Scattering of cyclotron radiation in a moving plasma

We consider the scattering of cyclotron radiation in a plasma moving along a homogeneous magnetic field. The equation of radiation transfer in a co-mo ving frame is derived and two limiting cases are pointed out. In the first case of a "small" velocity gradient, the total Doppler frequency shift due to variations in the plasma velocity over the flow is much smaller than the width of the line. The second, opposite, case of a "large" velocity gradie nt is analogous to the Sobolev approximation in the theory of moving stella r envelopes. The solution of the transfer equation for a wind-type flow ill uminated by radiation of a given intensity is obtained in the latter case, when the influence of the plasma motion on cyclotron scattering is most imp ortant. It is shown that cyclotron scattering in a moving plasma differs fr om the known land qualitatively similar) problems of resonance scattering i n moving stellar envelopes and cyclotron scattering in a motionless plasma permeated by an inhomogeneous magnetic field. In particular, a symmetric ab sorption band with residual intensity proportional to the velocity gradient appears in the spectrum of the outgoing radiation, while in these two othe r problems, the depth of the corresponding spectral features cannot exceed half the continuum level. Detailed qualitative analysis reveals that this d ifference is due to the particular form of the frequency redistribution for cyclotron scattering. (C) 2000 MAIK "Nauka/Interperiodica".