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

Abstract

[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.

On the altitude of the ELF/VLF source region generated during “beat‐wave” HF heating experiments

“…It thus not correct to assume that D ‐region absorption is negligible based on ionogram measurements that indicate little‐to‐no return from the D ‐region ionosphere. This conclusion further supports the analysis presented by Jin et al [2011], who specifically addressed this issue. Based on the observations presented in this paper, a collision‐dominated ionospheric region always exists, and a modulated HF power envelope can create an ELF/VLF source region within that altitude range.…”

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

“…These 2 days demonstrate a difference in generation conditions of ∼30 dB, likely due to either a weaker auroral electrojet on 3 March 2007 or an ionospheric profile less favorable for generation. Rietveld et al [] and Jin et al [] observe and model the connection between generated amplitudes and geophysical conditions, so we do not elaborate further here. All amplitudes presented here are uncorrected for any geophysical measurement, so the natural variation in signal strength is largely due to the natural variation in the generation conditions.…”

“…Using the rapid steering ability of HAARP to modulate the beam spatially, instead of pointing the beam vertically and modulating in time, generated 7–11 dB higher amplitudes and gave directional abilities [ Cohen et al , , , , ]. The connection between HAARP ELF/VLF amplitudes and natural conditions was investigated [ Jin et al , ], as were the properties of the communications channel [ Jin et al , ]. The HF beam can also be divided into two, and the so‐called dual‐beam modulated heating is being used for characterizing the D ‐region of the ionosphere [ Moore and Agrawal , ; Agrawal and Moore , ; Gołkowski et al , ].…”

Dual‐beam ELF wave generation as a function of power, frequency, modulation waveform, and receiver location

“…Extremely low frequency (ELF, 3–3000 Hz) and very low frequency (VLF, 3–30 kHz) waves can be generated by modulated high frequency (HF, 3–30 MHz) heating of the D ‐region ionosphere (∼60–100 km altitude) in the presence of naturally forming electric currents, such as the auroral electrojet [e.g., Getmantsev et al , 1974; Stubbe et al , 1982; Papadopoulos et al , 2003; Moore et al , 2007; Cohen et al , 2010]. The magnitude of ELF/VLF waves generated in this manner are dependent upon the ambient ionospheric conditions, such as the electron density and the electron temperature [e.g., Tomko et al , 1980; Stubbe et al , 1982; Barr and Stubbe , 1984; James et al , 1984; Rietveld and Stubbe , 1987; Papadopoulos et al , 1990; Barr and Stubbe , 1991a, 1991b; Moore , 2007; Cohen et al , 2010; Moore and Agrawal , 2011], as well as on the background geomagnetic conditions that drive the strength of the auroral electrojet [e.g., Stubbe et al , 1981; Rietveld et al , 1983; Papadopoulos et al , 2003; Payne , 2007; Jin et al , 2011]. Additionally, in an effort to understand the dynamics of high power radio wave heating of the ionosphere, a number of studies have investigated the dependence of the generated ELF/VLF signal strength on the HF transmission parameters, such as HF power, HF polarization, and modulation frequency [e.g., Ferraro et al , 1984; Barr and Stubbe , 1991a, 1991b; Villaseñor et al , 1996; Milikh et al , 1999; Moore et al , 2006; Fujimaru and Moore , 2011].…”