  Isoscalar dipole nuclear response reveals the low- and high-energy
resonances. The nature of isoscalar dipole resonances in heavy spherical nuclei
is studied by using the translation-invariant kinetic model of small
oscillations of finite Fermi systems. Calculations of the velocity field at the
centroid energy show a pure vortex character of the low-energy isoscalar dipole
resonance in spherical nuclei and confirm the anisotropic compression character
of the high-energy one. The evolution of the velocity field in dependence on
the excitation energy of the nucleus within the resonance width is studied. It
is found that the low-energy isoscalar dipole resonance retains a vortex
character while with increasing energy this collective excitation also involves
compression. The high-energy resonance keeps the compression character with a
change in the excitation energy within the resonance width but the
compression-expansion region of the velocity field related to this resonance
shifts inside the nucleus.
