric instabilities by O‐mode heating and to investigate the suppression of the FAIs by simultaneous X‐mode heating. We find that the threshold pump power for irregularity excitation was consistent with theoretical predictions and increased by approximately a factor of 2 when X‐mode heating was present. A modified version of the Another Model of the Ionosphere (SAMI2) ionospheric model was used to simulate the threshold experiments and suggested that the increase was entirely due to enhanced D region absorption associated with X‐mode heating. Additionally, a remarkable degree of fine structure possibly caused by natural gradient drift instability in the heater‐modified volume was observed in experiments performed during geomagnetically active conditions.

High latitude artificial periodic irregularity observations with the upgraded EISCAT heating facility

We present a recently developed ionospheric modification experiment that produces artificial periodic irregularities in the ionosphere and uses them to make observations of the spatiotemporal behaviour of the irregularities. In addition, the method can be used to measure Faraday rotation and vertical velocities. We also introduce a novel experiment that allows monitoring the formation of the irregularities during heating, in addition to observing their decay after heating. The first measurements indicate, contrary to existing theory, that the amplitude of the radar echoes from the periodic irregularities grows faster than they decay. We focus on the API effects in the D- and E-region of the ionosphere.

The main types of electron energy distribution determined by model fitting to optical emissions during HF wave ionospheric modification experiments

Enhanced optical emissions observed during HF pumping are induced by electrons accelerated by high‐power electromagnetic waves. Using measured emission intensities, the energy distribution of accelerated electrons can be inferred. Energy loss from the excitation of molecular nitrogen vibrational levels (the vibrational barrier) strongly influences the electron energy distribution (EED). In airglow calculations, compensation for electron depletion within the 2–3 eV energy range, induced by the vibrational barrier, can be achieved via electrons with an EED similar to a Gaussian distribution and energies higher than 3 eV. This EED has a peak within the 5–10 eV energy range. We show that the main EED features depend strongly on altitude and solar activity. An EED similar to a power law distribution can occur above 270–300 km altitude. Below 270 km altitude, a Gaussian distribution for energies between 3 eV and 10 eV, together with a power law distribution for energies higher than 10 eV, is indicated. A Gaussian distribution combined with an exponential function is needed below 230 km altitude. The transition altitude from Gaussian to power law distribution depends strongly on solar activity, increasing for high solar activity. Electrons accelerated during the initial collisionless stage can inhibit the depletion of fast electrons within the vibrational barrier range, an effect that strongly depends on altitude and solar activity. The approach, based on the effective root square electric field, enables EED calculation, providing the observed red‐line intensities for low and high solar activities.

First observations of minority ion (H+) structuring in stimulated radiation during second electron gyroharmonic heating experiments

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.

Anomalous ISR echoes preceding auroral breakup: Evidence for strong Langmuir turbulence

Experimental results obtained with the 449‐MHz Poker Flat Incoherent Scatter Radar (PFISR) show unusual features in both the ion line and plasma line measurements during an auroral breakup event. The features are a greatly enhanced flat ion acoustic spectrum (believed to indicate the presence of an additional peak at zero Doppler), and two peaks in the plasma line spectrum. Similar spectral morphologies are observed during active HF ionospheric modification experiments and are considered unmistakable indications of Strong Langmuir Turbulence (SLT). In SLT theory, the central peak in ion acoustic spectrum is caused by Bragg scattering from non‐propagating density fluctuations (cavitons), and the two peaks in the plasma line spectrum are associated with (1) Langmuir waves trapped in the cavitons, at the cold plasma frequency, and (2) a “free mode” at the Langmuir frequency. Free modes are radiated Langmuir waves from collapsing cavitons that follow the linear dispersion relation. The observed turbulence was confined to a thin layer (∼10‐km) centered at ∼230 km altitude.