Observations of intensity variations and harmonics of heater induced VLF waves

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

“…However, natural changes in the electrojet are often the dominant source of variability in ELF generation and concerns over the reliability of modulated heating as an ELF source center around this variability. Data presented by Rietveld et al [1987] and Oikarinen et al [1997] show natural amplitude fluctuations of at least 12 dB. It is usually assumed that natural changes in ELF amplitude correlate directly with changes in electrojet current strength.…”

“… Kamide et al [1982] showed the total overhead east‐west current is linearly proportional to Δ H , the difference between the measured value of H and its undisturbed value during quiet conditions. However, experimental data often shows periods where electrojet current strength and ELF amplitude are poorly or negatively correlated [ Rietveld et al , 1987; Oikarinen et al , 1997; Jin et al , 2009].…”

“…The earliest attempts to correlate ELF generation with other measurements were shown by Kapustin et al [1977], who noted a positive, linear correlation between ELF amplitude and magnetometer measurements but did not quantify the relationship. Rietveld et al [1983, 1987], and Oikarinen et al [1997] present many more examples from the ionospheric heater at Tromsø noting cases of positive, weak, and negative correlation between ELF intensity and magnetometer data. Rietveld et al [1987] attempt to correlate ELF intensity with riometer absorption and the electric field driving the electrojet as measured by STARE.…”

Features of amplitude and Doppler frequency variation of ELF/VLF waves generated by “beat‐wave” HF heating at high latitudes

“…The signals at the beat frequency of 2017 Hz were recorded at the Lovozero Geophysical Observatory, and no signals have been missed. Signal detections and measurements in southward direction from the EISCAT facility at several latitudes have earlier been reported [ Rietveld et al , ; Barr and Stubbe , ; Oikarinen et al , ; Barr , ], but there are no measurements eastward except for Oikarinen et al [] who measured 96 km away toward the southeast.…”

“…The experiment verified the theoretical prediction that intensities of ELF/VLF signals generated during AM ionospheric heating would be strongest in time of a substorm [ Getmantsev et al , ]. Subsequent studies showed a lot of examples with weak or negative correlation between ELF/VLF intensity and magnetometer data [ Rietveld et al , ; Barr and Stubbe , ; Oikarinen et al , ; Jin et al , ]. [ Jin et al , , ] showed using statistical and theoretical models that the modulated heating is a highly nonlinear process and a weak electrojet can support high ELF amplitudes.…”