Geometric modulation: A more effective method of steerable ELF/VLF wave generation with continuous HF heating of the lower ionosphere

Abstract

ELF/VLF radio waves (300 Hz–30 kHz) are difficult to generate with practical antennae, because of their extraordinarily long (10–1000 km) wavelengths, and the lossy nature of the Earth's surface at these frequencies. ELF/VLF waves have been successfully generated via amplitude modulated (AM) HF (2–10 MHz) heating of the lower ionosphere. Through the temperature‐dependent conductivity of the lower ionospheric plasma, a patch of the ionospheric current becomes a large radiating ‘antenna’. We implement a new method of ELF/VLF wave generation, herein named ‘geometric modulation’, involving scanning the HF heating beam in a geometric pattern without modulating its power. Utilizing results from the upgraded 3.6 MW radiated HAARP HF antenna array, we show that geometric modulation can enhance ELF/VLF wave generation by up to ∼11 dB over the conventional AM method. 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.

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

“…The 1 to 6 kHz range is the same as that explored by Cohen et al [2008]. Observations were made at a receiver 550 km from the HF transmitter, comparable to the 700 km distances of the receivers used by Cohen et al [2008].…”

“…Observations were made at a receiver 550 km from the HF transmitter, comparable to the 700 km distances of the receivers used by Cohen et al [2008]. Although the experimental results were first reported by Barr et al [1987] (as cited by Cohen et al [2008]), Barr et al [1988] provide a more elegant and complete theoretical analysis of the observations and is more appropriate for comparison with GM. At Tromsø, when the HF beam was directed toward the receiver (up to 37 degrees off-vertical), a significant increase (sometimes by more than 10 dB, and on average by 6 dB) in the ELF/ VLF signal amplitude was observed compared to vertical heating.…”

“…As stated, the increase in ELF/VLF wave generation efficiency is thus 4 -8 dB due to the two-fold HF power consumption of the CSGM technique. The farfield experimental measurements presented by Barr et al [1988] indicate that a possibly significant portion of the 4 -8 dB increase in ELF/VLF wave generation efficiency observed by Cohen et al [2008] is due to the 15°off-vertical heating angle employed during CSGM. It is therefore more appropriate to compare the GM experiments with oblique AM heating.…”

“…[3] Cohen et al [2008] show that the ELF/VLF radiation produced using the circle-sweep format 'geometric modulation' (CSGM) technique is higher in amplitude (by 7-11 dB) than that produced by vertical, amplitude-modulated HF (AM) beam. As stated, the increase in ELF/VLF wave generation efficiency is thus 4 -8 dB due to the two-fold HF power consumption of the CSGM technique.…”

“…[1] Cohen et al [2008] overlook observations presented by Barr et al [1988], which have a direct and possibly significant impact on two of their primary conclusions. The two conclusions of interest are: 1) that 'geometric modulation' in the form of circle-sweeping is significantly more efficient than amplitude modulation at generating ELF/VLF waves, and 2) that the ELF/VLF beam generated using sawtooth format 'geometric modulation' indicates the creation of a controllable ELF phased array with an ''unprecedented'' level of directionality.…”