  We study the structure of the sigma meson, the lowest-lying resonance of the
pi pi scattering in the scalar-isoscalar channel, through the softening
phenomena associated with the partial restoration of chiral symmetry. We build
dynamical chiral models to describe the pi pi scattering amplitude, in which
the sigma meson is described either as the chiral partner of the pion or as the
dynamically generated resonance through the pi pi attraction. It is shown that
the internal structure is reflected in the softening phenomena; the softening
pattern of the dynamically generated sigma meson is qualitatively different
from the behavior of the chiral partner of the pion. On the other hand, in the
symmetry restoration limit, the dynamically generated sigma meson behaves
similarly to the chiral partner.
