Quantitative Analysis of Wave Absorption near Cyclotron Resonance in an Inhomogeneous Magnetic Field

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Propagation and absorption of electromagnetic waves near cyclotron resonance is quantitatively investigated.We have analysed nonlinear motion of charged particles in a magnetic mirror geometry. The interaction with an electromagnetic wave near cyclotron resonance gives rise to stochastic motion of the particles. We have estimated a threshold of wave amplitude where the particle's motion becomes stochastic, and confirmed it by numerical computation.The self-consistent analysis of wave propagation and absorption along an inhomogeneous magnetic field requires kinetic formulation. Based on an unperturbed particle orbit in the inhomogeneous magnetic field, we have derived the wave equation in an integrodifferential form. We have solved it numerically to describe the electron cyclotron wave incident from the high field side of the cyclotron resonance.The spatial profiles of the wave field and the absorbed power as well as the reflection coefficient are calculated. When the gradient of the magnetic field is moderate, the incident wave is completely absorbed near the cyclotron resonance and no reflection occurs. This is consistent with a conventional cold plasma analysis. In the case of steep gradient, however, a part of the incident wave is reflected, since the wave number remains finite owing to the finite temperature. The density and temperature dependence of the power deposition profile is also examined.