HF Heating Research Articles

F2‐region atmospheric gravity waves due to high‐power HF heating and subauroral polarization streams

We report the first evidence of atmospheric gravity waves (AGWs) generated in the F2 region by high‐power HF heating and subauroral polarization streams. Data come from the CHAMP and GRACE spacecraft overflying the High‐frequency Active Auroral Research Program (HAARP) heating facility. These observations facilitate a new method of studying the ionosphere‐thermosphere coupling in a controlled fashion by using various HF‐heating regimes. They also reveal the subauroralF2 region to be a significant source of substorm AGWs, in addition to the well‐known auroralE region.

A new model for formation of artificial ducts due to ionospheric HF‐heating

We present the results of numerical simulations of artificial ducts during high‐power HF heating performed by a novel model accounting for the effect of self‐action. This effect interferes with the HF‐plasma matching in the heated region and hence with electron heating. The model satisfactorily explains recent experimental observations. It helps for choosing the heating parameters optimal for duct formation, such as proper duration of the heating pulse and its frequency. It also suggests that distortion of the ducts caused by the self‐action effect can be avoided by down‐chirping the heating frequency. The down‐chirping rates needed to suppress such distortions are evaluated.

Sounding of HF heating‐induced artificial ionospheric disturbances by navigational satellite radio transmissions

During experiments carried out in 2009–2011 the midlatitude ionosphere was modified by powerful HF pulses from the Sura heating facility located near Nizhny Novgorod (Russia) and operated by the Radio Physical Research Institute. GPS/GLONASS and Parus/Tsikada satellite radio transmissions responding to the heating‐induced disturbances in electron density were analyzed. The variations in the total electron content (TEC), which are proportional to the reduced phase of navigational signals, were studied for various schemes of radiation of the heating wave. The variations in TEC (their amplitudes and temporal behavior) caused by HF heating are identified in several examples. The TEC spectra contain frequency components corresponding to the modulation periods of the heating wave. For the first time, the spatial structure of the wave disturbances generated in the ionosphere by high‐power radio waves radiated by the Sura heating facility with a square wave modulation of the effective radiated power at a frequency lower than or of the order of the Brunt‐Vaisala frequency of the neutral atmosphere is imaged using the method of low‐orbital radio tomography and GPS/GLONASS data.

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.

Beating HF waves to generate VLF waves in the ionosphere

Beat‐wave generation of very low frequency (VLF) waves by two HF heaters in the ionosphere is formulated theoretically and demonstrated experimentally. The heater‐induced differential thermal pressure force and ponderomotive force, which dominate separately in the D and F regions of the ionosphere, drive an electron current for the VLF emission. A comparison, applying appropriate ionospheric parameters shows that the ponderomotive force dominates in beat‐wave generation of VLF waves. Three experiments, one in the nighttime in the absence of D and E layers and two in the daytime in the presence of D and E layers, were performed. X mode HF heaters of slightly different frequencies were transmitted at CW full power. VLF waves at 10 frequencies ranging from 3.5 to 21.5 kHz were generated. The frequency dependencies of the daytime and nighttime radiation intensities are quite similar, but the nighttime radiation is much stronger than the daytime one at the same radiation frequency. The intensity ratio is as large as 9 dB at 11.5 kHz. An experiment directly comparing VLF waves generated by the beat‐wave approach and by the amplitude modulation (AM) approach was also conducted. The results rule out the likely contribution of the AM mechanism acting on the electrojet and indicate that beat‐wave in the VLF range prefers to be generated in the F region of the ionosphere through the ponderomotive nonlinearity, consistent with the theory. In the nighttime experiment, the ionosphere was underdense to the HF heaters, suggesting a likely setting for effective beat‐wave generation of VLF waves by the HF heaters.

