  The magnon pumping effect in ferromagnetic insulators under an external
time-dependent transverse magnetic field is theoretically studied. Generation
of a magnon current is discussed by calculating the magnon source term in the
spin continuity equation. This term represents the non-conservation of magnons
arising from an applied transverse magnetic field. The magnon source term has a
resonance structure as a function of the angular frequency of the transverse
field, and this fact is useful to enhance the pumping effect.
