On the generation of ELF/VLF waves for long‐distance propagation via steerable HF heating of the lower ionosphere

Cohen, M. B.; Inan, U. S.; Gołkowski, Mark; Lehtinen, N. G.

doi:10.1029/2009ja015170

Cited by 40 publications

(26 citation statements)

References 34 publications

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“…The radiation pattern from sinusoidal power modulation is mostly isotropic around HAARP. There is some feature (most noticeable at 2 kHz) that looks like a spiral pattern, resulting from the non‐vertical component of the geomagnetic field, and the asymmetric response of the Hall and Pedersen conductivity, described in Figure 3 of Cohen et al [2010a]. Also evident is the interference pattern of multimode ELF/VLF propagation, appearing as concentric rings around HAARP, particularly at the higher frequencies where more modes propagate, and the free space wavelength is shorter.…”

Section: Resultsmentioning

confidence: 95%

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ELF/VLF wave generation from the beating of two HF ionospheric heating sources

Cohen¹,

Moore²,

Gołkowski³

et al. 2012

J. Geophys. Res.

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It is well established that Extremely Low Frequency (ELF, 0.3–3 kHz) and Very Low Frequency (VLF, 3–30 kHz) radio waves can be generated via modulated High Frequency (HF, 3–10 MHz) heating of the lower ionosphere (60–100 km). The ionospheric absorption of HF power modifies the conductivity of the lower ionosphere, which in the presence of natural currents such as the auroral electrojet, creates an ‘antenna in the sky.’ We utilize a theoretical model of the HF to ELF/VLF conversion and the ELF/VLF propagation, and calculate the amplitudes of the generated ELF/VLF waves when two HF heating waves, separated by the ELF/VLF frequency, are transmitted from two adjacent locations. The resulting ELF/VLF radiation pattern exhibits a strong directional dependence (as much as 15 dB) that depends on the physical spacing of the two HF sources. This beat wave source can produce signals 10–20 dB stronger than those generated using amplitude modulation, particularly for frequencies greater than 5–10 kHz. We evaluate recent suggestions that beating two HF waves generates ELF/VLF waves in the F‐region (>150 km), and conclude that those experimental results may have misinterpreted, and can be explained strictly by the much more well established D region mechanism.

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Section: Resultsmentioning

confidence: 95%

“…We utilize a pair of three‐dimensional models which together simulate the HF to ELF/VLF conversion and propagation process, described by Cohen et al [2010a]. The HF heating model is based on earlier work [ Tomko , 1981; Rodriguez , 1994; Moore , 2007; Payne et al , 2007].…”

Section: Model Descriptionmentioning

confidence: 99%

ELF/VLF wave generation from the beating of two HF ionospheric heating sources

Cohen¹,

Moore²,

Gołkowski³

et al. 2012

J. Geophys. Res.

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“…The HF heating component of the model is described in detail by Cohen et al [2010a] and builds off the work of Tomko et al [1980], Rodriguez [1994], Payne [2007], and Moore [2007]. To summarize, the three‐dimensional model uses a grid, which in this case is defined from 60–95 km in altitude and spans 160 km in each horizontal direction.…”

Section: Theoretical Modelingmentioning

confidence: 99%

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

et al. 2011

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[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.

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“…We use here a three‐dimensional theoretical model to try to roughly estimate the radiated power using short‐range observations of the magnetic field. We apply a theoretical model of the HF to ELF/VLF conversion process that has been used and validated several times in connection with HAARP‐generated ELF/VLF signals [ Cohen et al , , , , ], so we describe it only briefly here. The model takes as input an ionospheric background condition and a realistic HF radiation pattern from HAARP.…”

Section: Radiated Powermentioning

confidence: 99%

100 days of ELF/VLF generation via HF heating with HAARP

Cohen

Gołkowski

2013

JGR Space Physics

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Extremely low frequency/very low frequency (ELF/VLF) radio waves are difficult to generate with conventional antennas. Ionospheric high frequency (HF) heating facilities generate ELF/VLF waves via modulated heating of the lower ionosphere. HF heating of the ionosphere changes the lower ionospheric conductivity, which in the presence of natural currents such as the auroral electrojet creates an antenna in the sky when heating is modulated at ELF/VLF frequencies. We present a summary of nearly 100 days of ELF/VLF wave generation experiments at the 3.6 MW High Frequency Active Auroral Research Program (HAARP) facility near Gakona, Alaska, at a variety of ELF/VLF frequencies, seasons, and times of day. We present comprehensive statistics of generated ELF/VLF magnetic fields observed at a nearby site, in the 500–3500 Hz band. Transmissions with a specific HF beam configuration (3.25 MHz, vertical beam, amplitude modulation) are isolated so the data comparison is self‐consistent, across nearly 5 million individual measurements of either a tone or a piece of a frequency‐time ramp. There is a minimum in the average generation close to local midnight. It is found that generation during local nighttime is on average weaker but more highly variable, with a small number of very strong generation periods. Signal amplitudes from day to day may vary by as much as 20–30 dB. Generation strengthens by ∼5 dB during the first ∼30 min of transmission, which may be a signature of slow electron density changes from sustained HF heating. Theoretical calculations are made to relate the amplitude observed to the power injected into the waveguide and reaching 250 km. The median power generated by HAARP and injected into the waveguide is ∼0.05–0.1 W in this baseline configuration (vertical beam, 3.25 MHz, amplitude modulation) but may have generated hundreds of watts for brief durations. Several efficiency improvements have improved the ELF/VLF wave generation efficiency further.

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On the generation of ELF/VLF waves for long‐distance propagation via steerable HF heating of the lower ionosphere

Cited by 40 publications

References 34 publications

ELF/VLF wave generation from the beating of two HF ionospheric heating sources

ELF/VLF wave generation from the beating of two HF ionospheric heating sources

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

100 days of ELF/VLF generation via HF heating with HAARP

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