  In the thermodynamic limit, the time evolution of isolated long-range
interacting systems is properly described by the Vlasov equation. This equation
admits non-equilibrium dynamically stable stationary solutions characterized by
a zero order parameter. We show that the presence of external noise sources,
like for instance a heat bath, can induce at a specific time a dynamical phase
transition marked by a non-zero order parameter. This transition corresponds to
a restoring of the full ergodic properties of the system and may be used as a
distinctive experimental signature of the existence of non-equilibrium
Vlasov-stable states. In particular, we evidence for the first time a regime
characterized by an order parameter pulse. Our analytical results are
corroborated by numerical simulations of a paradigmatic long-range model.