RF-Induced Airglow Observed Using Composite Multispectral Imaging

Atmospheric airglow emissions accompanying artificial ionospheric plasmas occur when the bottom-side ionospheric F-region is exposed to high-power HF heating. These artificial emissions are spectrally similar to those which occur naturally as airglow and aurora, yet they have spatiotemporal behavior commensurate with the heater beam geometry. Interesting dynamics of both the artificial plasma layers and optical emissions have been observed, namely, the presence of multiple descending plasma layers distributed in altitude, mesoscale (10-100 km) bullseye-type airglow emission patterns, and, finally, small-scale (≲ 10 km) field-aligned filaments. We present visualizations of such features exhibited by the heated region using composite multispectral imaging.

First demonstration of HF-driven ionospheric currents

[1] The first experimental demonstration of HF driven currents in the ionosphere at low ELF/ULF frequencies without relying in the presence of electrojets is presented. The effect was predicted by theoretical/computational means in a recent letter and given the name Ionospheric Current Drive (ICD). The effect relies on modulated F-region HF heating to generate Magneto-Sonic (MS) waves that drive Hall currents when they reach the E-region. The Hall currents inject ELF waves into the Earth-Ionosphere waveguide and helicon and Shear Alfven (SA) waves in the magnetosphere. The proof-of-concept experiments were conducted using the HAARP heater in Alaska under the BRIOCHE program. Waves between 0.1–70 Hz were measured at both near and far sites. The letter discusses the differences between ICD generated waves and those relying on modulation of electrojets.

Characteristics of Pc4–5 pulsations obtained using the method of bistatic backscatter of HF radio waves, the EISCAT HF heating facility, and ground-based magnetometers

The results of studying the Pc4–5 pulsation parameters based on the method of bistatic backscatter of radio waves, using the EISCAT/Heating HF facility (Tromso, Norway) and IMAGE ground-based magnetometers (Scandinavia), are presented. The observations were performed during the morning hours on October 3, 2006, when a substorm developed on the nightside. An analysis of the observational data obtained from 1000 to 1020 UT indicated that wave-like disturbances with periods corresponding to Pc4–5 pulsations (80–240 s) existed at that time. The variations in the full vector of the ionospheric irregularity motion and the electric field strength in an artificially disturbed high-latitude ionospheric F region has been reconstructed based on simultaneous Doppler observations on two paths. A general conformity is observed among the time variations in Pc4–5 pulsations in the magnetic and ionospheric data: between the velocity amplitude (|V|) and the X component of the Earth’s magnetic field and between the irregularity motion azimuth and the Y component. Large-scale waves, corresponding to the natural resonances of magnetic field lines (small values of the azimuthal number |m| ∼ 2–4), and small-scale waves (large values |m| ∼ 17–20) were simultaneously registered during the experiment based on magnetic data. It has been indicated that the periods of wave-like processes registered using the method of bistatic backscatter and ground-based magnetometers were in agreement with one another. The formation of wave-like processes is explained by the nonstationary impact of the solar wind and IMF on the Earth’s magnetosphere. The variations in the IMF, according to the ACE satellite measurements, were characterized by a sharp increase in the solar wind plasma dynamic pressure that occurred at about 09 UT on October 3, 2006, and was accompanied by rapid polarity reversals of the north-ward-southward (Bz) and transverse (By) IMF components.

Downward ELF/VLF Waves Radiation Excited by Ionospheric Artificial Modulation

AbstractBy heating the ionosphere with ELF/VLF‐modulated high‐power HF wave, a virtual antenna is produced in the ionosphere, which is an effective means to launch ELF/VLF waves. This paper uses the modulated‐heating models of Wang Feng [2009] to calculate the strength of the LF radiation source produced by HF heating, and analyzes attenuation and reflection of the LF wave transmitting downward using full‐wave algorithm. With HAARP experiment parameter, this work simulates magnetic field of the LF signals on the sea, which is of PT order of magnitude, consistent with the experimental data.

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.

