  We report structural evidence of dynamic reorganization in vortex matter in
clean NbSe$_2$ by joint small angle neutron scattering and ac-susceptibility
measurements. The application of oscillatory forces in a transitional region
near the order-disorder transition results in robust bulk vortex lattice
configurations with an intermediate degree of disorder. These
dynamically-originated configurations correlate with intermediate pinning
responses previously observed, resolving a long standing debate regarding the
origin of such responses.
