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

Abstract

[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.

Excitation of zero-frequency magnetic field-aligned currents by ionospheric heating

“…According to Eq. (4), oscillations of the ionospheric current are coupled with oscillations of the magnetic field-aligned currents (FACs), which are carried by shear Alfvén waves in the magnetosphere (Yampolski et al, 1997;Robinson et al, 2000;Papadopoulos et al, 1990;Cohen et al, 2010). A recent study by Papadopoulos et al (2011) demonstrates that shear Alfvén waves can also be generated by variations in the plasma pressure in the ionospheric F-region caused by the heating.…”

Effects of the Hall Conductivity in Ionospheric Heating Experiments

“…It may also explain the more efficient wave generation observed in some experiments with the geometric modulation of heating reported by Cohen et al (2008Cohen et al ( , 2010. The effect of this current is quite significant, particularly, when Σ H ∕Σ P = 2.0.…”

“…Two of the most efficient methods described in the literature include "beam painting" and geometric modulation techniques. The injection of extremely-low frequency (ELF)/very-low frequency (VLF) waves into the magnetosphere by the modulated heating of the electrojet by the High Altitude Active Research Program (HAARP) facility in Alaska has been extensively studied by Papadopoulos et al (2003), Golkowski et al (2008, 2011), and Cohen et al (2010. The entire process is modulated with the frequency of the generated wave.…”

On the generation of ELF/VLF waves for long‐distance propagation via steerable HF heating of the lower ionosphere

“…This technique enables a larger area of the ionosphere to be heated, implying a larger antenna and stronger ELF/VLF generation. Cohen et al [2010b] experimentally compare beam painting with geometric modulation, as well as AM heating with an oblique HF beam. In this paper, we explore the comparison theoretically.…”