  We revisit the well-known Hoyle-Narlikar-Wheeler proposition that neutron
star emerging in the magnetic-flux-conserving process of core-collapse
supernova can convert the stored energy of Alfven vibrations into power of
magneto-dipole radiation. We show that the necessary requirement for the energy
conversion is the decay of internal magnetic field. In this case the loss of
vibration energy of the star causes its vibration period, equal to period of
pulsating emission, to lengthen at a rate proportional to the rate of magnetic
field decay. These prediction of the model of vibration powered neutron star
are discussed in juxtaposition with data on pulsating emission of magnetars
whose radiative activity is generally associated with the decay of ultra strong
magnetic field.
