HAARP Research Articles

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.

Analysis of time-of-arrival observations performed during ELF/VLF wave generation experiments at HAARP

[1] Modulated high frequency (HF) heating of the lower ionosphere in the presence of auroral electrojet currents has become an important method for generating electromagnetic waves in the extremely-low frequency (ELF) and very-low frequency (VLF) bands. Recent research efforts focus on improving the efficiency of ELF/VLF wave generation. One method to do so involves the spatial mapping of modulated currents that result from HF heating for comparison with HF heating models. As a first step toward providing a spatial map of the modulated ionospheric currents, we introduce time-of-arrival (TOA) observations performed during a series of experimental research campaigns conducted at the High-frequency Active Auroral Research Program (HAARP) in Gakona, Alaska. The TOA method provides a measurement of the ELF/VLF amplitude and phase detected at a ground-based receiver as a function of time, and this information may be used to estimate the distribution of ELF/VLF source currents within the HF heated region. In an effort to test and improve the TOA method, the University of Florida conducted ELF/VLF wave generation experiments using the HAARP HF transmitter under varying ionospheric conditions and using various transmission formats. In this paper, we summarize our experimental results and compare observations with the predictions of a theoretical model.

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.

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.

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.

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.

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.

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.

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.

Relationship between electrojet current strength and ELF signal intensity in modulated heating experiments

The High Frequency Active Auroral Research Program (HAARP) facility is used to generate waves in the extremely low frequency (ELF) range via modulated HF heating of the ionosphere. This HF heating modulates the electron temperature in the D region ionosphere and leads to modulated conductivity and a time‐varying current which then radiates at the modulation frequency. We investigate the relationship between the intensity of the HAARP‐generated ELF signal and the strength of the east‐west component of the auroral electrojet as measured by a ground‐based magnetometer. We find that under all magnetic conditions, HAARP can generate ELF radiation detectable 37 km away with 73% of tones having an amplitude exceeding 0.15 pT. While strong ELF amplitudes (>1.5 pT) were most common during an enhanced electrojet, a weak electrojet can also support equally high ELF amplitudes. The relative change in ELF amplitude per unit change in electrojet current strength is inversely proportional to the absolute current strength. We interpret the dynamic relationship between ELF amplitude and electrojet current strength in terms of the time‐variable ionospheric parameters and HF heating efficiency.

New observations of HF-induced optical emissions from the ionosphericEregion

[1] A new independent instance of highly localized artificial 557.7 nm optical emissions induced by high-power HF waves incident upon a precipitation-produced E layer at High Frequency Active Auroral Research Program (HAARP) confirms the original discovery of this phenomenon and adds a number of new observational facts. The features are excited by O mode transmissions but not X mode transmissions and disappear entirely during deliberate off periods. The specific transmitter pulsing period does not appear to be a critical factor, ruling out ULF resonances as a possible mechanism. The observed locations of the enhanced emissions are consistent with a small number of long-lived inhomogeneities traveling steadily across the field of view over several minutes. These persistent regions are preferentially excited even at a small fraction of peak power well away from the beam center and can reappear along the same trajectory after gaps of more than 1 min. The absence of any detectable enhancements in directly coincident 630.0 nm data or side-looking 557.7 and 630.0 nm images focused on the F region portion of the field line conclusively locates these artificial emissions in the ionospheric E region. We suggest possible hypotheses that might explain the new observations and the earlier results.

Unprecedentedly Strong and Narrow Electromagnetic Emissions Stimulated by High-Frequency Radio Waves in the Ionosphere

Experimental results of secondary electromagnetic radiation, stimulated by high-frequency radio waves irradiating the ionosphere, are reported. We have observed emission peaks, shifted in frequency up to a few tens of Hertz from radio waves transmitted at several megahertz. These emission peaks are by far the strongest spectral features of secondary radiation that have been reported. The emissions are attributed to stimulated Brillouin scattering, long predicted but hitherto never unambiguously identified in high-frequency ionospheric interaction experiments. The experiments were performed at the High-Frequency Active Auroral Research Program (HAARP), Alaska, USA.

Radio Pumping of Ionospheric Plasma with Orbital Angular Momentum

Experimental results are presented of pumping ionospheric plasma with a radio wave carrying orbital angular momentum (OAM), using the High Frequency Active Auroral Research Program (HAARP) facility in Alaska. Optical emissions from the pumped plasma turbulence exhibit the characteristic ring-shaped morphology when the pump beam carries OAM. Features of stimulated electromagnetic emissions (SEE) that are attributed to cascading Langmuir turbulence are well developed for a regular beam but are significantly weaker for a ring-shaped OAM beam in which case upper hybrid turbulence dominates the SEE.

Magnetospheric amplification and emission triggering by ELF/VLF waves injected by the 3.6 MW HAARP ionospheric heater

The HF dipole array of the High Frequency Active Auroral Research Program (HAARP) in Gakona, Alaska, was recently upgraded to 180 elements, facilitating operations at a total radiated power level of 3.6 MW and an effective radiated power of ∼575 MW. In the first experiments at the new power level, the HAARP array is used for magnetospheric wave injection. Modulated heating of auroral electrojet currents in the ionosphere yields radiation in the ELF/VLF frequency range. The HAARP‐generated signals are injected into the magnetosphere, where they propagate in the whistler mode in field‐aligned “ducts,” allowing them to be observed at the conjugate point on a ship‐borne receiver and on autonomous buoy platforms. The observation of the 1‐hop signals is accompanied by the observation of associated 2‐hop components in the northern hemisphere, which have reflected from the ionospheric boundary in the southern hemisphere. The observed signals are accompanied by triggered emissions and exhibit temporal amplification of 15–25 dB/s and bandwidth broadening to ∼50 Hz. Amplification occurs at injected signal frequencies selected in near real time on the basis of observations of natural emission activity, and only certain components of the frequency‐time formats transmitted are amplified. Observations at multiple sites and dispersion analysis show that the signals are injected into the magnetosphere directly above the HF heater. The duration of echo observation and the prevalence of 1‐hop observations are consistent with statistics from 1986 Siple Station experiments. The particle‐trapping wave amplitude near the magnetic equator is estimated in the range 0.1–0.4 pT and gyroresonance with 10 keV–100 keV electrons.