ELF/VLF wave generation using simultaneous CW and modulated HF heating of the ionosphere

Abstract

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 singleand 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.Citation: Moore, R. C., and D. Agrawal (2011), ELF/VLF wave generation using simultaneous CW and modulated HF heating of the ionosphere,

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

“…This result may constitute the first direct experimental evidence that HF heating (even when modulated) changes the electron density profile in the D ‐region, apparently in a way that improves the generation by ∼5 dB over ∼30 min. The 5 dB intensification is somewhere in between the 7 dB predicted by Milikh and Papadopoulos [] and the 3 dB changes from simultaneous continuous wave heating discussed by Moore and Agrawal []. Milikh and Papadopoulos [] predicted that the electron density changes would occur over the time span of a few minutes, which is shorter than the 30 min of improving generation conditions observed here.…”

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].…”

“…Additionally, we perform the experiment using five different modulation waveforms and present observations performed at three significantly different distances from HAARP (3 km, 33 km, and 98 km). The experimental observations are compared to the predictions of a dual‐beam ionospheric HF heating model [ Moore and Agrawal , 2011], demonstrating that the model properly characterizes the ELF wave magnitude dependence on the transmission parameters. The dual‐beam HF heating model is further employed to predict the dependence of ELF wave magnitude on the polarization of the CW beam and on the modulation frequency of the modulated HF beam.…”

Observations of Ionospheric ELF and VLF Wave Generation by Excitation of the Thermal Cubic Nonlinearity

“…Comparison with theory.-The dual-beam ionospheric HF heating model presented in this work is based on past work [9,22,23], but accounts for electron density changes as a function of HF heating [24]. The model accounts for the HF phasing effect that results from the physical separation of the HF sources [21,22].…”

“…4. These profiles have been used in previous work [9,23]. The ambient electron temperature and neutral density profiles are provided by the MSISE-90 Atmosphere Model [26][27][28].…”