Abstract
Abstract. Artificial E region field aligned plasma density irregularities (FAIs) were generated using HAARP in four different experimental modes and observed with a coherent scatter radar imager located 450 km to the southwest where it could detect field-aligned backscatter. The experiments were conducted in July of 2008, during the Polar Aeronomy and Radio Science Summer School (PARS), during quiet conditions in the daytime when the E layer was dense and absorption was modest. The echoes observed during zenith and magnetic zenith heating experiments were deflected from their nominally anticipated horizontal positions toward the midpoint position. The occurrence of hysteresis when heating with amplitude modulated pulses implied the development of the resonance instability, although the threshold for the onset of instability appeared to be higher than what has been predicted theoretically. Heating experiments involving pump frequencies slightly above and below the second electron gyroharmonic frequency produced no significant differences in the observed echoes. Finally, heating with a pump frequency slightly above the E region critical frequency appears to have produced FAIs at two distinct altitudes where the upper-hybrid resonance condition could be satisfied.
Highlights
Studies of artificial ionospheric effects in the E region were proposed by Bailey (1959), but Fialer (1974) was the first to report on E region artificial field-aligned ionospheric irregularities (FAIs) observed by coherent scatter radar
Grach et al (1978) and Dysthe et al (1982) anticipated a hysteresis effect associated with the resonance instability and wave trapping, such that irregularities can be maintained by pump waves with amplitudes below their excitation threshold
Observations of E region FAIs presented in this paper generally support prevailing theories regarding irregularity generation in ionospheric modification experiments, a number of remaining problems and some inconsistencies are evident
Summary
Studies of artificial ionospheric effects in the E region were proposed by Bailey (1959), but Fialer (1974) was the first to report on E region artificial field-aligned ionospheric irregularities (FAIs) observed by coherent scatter radar. Djuth et al (1985) presented evidence of irregularity generation near the upper-hybrid resonance height, where the resonance instability is thought to produce density striations (see Mishin et al, 2005, and Gurevich, 2007, for review). Grach et al (1978) and Dysthe et al (1982) anticipated a hysteresis effect associated with the resonance instability and wave trapping, such that irregularities can be maintained by pump waves with amplitudes below their excitation threshold. Kosch et al (2000) and Pedersen and Carlson (2001) have reported on experiments involving zenith heating where the artificial emissions exhibited elongation and displacement towards the magnetic zenith. Experiments involving heating at frequencies just above and below the second electron gyroharmonic are presented, these are analyzed in detail in a companion paper (Hysell and Nossa, 2009)
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