Magnetospheric injection of ELF/VLF waves with modulated or steered HF heating of the lower ionosphere

[1] ELF/VLF waves have been generated via steerable HF heating of the lower ionosphere. The temperature-dependent conductivity of the lower ionospheric plasma enables HF heating (and subsequent recovery) to modulate natural current systems such as the auroral electrojet, thus generating an antenna embedded in the ionospheric plasma. We apply a realistic three-dimensional model of HF heating and ionospheric recovery, as well as ELF/VLF wave propagation in and below the ionosphere, to derive the radiation pattern into the magnetosphere as a result of steerable HF heating. It is found that modulated HF heating preferentially directs signals upward into space because of the phasing effect of the upward HF wave propagation. We find that the steering techniques such as the geometric modulation “circle sweep” enhances the total ELF/VLF power injected into the magnetosphere by 5–7 dB compared to amplitude modulated heating, with a few dB enhancement in the peak magnetic field value. Another technique known as beam painting enhances the total injected power by 1–3 dB but produces weaker peak magnetic fields due to the power being spread over a larger area. Observations on the DEMETER spacecraft are presented and compared with theoretical predictions. DEMETER observations show that the signal produced with geometric modulation can be stronger than the signal from AM under the same conditions.

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.

HF-driven currents in the polar ionosphere

[1] Polar ionospheric heaters have generated ULF/ELF/VLF waves by modulating the auroral electrojet at D/E region altitudes. We present theoretical/computational results indicating that modulated F-region HF heating can generate ionospheric currents even in the absence of electrojet currents. The ELF currents are driven in a two-step process. First, the pressure gradient associated with F-region electron heating drives a local diamagnetic current. This acts as an antenna to inject Magneto-Sonic (MS) waves in the ionospheric plasma. Second, the electric field of the magneto-sonic wave drives Hall currents when it reaches the E region of the ionosphere. The Hall currents act as a secondary antenna that injects waves in the Earth-Ionosphere Waveguide below and Shear Alfven waves upwards to the conjugate regions. The paper examines the scaling and limitations of the concept and suggests proof-of-principle experiments using the HAARP ionospheric heater.

VLF wave generation by beating of two HF waves in the ionosphere

[1] Theory of a beat-wave mechanism for very low frequency (VLF) wave generation in the ionosphere is presented. The VLF current is produced by beating two high power HF waves of slightly different frequencies through the nonlinearity and inhomogeneity of the ionospheric plasma. Theory also shows that the density irregularities can enhance the beat-wave generation. An experiment was conducted by transmitting two high power HF waves of 3.2 MHz and 3.2 MHz + f, where f = 5, 8, 13, and 2.02 kHz, from the HAARP transmitter. In the experiment, the ionosphere was underdense to the O-mode heater, i.e., the heater frequency f0 > foF2, and overdense or slightly underdense to the X-mode heater, i.e., f0 < fxF2 or f0 ≥ fxF2. The radiation intensity increased with the VLF wave frequency, was much stronger with the X-mode heaters, and was not sensitive to the electrojet. The strongest VLF radiation of 13 kHz was generated when the reflection layer of the X-mode heater was just slightly below the foF2 layer and the spread of the O-mode sounding echoes had the largest enhancement, suggesting an optimal setting for beat-wave generation of VLF waves by the HF heaters.

Artificial small-scale field-aligned irregularities in the high latitude F region of the ionosphere induced by an X-mode HF heater wave

[1] The effects on the high-latitude F region of the ionosphere by X-mode powerful HF radio waves injected towards the magnetic zenith (MZ) are analysed. The experiments were conducted using the EISCAT/Heating facility and UHF radar at Tromso, Norway, the CUTLASS (SuperDARN) radar and the EISCAT ionosonde (dynasonde). The results show that the X-mode HF pump wave, radiated into the magnetic zenith from the HF heater, can generate very strong small-scale artificial field aligned irregularities (AFAIs) in the F-region of the high-latitude ionosphere. These irregularities, with spatial scales across the geomagnetic field of the order of 8–15 m, are generated when the heater frequency is above the ordinary-mode critical frequency but comparable with the extraordinary-mode critical frequency. The generation of the X-mode AFAIs was accompanied by electron temperature (Te) enhancements up to 50% above the background level and an increase in the electron density (Ne) by up to 30%.

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.

