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

摘要

Generation of ELFNLF radio waves (300 Hz to 10 kHz) is achievable via modulationof 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 HFpower modulation, therein referred to as geometric modulation, in which the HF ionosphericheating 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 generatedELF/VLF waves, compared to vertical amplitude modulation. In this paper, we quantitativelycompare amplitude modulation, geometric modulation, and a previously proposedtechnique known as beam painting, wherein the HF beam is rapidly moved over a widearea during the on portion of amplitude modulation in order to create a larger heated regionin the ionosphere. We experimentally analyze both the total generation and thedirectionality, i.e., the suitability of each technique to direct signals along a chosenazimuth. Among the three methods, geometric modulation is found to be uniquelywell suited for both goals. We also conduct experiments to investigate two particularphysical effects and their role in generation efficacy: that of heat-cool duty cycle and theoblique angle of the HF heating beam. It is found that both duty cycle and the obliqueangle of the beam have small but counteracting impacts, consistent with the notion that theprimary physical process responsible for generation enhancement in geometric modulationis that of formation of an effective multielement phased array.