Abstract
We present the observations of the artificial ionospheric heating experiment of EISCAT (European Incoherent Scatter Scientific Association) on 22 February 2012 in Tromsø, Norway. When the pump is operating near the fourth electron gyrofrequency, the UHF radar observation shows some strong enhancements in electron temperature, electron density, ion line, and the outshifted plasma lines. Based on some existing theories, we find the following: first, Langmuir waves scattering off lower hybrid density fluctuations and strong Langmuir turbulence (SLT) in the Zakharov model cannot completely explain the outshifted plasma lines, but the data suggest that this phenomenon is related to the cascade of the pump wave and should be researched further; second, the spatiotemporal consistency between the enhancement in electron density/electron temperature reaches up to three to four times that of the undisturbed state and HF-enhanced ion lines (HFILs) suggest that SLT excited by parametric instability plays a significant role in superthermal electron formation and electron acceleration; third, some enhancements in HFILs and HF-induced plasma lines (HFPLs) are generated by parametric decay instability (PDI) during underdense heating in the third cycle, we suggest that this is due to the existence of a second cut-off in the upper hybrid dispersion relation as derived from a kinetic description.
Highlights
The interaction of high-power high-frequency (HF) waves with the ionosphere can excite a series of complex phenomena due to parametric instability.According to the different excitation height, parametric instability can be divided intoLangmuir parametric instability and thermal parametric instability (TPI)
As the high-power HF O-mode radio waves reach the regions of their resonant interactions, complex phenomena arise, e.g., HF-enhanced ion lines (HFILs) and HF-induced plasma lines (HFPLs), an increase in electron temperature, and the perturbation of electron density
The results given by Robinson et al [6] indicate that there are strong minima in the responses of both anomalous absorption and electron temperature when pump frequency is operating in the vicinity of the third and fourth electron gyrofrequencies and that anomalous electron heating in the presence of small scale field aligned plasma irregularities dominates over collisional heating at high latitudes
Summary
The interaction of high-power high-frequency (HF) waves (usually termed as a pump) with the ionosphere can excite a series of complex phenomena due to parametric instability.According to the different excitation height, parametric instability can be divided intoLangmuir parametric instability and thermal (resonance) parametric instability (TPI). The results given by Robinson et al [6] indicate that there are strong minima in the responses of both anomalous absorption and electron temperature when pump frequency is operating in the vicinity of the third and fourth electron gyrofrequencies and that anomalous electron heating in the presence of small scale field aligned plasma irregularities dominates over collisional heating at high latitudes. Borisova et al [7] found that the small-scale field-aligned irregularities are suppressed near the fourth electron gyrofrequency This effect is a typical feature for electron temperature during frequency-stepping experiments around n fce, and Mjolhus et al proposed that this feature is related to the second cut-off in the upper hybrid dispersion relation as derived from a kinetic description [4]
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