Analysis of D-Region Absorption via HF Cross-Modulation Experiments at HAARP

Details Analysis of D-Region Absorption via HF Cross-Modulation Experiments at HAARP Analysis of D-Region Absorption via HF Cross-Modulation Experiments at HAARP

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsa33a1764b&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SA33A-1764

Computer Science

Sound

[0654] Electromagnetics / Plasmas, [2403] Ionosphere / Active Experiments, [2487] Ionosphere / Wave Propagation, [6974] Radio Science / Signal Processing

Scientific paper

Experimental observations performed near the High-frequency Active Auroral Research Program (HAARP) HF transmitter in Gakona, Alaska are used to implement a new method quantifying the rate of absorption of HF radio waves in the D-region ionosphere. Quantifying the ambient and HF-modified characteristics of the D-region ionosphere in the vicinity of ionospheric HF heaters has historically proven to be a difficult task. For example, the electron density in the 60-90 km altitude range is typically too low to employ radio-sounding techniques; LIDAR observations typically require significant temporal averaging, precluding the investigation of physical processes that occur on sub-millisecond time scales; ELF/VLF wave generation experiments typically have difficulty providing reliable spatial resolution and also cannot experimentally distinguish between the spatial distribution of the ionospheric conductivity modulation produced by modulated HF heating and that of the current-driving electric fields associated with the auroral electrojet. Yet, the majority of HF signal absorption occurs in this region of the ionosphere, and the ability to characterize HF absorption in this region benefits a wide range of ionospheric HF heating experiments. The technique described and demonstrated in this paper combines ionosonde-style radio sounding with ELF/VLF cross-modulation experiments to identify the altitude of maximum D-region absorption as a function of HF frequency. Observations are presented and compared with the predictions of a theoretical model, demonstrating excellent agreement between experiment and theory and indicating that the technique may be used successfully in practice. Based on the success of this first experiment, another HF cross-modulation experiment has been performed at HAARP and analyzed theoretically. Pulsed-modulation experiments are used to assess the relative absorption as a function of altitude within the D-region ionosphere, and a method to chart the relative levels of absorption as a function of altitude is demonstrated. An initial analysis of absorption as a function of time is also discussed.

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