High Frequency Active Auroral Research Research Articles

Modulation of auroral electrojet currents using dual modulated HF beams with ELF phase offset, a potential D-region ionospheric diagnostic

[1] Experiments at the ionospheric heating facility of the High Frequency Active Auroral Research Program (HAARP) are performed employing dual HF beams amplitude modulated at ELF/VLF with a phase offset between the two modulation waveforms. The amplitude of the observed ELF/VLF waves is strongly dependent on the imposed ELF/VLF phase offset, the modulation waveform, and the orientation of the HF beams. Data from two ground stations are interpreted using simulations of modulated heating power envelopes as well as a comprehensive model of ionospheric ELF/VLF generation. It is found that two colocated vertical beams HF beams excite a single ionospheric ELF/VLF source, but independent ELF/VLF sources can be induced in the ionospheric region above the heater if the HF beams are offset from zenith to intersect at their 3 dB points. Furthermore, the use of two vertical HF beams with ELF phase offset is found to be a potential diagnostic method for the ionospheric D region.

Ion gyro‐harmonic structuring in the stimulated radiation spectrum and optical emissions during electron gyro‐harmonic heating

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.

HF beam parameters in ELF/VLF wave generation via modulated heating of the ionosphere

ELF/VLF (0.3–30 kHz) wave generation is achievable via modulated HF (3–30 MHz) heating of the lower ionosphere in the presence of natural currents such as the auroral electrojet. Using the 3.6 MW High Frequency Active Auroral Research Program (HAARP) facility near Gakona, AK, we investigate the effect of HF frequency and beam size on the generated ELF/VLF amplitudes, as a function of modulation frequency, and find that generation in the Earth‐ionosphere waveguide generally decreases with increasing HF frequency between 2.75–9.50 MHz. HAARP is also capable of spreading the HF power over a wider area, and we find that a larger beam area yields larger generated amplitudes on the ground. Measurements are shown to generally agree with a theoretical model, which is then applied to also predict the effect of HF beam parameters on magnetospheric injection with HAARP.

Observations and effects of artificial density layers on oblique high‐frequency backscatter

An improved method of estimating ground‐scattered power using high‐frequency (HF) ray tracing techniques that overcomes the limitations of the derivation presented by Bristow and Greenwald (1995) is presented. The improved method is applied toward identifying the effects of an artificial ionospheric density layer on measured ground scatter power. The presence of artificial density layers induced at the High Frequency Active Auroral Research Program (HAARP) station in Gakona, Alaska, are observed through the ground‐scattered power received by the Kodiak Super Dual Auroral Radar Network (SuperDARN) HF radar. The location and physical dimensions of the artificial layers are estimated by simulating radar returns using HF ray tracing through a model ionosphere that includes a model artificial density layer. Simulation results of ground‐scattered power as a function of range are compared to the measured ground‐scattered power as a function of range during a time period when artificial layers were evident in ionogram data. It is shown that a model artificial density layer based on research by Pedersen et al. (2009) produces simulation results that approximate the mean of the measured results.

Excitation of the ionospheric Alfvén resonator from the ground: Theory and experiments

[1] We report results from numerical and experimental studies of the excitation of ULF shear Alfvén waves inside the ionospheric Alfvén resonator (IAR) by heating the ionosphere with powerful HF waves launched from the High Frequency Active Auroral Research Program (HAARP) facility in Alaska. Numerical simulations of the two-fluid MHD model describing IAR in a dipole magnetic field geometry with plasma parameters taken from the observations at HAARP during the October–November 2010 experimental campaign reveal that the IAR quality is higher during nighttime conditions, when the ionospheric conductivity is very low. Simulations also reveal that the resonance wave cannot be identified from the magnetic measurements on the ground or at an altitude above 600 km because the magnetic field in this wave has nodes on both ends of the resonator, and the best way to detect IAR modes is by measuring the electric field on low Earth orbit satellites. These theoretical predictions are in good, quantitative agreement with results from observations: In particular, (1) observations from the ground-based magnetometer at the HAARP site demonstrate no significant difference in the amplitudes of the magnetic field generated by HAARP in the frequency range from 0 to 5 Hz, and (2) the DEMETER satellite detected the electric field of the IAR first harmonic at an altitude of 670 km above HAARP during the heating experiment.

