An analytical study is presented of the interaction between a fast Alfvén wave having a frequency larger than the ion-cyclotron frequency, and low frequency density fluctuations. When appropriate k, ω matching conditions are satisfied, the interaction results in a beat current that acts as an in-situ antenna, exciting ion Bernstein modes resonantly. Since such modes have relatively small damping for large parallel phase velocities, their excitation can significantly enhance the damping of fast waves having small k∥, particularly in those regions of the plasma where the temperature is relatively low. The interaction with coherent, as well as with broad band density fluctuations is investigated, and the role of plasma nonuniformities is examined. Specific features of relevance to ion-cyclotron range of frequency heating of tokamak plasmas are considered.
We investigate direct conversion of ordinary mode into upper hybrid (UH) waves by pre‐existing electron density irregularities during ionospheric heating. The electric field amplitude and the average power absorbed by the excited UH waves are calculated when the irregularity distribution is (i) coherent, and (ii) incoherent with a broad band power spectrum modelled by a drift wave type scaling. For the typical parameters of the experiments, we find that the direct conversion process can be quite efficient.
Scattering of a coherent whistler from random density fluctuations is treated by a perturbation procedure. The attenuation length is calculated for scattering by a drift-wave type fluctuation model and is simply expressed as a function of the propagation angle in the limit where the whistler wavelength is long compared to a correlation length. For typical tokamak and space plasmas, this scattering becomes most important at larger angles.
Excitation of upper hybrid (UH) waves from ordinary‐mode (O‐mode) electromagnetic waves via large‐scale electron density gradient in the upper hybrid resonance (UHR) region of the ionosphere is investigated. The model used is that of a driven harmonic oscillator in which the electron drift velocity induced by the O‐mode beats with the local electron density to produce a source current that drives the excited UH waves. It is found that strong excitation of UH waves occurs in the UHR region, implying enhanced damping of the incident O‐mode.
Abstract.In this paper we investigate the role of dissipative drift instability in exciting "needle-like" short scale density structures recently observed during
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