Cite abstracts as Author(s) (2006), Title, Eos Trans. AGU,
87(52), Fall Meet. Suppl., Abstract xxxxx-xx

R:  0800h
AN: SA11A-0292
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