Electron cyclotron absorption at downshifted frequencies in the presence of a current-carrying superthermal tail

The absorption of the extraordinary mode at frequencies downshifted with respect to either the fundamental or the second harmonic electron cyclotron resonance and such that the relevant resonance lies outside the plasma is evaluated for a Maxwellian distribution with a current-carrying superthermal tail. The contribution from the current-carrying electrons to the absorption is obtained by Lorentz-transforming the dielectric tensor from the comoving to the laboratory frame. The dielectric tensor accounts for the fully relativistic cyclotron resonance condition and the corresponding Hermitian part includes the relevant thermal effects. The spatial dependence of the drift momentum, the density, and the temperature of both the bulk and the superthermals is accounted for in the evaluation of the absorption profile. The relevance of the relativistic effects is discussed with respect to both the profile of the absorption and the overall absorption.