The transformation of a circularly polarized electromagnetic wave in a magnetoplasma medium with increasing plasma density is considered. The wave propagates along the static magnetic field. Complete analysis, including ion motion, is given both for slow (compared to the wave frequency) and rapid ionization rate. In the case of slow temporal variation of the plasma density, a relation between the energy of the wave and its frequency, which is conserved during the plasma creation process (adiabatic invariant), is found. The existence of significant energy losses follows from the invariant. The dissipative mechanism is explained via consideration of the case of a sudden growth of plasma density in time from one value to another. It is shown that energy transforms into the kinetic energy of carriers, and preionization of the medium plays a principal role in the dissipation process. In the special case of a whistler wave, up to 50% of the energy may be transformed into an ion-cyclotron wave when dense plasma is rapidly created. [References: 32]
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