  We investigated the role of the cyclotron emission (CE) associated to cosmic
magnetic fields (MF) on the evolution of cosmic microwave background (CMB)
spectral distortions. We computed the photon and energy injection rates by
including spontaneous and stimulated emission and absorption. These CE rates
have been compared with those of bremsstrahlung (BR) and double Compton
scattering (DC), for realistic CMB distorted spectra at various cosmic epochs.
For reasonable MF strengths we found that the CE contribution to the evolution
of the CMB spectrum is much smaller than the BR and DC contributions. The
constraints on the energy exchanges at various redshifts can be then derived,
under quite general assumptions, by considering only Compton scattering (CS),
BR, and DC, other than the considered dissipation process. Upper limits to the
CMB polarization degree induced by CE have been estimated.
