  We investigate a dynamical magnetoelectric effect due to a magnetic resonance
in helical spin structures through the coupling between magnetization and
electric polarization via a spin current mechanism. We show that the magnon has
both the dynamical magnetic moment $\Delta M^\omega$ and the electric moment
$\Delta P^\omega$ ($\perp \Delta M^\omega$), i.e., a dynamical toroidal moment,
under external magnetic fields, and thus it is named the {\em toroidalmagnon}.
The toroidalmagnon exists in most conical spin structures owing to the
generality of the spin current mechanism. In the absorption of electromagnetic
waves, the toroidalmagnon excitation process generally induces a nonreciprocal
directional dichroism as a consequence of an interference of the magnetic and
electric responses.