The relationship between geophysical conditions and ELF amplitude in modulated heating experiments at HAARP: Modeling and experimental results

[1] Experiments for generating extremely low frequency (ELF) radio waves using modulated HF heating of the auroral ionosphere have been conducted and refined at the High Frequency Active Auroral Research Program (HAARP) facility at Gakona, Alaska. Because this technique is dependent on strength of the naturally generated electrojet current system, the amplitude of the generated ELF changes with geophysical conditions. Past work has shown that electrojet current strength as measured by magnetometers often correlates with generated ELF amplitude, but there are periods of poor or negative correlation. We attempt to use additional diagnostics from a radar, riometer, ionosonde, and magnetometer chain to understand how ionospheric conditions affect ELF generation. We then present the results of a statistical model that shows that ELF amplitude is roughly proportional to magnetometer measurements for a fixed value of riometer absorption and that the proportionality constant decreases as riometer absorption increases. Theoretical simulations of modulated heating are conducted for a variety of ionospheric density profiles to verify that denser profiles result in smaller gains for ELF generation as a function of electrojet current at a given electric field.

DEMETER observations of the ionospheric trough over HAARP in relation to HF heating experiments

[1] Plasma density variations observed aboard the DEMETER satellite in the topside ionospheric F layer are analyzed in relation to high-frequency transmitter operations. The main interest is the high-latitude region. One hundred cases with operating and nonoperating High Frequency Active Auroral Research Program HF transmitter during day and night are examined. It is found that most large-scale variations can be attributed to the presence of the main ionospheric trough and that such natural variations complicate the detection of HF transmitter effects on a case-by-case basis. From statistics, no correlation between the HF transmissions and the presence of the irregularities has been established. A comparison of our observations with two recent works on electron density ducts created by HF transmitters and detected by DEMETER shows that in those works the main ionospheric trough is the major factor in density variations, and it is not clear how to distinguish density variations created by the HF heater from natural variations in such cases. Finally, possible experimental techniques for duct formation by HF heaters are discussed.

In-situ measurements of topside ionosphere electron density enhancements during an HF-modification experiment

[1] A Defense Meteorological Satellite Program (DMSP) satellite measured an electron density enhancement of approximately 30% at 840 km altitude on 25 February 2008 during an overpass of the High frequency Active Auroral Research Program (HAARP) research station in Alaska where ionosphere modification experiments were being conducted. An upward ion velocity enhancement of 200 m/s was also observed. Simulation results from a one-dimensional self-consistent ionosphere model indicate that topside electron density enhancements similar in magnitude to the observed enhancements at HAARP follow from electron temperature enhanced ambipolar diffusion, lifting atomic oxygen ions from the peak density layer along the geomagnetic field line up to the DMSP satellite orbit altitude. Assuming the HF pump heats the ionosphere electrons uniformly over a 10 km layer, the effective volume heating rate inferred from the model calculations is approximately 1 nW/m3.

ELF/VLF wave generation using simultaneous CW and modulated HF heating of the ionosphere

[1] Experimental observations of ELF/VLF waves generated using the dual-beam heating capability of the High frequency Active Auroral Research Program (HAARP) HF transmitter in Gakona, Alaska, are compared with the predictions of an ionospheric HF heating model that accounts for the simultaneous propagation and absorption of multiple HF beams. The model output is used to assess three properties of the ELF/VLF waves observed on the ground: the ELF/VLF signal magnitude, the ELF/VLF harmonic ratio, and the ELF/VLF power law exponent. Ground-based experimental observations indicate that simultaneous heating of the ionosphere by a CW HF wave and a modulated HF wave generates significantly lower ELF/VLF magnitudes than during periods without CW heating, consistent with model predictions. Further modeling predictions demonstrate the sensitive dependence of ELF/VLF magnitude on the frequency and power of the CW signal. The ratio of ELF/VLF harmonic magnitudes is also shown to be a sensitive indicator of ionospheric modification, although it is somewhat less sensitive than the ELF/VLF magnitude. Last, the peak power level of the modulated HF beam was varied in order to assess the power dependence of ELF/VLF wave generation under both single- and dual-beam heating conditions. Experimental and theoretical results indicate that accurate evaluation of the ELF/VLF power law index requires high signal-to-noise ratio; it is thus a less sensitive indicator of ionospheric modification than either ELF/VLF magnitude or the ELF/VLF harmonic ratio.

On the occurrence of ground observations of ELF/VLF magnetospheric amplification induced by the HAARP facility

[1] The ionospheric heating facility of the High Frequency Active Auroral Research Program (HAARP) has been used extensively in the last 3 years for injection of ELF/VLF waves into the magnetosphere via modulated heating of the overhead auroral electrojet currents. Of particular interest are waves that are observed to be nonlinearly amplified after interaction with hot plasma electrons in the Earth's radiation belts. Past results have shown HAARP to be an effective platform for controlled studies of wave particle interactions in the Earth's magnetosphere. A summary of the experimental results is provided in the context of dependencies on geomagnetic conditions and transmitter parameters. It is deduced that the primary variable that is associated with successful ground observations of HAARP-induced magnetospheric amplification is availability of magnetospheric wave guiding structures. Such structures are found to be most prevalent under quiet geomagnetic conditions following a disturbance when the plasmapause extends to the latitude of the HAARP facility or higher. Strong electrojet currents and high amplitudes of generated ELF/VLF signals observed on the ground are poor indicators of observation probability on a day to day basis although variation of these variables can be important on minute and second timescales. Frequency-time formats with continuously increasing ELF/VLF frequency show preferential amplification as predicted by nonlinear theory of electron trapping. Amplification of signals is also found to be possible for signals with noncoherent bandwidths of up to 30 Hz.

