Effects of Lower Hybrid Turbulence During Injections of HF Waves at the Magnetic Zenith

Details Effects of Lower Hybrid Turbulence During Injections of HF Waves at the Magnetic Zenith Effects of Lower Hybrid Turbulence During Injections of HF Waves at the Magnetic Zenith

May 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agusmsa51a..02m&link_type=abstract

American Geophysical Union, Spring Meeting 2007, abstract #SA51A-02

Other

2403 Active Experiments, 2471 Plasma Waves And Instabilities (2772), 2483 Wave/Particle Interactions (7867)

Scientific paper

Recent optical and ionospheric radar observations during HF heating experiments at the High-frequency Active Auroral Research Program (HAARP) facility are analyzed. The observations indicate that both the thermal and parametric instabilities can coexist at magnetic zenith [Mishin et al., GRL, 32, L23106, doi:10.1029/2005GL023864, 2005] and that up to three mechanisms of electron acceleration can be acting, depending on the pump frequency relative to the electron gyro-harmonic [Kosch et al., JGR, 2007 (in press)]. Clear evidence is found for the presence of lower hybrid (LH) waves. With regards to HF heating, LH-waves have mainly been discussed in association with the downshifted maximum in the SEE spectrum. However, it is well known that nonlinear coupling of LH waves can result in electron heating and magnetic field-aligned acceleration. We focus on this aspect for the case when the dynamics of low-frequency LH waves are dominated by the lower hybrid collapse, which threshold energy density is quite low and is surely exceeded in the conditions in question. In the course of collapse, the longitudinal and transverse dimensions of cavities decrease, while the density variation increases. Ultimately, the LH-wave energy in collapsing cavities is absorbed by cold-plasma particles via Landau or transit-time damping. This produces suprathermal tails in the field-aligned electron distribution and in the transverse ion distribution. The density of accelerated electrons can be determined by assuming that resonant particles carry away all of the energy pumped into collapsing cavities. We have shown that this mechanism is effective in the energy range below 10 eV, and can thus contribute to the red- and green-line emissions. Another observable outcome of LH-cavities is coherent scattering of HF radio waves, which has likely been observed by the SuperDARN radar.

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