  The emission by an initially completely inverted atomic ensemble in the
long-wavelength regime is simultaneously enhanced by both collective effects
(Dicke effect) and dielectric environments (Purcell effect), thus giving rise
to a combined Purcell-Dicke effect. We study this effect by treating the
ensemble of N atoms as a single effective N+1-level 'Dicke atom' which couples
to the environment-assisted quantum electrodynamic field. We find that an
environment can indeed alter the superradiant emission dynamics, as exemplified
using a perfectly conducting plate. As the emission acquires an additional
anisotropy in the presence of the plate, we find an associated resonant
Casimir-Polder potential for the atom that is collectively enhanced and that
exhibits a superradiant burst in its dynamics. An additional tuneability of the
effect is introduced by applying an external driving laser field.
