  In order to clarify the ``superradiant'' conditions for the moving Josephson
vortices to excite in-phase AC electromagnetic fields over all junctions, we
perform large scale simulations of realistic dimensions for intrinsic Josephson
junctions under the layer parallel magnetic field. Three clear step-like
structures in the I-V curve are observed above a certain high field ($H > 1T$
in the present simulations), at which we find structural transitions in the
moving flux-line lattice. The Josephson vortex flow states are accordingly
classified into four regions (region I $\sim $ IV with increasing current), in
each of which the power spectrum for the electric field oscillations at the
sample edge are measured and typical snapshots for Josephson vortex
configurations are displayed. Among the four regions, especially in the region
III, an in-phase rectangular vortex lattice flow state emerges and the power
spectrum shows remarkably sharp peak structure, i.e., superradiant state.
Comparison of the simulation results with an eigenmode analysis for the
transverse propagating Josephson plasma oscillations reveals that the
resonances between Josephson vortex flow states and some of the eigenmodes are
responsible for the clear flux lattice structural transitions. Furthermore, the
theoretical analysis clarifies that the width of the superradiant state region
in the I-V characteristics enlarges with decreasing both the superconducting
and insulating layer thickness.