Ionospheric modification from under-dense heating by high-power HF transmitter

[1] Under-dense HF heating experiments were conducted near local solar noon as well as in the nighttime with the HF heater transmitting at 9.1 MHz directed along the geomagnetic zenith and run at 2 min on and 2 min off. The effective isotropic radiated power of the HF transmitter exceeded 3 GW. The Digisonde operated in a fast mode was used to monitor the temporal evolution of the ionospheric electron density distributions in the bottomside of the ionosphere (in the ranges from 90 to 190 km in the noontime and from 230 to 350 km in the nighttime). The electron temperature distributions were then evaluated. The results show that the electron density distributions are modified continuously over the experimental periods. In the noontime, the electron density decreases/increases in time in the region below/above a height at about 140 km, manifesting the change of the balance between the photoionization and the electron-ion recombination and the electron-oxygen dissociative attachment losses by the heating. In the nighttime, the ionosphere was lifted by 30 to 50 km through continuously upward expansion, resulting in the drop of the electron density in the bottomside of the ionosphere in time. A comparison with the ionogram, height, and electron density distribution of unheated ionosphere with similar background conditions further elaborates the observation of thermal expansion.

Microwave Magnetic Field Heating of a Cobalt-Based Amorphous Ribbon

The relationship between the magnetic properties of a Co-based amorphous ribbon and its heating by a microwave magnetic field (hf) was investigated by using a 5.8-GHz single-mode applicator. The amorphous ribbon has a magnetic anisotropy in the ribbon plane. Heating efficiency of hf which was applied in the direction perpendicular to the casting direction was better than that of it applied along the casting direction. The contribution of magnetic loss to the hf heating was evaluated quantitatively when the hf was applied to both directions. The contribution of magnetic loss was more than 80% when the microwave hf was applied perpendicular to the casting direction, and less than 20% when it was applied parallel to the casting direction.

Generation of ionospheric ducts by the HAARP HF heater

We report an investigation of ionospheric ducts having the shape of large plasma sheets, generated by vertically transmitted HAARP HF heater waves in several experiments conducted in Gakona, Alaska. Theory predicts that O-mode heater wave-created ionospheric ducts form parallel-plate waveguides within the meridional plane, and those generated by the X-mode heater waves are orthogonal to the meridional plane. Our theoretical prediction is supported by measurements of ionosonde data (namely ionograms), range–time–intensity (RTI) plots of UHF and HF backscatter radars, as well as magnetometer data analyses. When these plasma sheets experienced E×B drifts, they were intercepted by the HAARP UHF radar and seen as slanted stripes in the RTI plots. This striking feature was also observed in our earlier experiments using the Arecibo UHF radar.

A method for improving plasma temperature estimates from incoherent scatter analysis during artificial ionospheric modification experiments

Spectral features in incoherent scatter data, such as those caused by the purely growing mode (PGM), can often be strongly enhanced during the first few seconds of artificial ionospheric heating experiments, such as those carried out using the high‐power European Incoherent Scatter HF heater at Tromsø. These features, often referred to as “overshoot” effects, are indicators of turbulent non‐Maxwellian plasma, and the analysis of these spectra using standard incoherent scatter data analysis software leads to a poor estimation of the plasma parameters (particularly electron and ion temperature) during RF heating experiments. In this study, a procedure is developed to derive a more reliable estimate of plasma temperature during periods when the incoherent scatter spectrum is affected by contamination from the PGM. This is achieved by removing the PGM from the measured spectrum and then analyzing the modified spectrum using standard software. The results are compared to those obtained from the analysis of the original, contaminated spectra. It is found that the differences between the results obtained from the corrected and uncorrected spectra are strongly proportional to the magnitude of the PGM feature. We also show that the bulk temperatures during the remainder of the “heater on” period after the overshoot can generally be estimated reliably by the standard analysis software, though with some important exceptions. These results are important since the plasma temperatures play a crucial role in governing thermal conduction processes, and their correct estimation is thus very important to understanding the underlying physical processes which occur during ionospheric heating.