Comment on “Geometric modulation: A more effective method of steerable ELF/VLF wave generation with continuous HF heating of the lower ionosphere” by M. B. Cohen, U. S. Inan, and M. A. Golkowski

Moore, R. C.; Rietveld, M. T.

doi:10.1029/2008gl036002

Cited by 6 publications

(18 citation statements)

References 5 publications

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“…The lack of ability to control the altitude of ionospheric electron temperature modulation using HF carrier frequency or X versus O mode polarization confirms the findings of ] who found no significant altitude difference for different HF carriers using a time of arrival technique. The ability of HF heating techniques to generate independent ELF sources was a subject of recent discussion in the literature [ Moore and Rietveld , ; Cohen et al , ]. The results shown herein support the claim that subregions of the ionosphere can be treated independently, thus implying a coherence length of conductivity modification smaller than the dimensions of the heated region.…”

Section: Discussion and Summarysupporting

confidence: 72%

Modulation of auroral electrojet currents using dual modulated HF beams with ELF phase offset, a potential D‐region ionospheric diagnostic

2013

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Experiments at the ionospheric heating facility of the High Frequency Active Auroral Research Program (HAARP) are performed employing dual HF beams amplitude modulated at ELF/VLF with a phase offset between the two modulation waveforms. The amplitude of the observed ELF/VLF waves is strongly dependent on the imposed ELF/VLF phase offset, the modulation waveform, and the orientation of the HF beams. Data from two ground stations are interpreted using simulations of modulated heating power envelopes as well as a comprehensive model of ionospheric ELF/VLF generation. It is found that two colocated vertical beams HF beams excite a single ionospheric ELF/VLF source, but independent ELF/VLF sources can be induced in the ionospheric region above the heater if the HF beams are offset from zenith to intersect at their 3 dB points. Furthermore, the use of two vertical HF beams with ELF phase offset is found to be a potential diagnostic method for the ionospheric D region.

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Section: Discussion and Summarysupporting

confidence: 72%

Modulation of auroral electrojet currents using dual modulated HF beams with ELF phase offset, a potential D‐region ionospheric diagnostic

2013

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“…The resulting phase gradient appearing across the heated region causes the radiation to be directed toward the beam tilt. Moore and Rietveld [2009] suggest that the observations by Cohen et al [2008b] may possibly be attributed in large part to the tilted beam nature inherent to geometric modulation. On the other hand, Cohen et al [2009] suggest that this effect more likely responsible for only a small portion of the amplitude enhancement and directionality associated with geometric modulation.…”

Section: Physical Mechanismsmentioning

confidence: 95%

ELF/VLF wave generation via ionospheric HF heating: Experimental comparison of amplitude modulation, beam painting, and geometric modulation

et al. 2010

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[1] Generation of ELF/VLF radio waves (300 Hz to 10 kHz) is achievable via modulation of natural currents in the lower ionosphere with high-power HF (2-10 MHz) heating. Recently, Cohen et al. (2008b) put forth an alternative to conventional amplitude HF power modulation, therein referred to as geometric modulation, in which the HF ionospheric heating beam is geometrically steered at the desired ELF/VLF frequency, and found 7-11 dB enhanced amplitudes, and ∼14 dB directional dependence for the thus generated ELF/VLF waves, compared to vertical amplitude modulation. In this paper, we quantitatively compare amplitude modulation, geometric modulation, and a previously proposed technique known as beam painting, wherein the HF beam is rapidly moved over a wide area during the on portion of amplitude modulation in order to create a larger heated region in the ionosphere. We experimentally analyze both the total generation and the directionality, i.e., the suitability of each technique to direct signals along a chosen azimuth. Among the three methods, geometric modulation is found to be uniquely well suited for both goals. We also conduct experiments to investigate two particular physical effects and their role in generation efficacy: that of heat-cool duty cycle and the oblique angle of the HF heating beam. It is found that both duty cycle and the oblique angle of the beam have small but counteracting impacts, consistent with the notion that the primary physical process responsible for generation enhancement in geometric modulation is that of formation of an effective multielement phased array.

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“…We thank Moore and Rietveld [2009] (hereafter referred to as MR09) for their comments to Cohen et al [2008] (hereafter referred to as CA08). MR09 consider the two following conclusions of CA08 concerning the generation of ELF/VLF (0.3–30 kHz) waves with typical amplitude modulation (AM) and geometric modulation of ionospheric High‐Frequency (HF) heating: 1) “Geometric modulation can enhance ELF/VLF wave generation by up to ∼11 dB over the conventional AM method”, and 2) “Geometric modulation also allows directional launching of the signal into the Earth‐ionosphere waveguide, forming an unprecedented steerable large‐element ELF/VLF ionospheric phased array”.…”

Section: Mechanisms Of Elf/vlf Directionalitymentioning

confidence: 99%

“…MR09 refer to Figure 6 of Barr et al [1988], which appears to show that 3–5 dB amplitude gains at long‐distances can be achieved by tilting an AM HF beam by 15° toward that receiver (and conversely, a 3–5 dB reduction results from an HF beam tilted 15° away from the receiver), i.e., oblique AM HF heating. These observations are a straightforward consequence of phasing of ELF/VLF radiating currents across the heated region as a result of the variable propagation delay of the HF signal to the ionosphere [see Barr et al , 1988, Figure 7].…”

Section: Mechanisms Of Elf/vlf Directionalitymentioning

confidence: 99%

Reply to comment by R. C. Moore and M. T. Rietveld on “Geometric modulation: A more effective method of steerable ELF/VLF wave generation with continuous HF heating of the lower ionosphere”

Cohen

Inan

Gołkowski

2009

Geophysical Research Letters

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Comment on “Geometric modulation: A more effective method of steerable ELF/VLF wave generation with continuous HF heating of the lower ionosphere” by M. B. Cohen, U. S. Inan, and M. A. Golkowski

Cited by 6 publications

References 5 publications

Modulation of auroral electrojet currents using dual modulated HF beams with ELF phase offset, a potential D‐region ionospheric diagnostic

Modulation of auroral electrojet currents using dual modulated HF beams with ELF phase offset, a potential D‐region ionospheric diagnostic

ELF/VLF wave generation via ionospheric HF heating: Experimental comparison of amplitude modulation, beam painting, and geometric modulation

Reply to comment by R. C. Moore and M. T. Rietveld on “Geometric modulation: A more effective method of steerable ELF/VLF wave generation with continuous HF heating of the lower ionosphere”

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