Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsa33a1761f&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SA33A-1761
Other
[0654] Electromagnetics / Plasmas, [2403] Ionosphere / Active Experiments, [2487] Ionosphere / Wave Propagation, [6934] Radio Science / Ionospheric Propagation
Scientific paper
A time-of-arrival (TOA) analysis is applied to observations during ELF/VLF wave generation experiments performed at the High-frequency Active Auroral Research Program (HAARP) HF transmitter in Gakona, Alaska. This TOA technique provides measurements of the ELF/VLF current source with a ranging resolution of ˜2~km. Only recently has this TOA technique been applied to ELF/VLF waves generated by modulated HF heating of the ionosphere, although it has the potential to provide very useful measurements regarding the spatial distribution of the ELF/VLF source region. Over the course of several experiment campaigns performed at HAARP over the last two years, ELF/VLF signals have been generated with a specific frequency-time format, permitting the TOA analysis to be performed on ELF/VLF signals generated under varying ambient conditions. In this paper, we summarize these experimental results, demonstrating that: 1) the TOA technique is a valid experimental measure of the magnitude and phase of the received ELF/VLF signal as a function of time, 2) assuming speed-of-light propagation, the TOA technique may be used to estimate the location of the dominant ELF/VLF source region, and 3) the TOA analysis has the capability to distinguish between "direct" and "ionospherically-reflected" signal paths. We provide analyses of ELF/VLF wave generation as a function of modulation frequency, HF frequency, and HF power. During specific experiments, the direction of the HF beam was pointed toward and away from the receiver, producing different TOA results. Particularly interesting TOA observations occur when sporadic-E layers are present. It is observed, for instance, that the peak amplitude is significantly (>150~μ sec) delayed compared to ELF/VLF waves generated in the absence of sporadic-E. It is at present unknown whether the ELF/VLF source region is at a higher altitude (within the sporadic-E layer itself, for instance) or whether the HF wave reflected from the sporadic-E layer produces additional conductivity modulation within the D-region, amplifying and delaying the received signal. These TOA results, and others, are discussed in detail in a context provided by a modulated HF heating and wave propagation model.
Fujimaru S.
Moore Robert C.
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