X‐mode suppression of artificial E region field‐aligned plasma density irregularities

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.

Focusing of HF radio-waves by ionospheric ducts

This paper presents the first direct observations of HF focusing induced by natural and artificial ionospheric ducts along with a simple theoretical model. The experiments were conducted by injecting HF radio-waves using the Ionospheric Research Instrument of the High Frequency Active Auroral Research Program located in Gakona, Alaska and detecting them with instruments on the overflying French micro-satellite DEMETER. The latter observed a multiple frequency band structure, which is characteristic of a strong HF signal exceeding the detector's saturation level. Analysis of the O + density measured by DEMETER along its orbit shows that the strong radio signal coincides with the presence of a “negative” duct in the ionosphere. “Negative” refers to the presence of a plasma density depletion with the peak depletion located near the center of the duct. Such ducts induce changes in the index of refraction leading to the focusing of HF waves in a manner equivalent to a “thick” plasma lens. Examination of the data along with a simple plasma lens model indicates the presence of focal node(s) in the vicinity of the overflying satellite. Two examples, one corresponding to focusing by a natural duct and one by an artificial one are presented.

Ionizing wave via high-power HF acceleration

Recent ionospheric modification experiments with the 3.6 MW transmitter at the High Frequency Active Auroral Research Program (HAARP) facility in Alaska led to discovery of artificial ionization descending from the nominal interaction altitude in the background F-region ionosphere by ~60 km. This paper presents a physical model of an ionizing wavefront created by suprathermal electrons accelerated by the HF-excited plasma turbulence.

Production of artificial ionospheric layers by frequency sweeping near the 2nd gyroharmonic

Abstract. Artificial ionospheric plasmas descending from the background F-region have been observed on multiple occasions at the High Frequency Active Auroral Research Program (HAARP) facility since it reached full 3.6 MW power. Proximity of the transmitter frequency to the 2nd harmonic of the electron gyrofrequency (2fce) has been noted as a requirement for their occurrence, and their disappearance after only a few minutes has been attributed to the increasing frequency mismatch at lower altitudes. We report new experiments employing frequency sweeps to match 2fce in the artificial plasmas as they descend. In addition to revealing the dependence on the 2fce resonance, this technique reliably produces descending plasmas in multiple transmitter beam positions and appears to increase their stability and lifetime. High-speed ionosonde measurements are used to monitor the altitude and density of the artificial plasmas during both the formation and decay stages.

Propagation of whistler mode waves with a modulated frequency in the magnetosphere

This paper presents results from experimental and numerical studies of the propagation of whistler mode waves in the Earth's magnetosphere. An experiment conducted at the High Frequency Active Auroral Research Program (HAARP) on 16 March 2008 demonstrates that ionospherically generated waves with particular frequency‐time formats are amplified on their pass from HAARP to the conjugate location in the South Pacific Ocean more efficiently than waves with a constant frequency. Numerical simulations of a one‐dimensional electron magnetohydrodynamics (MHD) model in the dipole magnetic field geometry reveal that the amplification takes place more efficiently when the frequency of the whistler mode waves (in the frequency range from 0.5 to 1.0 kHz) changes in the equatorial magnetosphere with the rate from 0.25 to 0.47 kHz/s. The maximum amplification occurs when this rate is 0.33 kHz/s and no/very little amplification is observed when this gradient is equal to 0 or when it is larger than 0.78 kHz/s.

Decameter structure in heater‐induced airglow at the High frequency Active Auroral Research Program facility

On 28 October 2008, small‐scale rayed artificial airglow was observed at the High frequency Active Auroral Research Program (HAARP) heating facility by the HAARP telescopic imager. This airglow occurred during an experiment at twilight from 0255–1600 UT (1855–2000 LT) and with estimated scale sizes of 100 m (at assumed 225 km altitude) constitutes the smallest structure observed in artificial airglow to date. The rays appeared to be oriented along the geomagnetic field lines. During this period, other instruments, SuperDARN, GPS receivers, stimulated electromagnetic emissions receivers, also recorded unusual data sets with the general characteristic of time scales longer than anticipated for features to form. The experiment took place at the commencement of a small geomagnetic disturbance (Kp of 4.3). This unique observation is as yet unexplained. The airglow features start as large scale structures and then become smaller as heating continues in apparent contradiction to current theories on irregularity development. A thermal gradient instability at boundary of the ionospheric footprint of the plasmapause may be responsible for causing the small‐scale structuring. Observations of 427.8 nm N2+ (first negative group) emissions indicate the presence of ionization.

