  We present a terahertz spectroscopic study of magnetic excitations in
ferroelectric antiferromagnet BiFeO3. We interpret the observed spectrum of
long-wavelength magnetic resonance modes in terms of the normal modes of the
material's cycloidal antiferromagnetic structure. We find that the modulated
Dzyaloshinski-Moriya interaction leads to a splitting of the out-of-plane
resonance modes. We also assign one of the observed absorption lines to an
electromagnon excitation that results from the magnetoelectric coupling between
the ferroelectric polarization and the cycloidal magnetic structure of BiFeO3.
