Abstract
Abstract. E region ionospheric modification experiments have been performed at HAARP using pump frequencies about 50 kHz above and below the second electron gyroharmonic frequency. Artificial E region field-aligned plasma density irregularities (FAIs) were created and observed using the imaging coherent scatter radar near Homer, Alaska. Echoes from FAIs generated with pump frequencies above and below 2Ωe did not appear to differ significantly in experiments conducted on summer afternoons in 2008, and the resonance instability seemed to be at work in either case. We argue that upper hybrid wave trapping and resonance instability at pump frequencies below the second electron gyroharmonic frequency are permitted theoretically when the effects of finite parallel wavenumbers are considered. Echoes from a sporadic E layer were observed to be somewhat weaker when the pump frequency was 50 kHz below the second electron gyroharmonic frequency. This may indicate that finite parallel wavenumbers are inconsistent with wave trapping in thin sporadic E ionization layers.
Highlights
The production of magnetic field-aligned plasma density irregularities (FAIs) is a signature feature of RF ionospheric modification experiments
In the thermal parametric instability, or the thermal oscillating two-stream instability (OTSI) more precisely, the pump electromagnetic wave is resonantly converted in the presence of field-aligned plasma density irregularities into upper hybrid waves which heat the plasma collisionally and, in turn, intensify the irregularities through differential thermal forcing
If the amplitude of the irregularities becomes sufficiently large, upper hybrid waves may be trapped within them, causing additional mode conversion, heating, and trapping in tandem with the formation of thermal cavitons. This is the condition for the resonance instability, which is characterized by explosive wave growth
Summary
The production of magnetic field-aligned plasma density irregularities (FAIs) is a signature feature of RF ionospheric modification experiments. A number of closely related phenomena, including stimulated electromagnetic emission, electron and ion heating, optical emissions, and irregularity generation are affected or interrupted when pumping at or near gyroharmonic frequencies (see Kosch et al, 2005; Mishin et al, 2005 for experimental and theoretical summaries).
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