Ionospheric feedback instability and substorm development

We report on ground magnetic and optical observations performed during an ionospheric heating experiment at the High Frequency Active Auroral Research Program (HAARP) facility in Alaska on 29 October 2008. The experiment was aimed at generation of large‐amplitude ULF electromagnetic waves by triggering and facilitating development of the ionospheric feedback instability (IFI) in the region adjacent to a bright auroral arc. In this region the downward/return magnetic field‐aligned current decreases plasma density and enhances the electric field in the ionosphere. A combination of these two effects creates favorable conditions for the instability. The experiment occurred during a period of substorm activity, but effects from the HAARP transmitter were not sufficiently intense to be detected against the background of strong natural oscillations occurring farther north from the HAARP site. Thus the experiment did not provide concrete evidence that heating of the ionosphere with powerful HF transmitters can affect IFI development or generate intense ULF electromagnetic waves. However, during the experiment ground‐based magnetometers in Alaska and Canada detected large‐amplitude ULF waves in regions where the substorm onset auroral arcs interacted with the ionosphere. The frequencies of these waves closely matched frequencies predicted by simulations of IFI for these particular geophysical conditions. These observations support the hypothesis that geomagnetic substorms, the corresponding dynamics of discrete auroral arcs, and the ionospheric feedback instability are closely connected phenomena.

Amplitude and phase of nonlinear magnetospheric wave growth excited by the HAARP HF heater

The High Frequency Active Auroral Research Program (HAARP) HF ionospheric heater is used to inject ELF/VLF signals into the magnetosphere to study wave‐particle interactions that lead to nonlinear amplification. HAARP‐generated whistler mode “echoes” are observed after twice crossing the equatorial plane in magnetospheric ducts. The magnetospheric paths traversed by the signals are determined to be in the range 5.06 < L < 5.19 with associated cold plasma densities of 177 cm−3 < Neq < 185 cm−3. The amplitude and phase of the echoes are observed to exhibit exponential temporal increase. A decrease in input amplitude by 14 dB is observed to yield a delay in onset of nonlinear growth by ∼1 s. Nonlinear theory of cyclotron resonant currents formed by electrons phase trapped in the potential well of the input wave is used to analyze the simultaneous evolution of amplitude and phase of the observations. The average nonlinear resonant current vector is found to rotate in relation to the input wavefields during growth, and the magnetospheric linear growth rate is estimated to be 31–45 dB/s.

Effects of substorm dynamics on magnetic signatures of the ionospheric Alfvén resonator

Spectral resonance structures (SRS) of the ionospheric Alfvén resonator (IAR) measured by the induction magnetometer at the High Frequency Active Auroral Research Program (HAARP) ionospheric observatory during a substorm on 28 February 2006 are presented. The evolution of IAR SRS is compared to ionospheric parameters measured by the colocated Digisonde, riometer and all‐sky imager at HAARP. Initially, the magnetic IAR signatures (spectral resonance structures) exhibited an expected variation that can be attributed to typical diurnal changes in ionospheric structure. At substorm onset, the signatures disappeared because of either a suppression of resonance conditions by substorm‐related particle precipitation or enhanced power in the Pc1 spectrum that concealed continuing IAR SRS. After the substorm, the SRS reappeared; however the harmonics had shifted to lower frequencies with tighter frequency spacing. For the first time, we show that this time‐dependent behavior in IAR SRS is explained by increased F region densities resulting from electron precipitation. Similarities between observed IAR harmonic frequencies and those calculated with a model suggest that variations in F region density, especially foF2, may often dominate the evolution of IAR eigenfrequencies. This could potentially provide a mechanism for monitoring topside dynamics using IAR SRS.

Cross modulation of whistler mode and HF waves above the HAARP ionospheric heater

[1] The HF facility of the High Frequency Active Auroral Research Program (HAARP) is employed to generate ELF/VLF radiation by modulation of overhead auroral electrojet currents for magnetospheric wave injection and propagation in the Earth-ionosphere waveguide. HAARP induced ducted magnetospherically amplified whistler mode signals are observed to return to the vicinity of the HF facility and experience cross modulation with simultaneous HF transmissions. The cross modulation effect results from modification of the attenuation properties of the ionospheric medium by HAARP HF waves in the daytime ionosphere. The cross modulation concept is subsequently investigated as a new method for ELF/VLF wave generation with the HAARP heating facility using two co-located HF beams. The new generation method is observed to generate radiation from 630 Hz–37 kHz. Observed amplitudes are an order of magnitude lower than fundamental frequencies generated using conventional AM, but of the same order as AM second harmonics.