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, M. B.; Inan, U. S.; Gołkowski, Mark

doi:10.1029/2008gl036519

Cited by 7 publications

(7 citation statements)

References 4 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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“… 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: 92%

“…Since the effect of duty cycle and phasing from oblique heating appear to be small and counteracting, these experiments appear to be consistent with the notion that geometric modulation is able to produce stronger and directable signals in significant part due to the effective creation of a large‐element ELF/VLF phased array. Individual beam locations create separate and independently radiating regions, whose phases can be controlled by the order in which those regions are heated, as discussed by Cohen et al [2008b, 2009].…”

Section: An Elf/vlf Phased Arraymentioning

confidence: 99%

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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“…In response to this, Cohen et al [34] explained the difference between GM and oblique AM modulation in terms of the generation efficiency and geometric effects, and argued that the mechanism of GM is close to the twoelement phased array modulation mentioned by Barr et al [35]. Furthermore, by utilizing the new upgrade ability at HAARP, it has been proven that GM is more complicated than both oblique AM and two-element phased array modulation [34]. Therefore, Cohen et al [34] emphasized that GM is an "unprecedented technique".…”

Section: Geometric Modulationmentioning

confidence: 99%

The Generation of ULF/ELF/VLF Waves in the Ionosphere by Modulated Heating

2021

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One of the most important effects of ionospheric modification by high power, high frequency (HF) waves is the generation of ultra low frequency/extremely low frequency/very low frequency (ULF/ELF/VLF) waves by modulated heating. This paper reviews the scientific achievements of the past five decades regarding the main mechanisms of excitation of ULF/ELF/VLF waves and discusses their characteristics, such as their electrojet dependency, the location of the source region, continuous and discontinuous waves, the number of HF arrays, and the suitable range of the modulation frequency for different proposed mechanisms. Finally, the outlook for future research in this area is presented.

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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”

Cited by 7 publications

References 4 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

The Generation of ULF/ELF/VLF Waves in the Ionosphere by Modulated Heating

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