  Low-energy dipole excitations are analyzed for the stable isotopes $^{40}$Ca
and $^{48}$Ca in the framework of the Skyrme-second random-phase approximation.
The corresponding random-phase approximation calculations provide a negligible
strength distribution for both nuclei in the energy region from 5 to 10 MeV.
The inclusion and the coupling of 2 particle-2 hole configurations in the
second random-phase approximation lead to an appreciable dipole response at low
energies for the neutron-rich nucleus $^{48}$Ca. The presence of a neutron skin
in the nucleus $^{48}$Ca would suggest the interpretation of the low-lying
response in terms of a pygmy excitation. The composition of the excitation
modes (content of 1 particle-1 hole and 2 particle-2 hole configurations),
their transition densities and their collectivity (number and coherence of the
different contributions) are analyzed. This analysis indicates that, in
general, these excitations cannot be clearly interpreted in terms of
oscillations of the neutron skin against the core with the exception of the
peak with the largest $B(E1)$ value, which is located at 9.09 MeV. For this
peak the neutron transition density dominates and the neutron and proton
transition densities oscillate out of phase in the internal part of the nucleus
leading to a strong mixing of isoscalar and isovector components. Therefore,
this state shows some features usually associated to pygmy resonances.
