  Using scanning susceptibility microscopy, we shed new light on the dynamics
of individual superconducting vortices and examine the hypotheses of the
phenomenological models traditionally used to explain the macroscopic ac
electromagnetic properties of superconductors. The measurements, carried out on
a 2H-NbSe$_2$ single crystal at relatively high temperature $T=6.8$ K, show a
linear amplitude dependence of the global ac-susceptibility for excitation
amplitudes between 0.3 and 2.6 Oe. We observe that the low amplitude behavior,
typically attributed to the shaking of vortices in a potential well defined by
a single, relaxing, Labusch constant, corresponds actually to strongly
non-uniform vortex shaking. This is particularly accentuated in the
field-cooled disordered phase, which undergoes a dynamic reorganization above
0.8 Oe as evidenced by the healing of lattice defects and a more uniform
oscillation of vortices. These observations are corroborated by molecular
dynamics simulations when choosing the microscopic input parameters from the
experiments. The theoretical simulations allow us to reconstruct the vortex
trajectories providing deeper insight in the thermally induced hopping dynamics
and the vortex lattice reordering.

Superconductivity