HR: 0800h
AN: SA11A-0292    [Abstracts]
TI: Geometric modulation: A new method of ELF/VLF wave generation with continuous HF heating of the auroral electrojet
AU: * Cohen, M B
EM: mcohen@stanford.edu
AF: Stanford University STAR Laboratory, 350 Serra Mall Packard Room 356, Stanford, CA 94305, United States
AU: Inan, U S
EM: mag41@stanford.edu
AF: Stanford University STAR Laboratory, 350 Serra Mall Packard Room 356, Stanford, CA 94305, United States
AU: Golkowski, M
EM: mag41@stanford.edu
AF: Stanford University STAR Laboratory, 350 Serra Mall Packard Room 356, Stanford, CA 94305, United States
AU: Lehtinen, N
EM: nleht@stanford.edu
AF: Stanford University STAR Laboratory, 350 Serra Mall Packard Room 356, Stanford, CA 94305, United States
AB: ELF (300 – 3000 Hz) and VLF (3 – 30 kHz) radio waves are very difficult to generate with practical antennae, because of their extraordinarily long (10 – 1000 km) wavelengths, and the lossiness of the Earth's surface at these frequencies. In recent decades, ELF and VLF waves have been successfully generated via amplitude modulated HF (2-10 MHz) heating of the auroral electrojet. Through the temperature dependent conductivity of the lower ionospheric plasma, a patch of the auroral electrojet becomes a large radiating body in the presence of modulated heating. Of particular note have been facilities near Tromso, Norway, and more recently, the HAARP facility near Gakona, Alaska, each of which have successfully generated ELF/VLF waves with AM HF heating, and detected the generated signal as far as 4400 km away. In this paper, we introduce a new and more powerful method of ELF/VLF wave generation, geometric modulation, which involves steering the HF heating beam in a geometric pattern without modulating its power. Utilizing results obtained from the HAARP facility, a phased HF antenna array recently upgraded to 3.6 MW of radiated power, we show that geometric modulation can strengthen ELF/VLF generation by 4-10 dB. We explore the effect of different geometric configurations, sizes and shapes, and observe signal parameters such as amplitude, phase, and polarization, for both nearby and long distance observations. Observations are placed in the context of a realistic, quantitative physical model of the HF-ELF conversion process, along with a discussion of ELF propagation in the Earth-ionosphere waveguide.
DE: 2403 Active experiments
DE: 2407 Auroral ionosphere (2704)
DE: 2409 Current systems (2721)
DE: 2487 Wave propagation (0689, 3285, 4275, 4455, 6934)
DE: 6984 Waves in plasma (7867)
SC: SPA-Aeronomy [SA]
MN: 2007 Fall Meeting