Cutoff frequencies and excitation threshold of artificial E region field aligned irregularities

Details Cutoff frequencies and excitation threshold of artificial E region field aligned irregularities Cutoff frequencies and excitation threshold of artificial E region field aligned irregularities

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsa11b..07h&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SA11B-07

Physics

[2439] Ionosphere / Ionospheric Irregularities, [2471] Ionosphere / Plasma Waves And Instabilities

Scientific paper

Artificial E region field aligned irregularities generated by HAARP have been observed using a coherent scatter radar imager in Homer, Alaska, in August of 2009. As in previous experiments, irregularities could be generated using O-mode pump frequencies both above and below the second electron gyroharmonic frequency, with no obvious differences in the irregularity properties in either case. However, echo suppression was observed when the pump frequency was swept across the second electron gyroharmonic frequency in small steps. This finding is consistent with the theory of thermal oscillating two-stream instability and the role of upper-hybrid waves, which are damped near gyroharmonic frequencies. The small frequency steps permit us to identify the width of the suppressed frequency band. Suppression was not complete, most likely because the double resonance matching condition cannot be met everywhere in the modified ionospheric volume. In separate experiments, the pump wave power was also varied in gradual, quadratic steps in order to identify the threshold in situ electric field amplitude for irregularity generation. The threshold field found this way, which was of the order of 100 mV/m, was smaller than previous experimental estimates and also approximately consistent with theory, although a great deal of uncertainty remains in both the contemporary theoretical formulation of the problem and the experimental conditions in the ionosphere.

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