We report observations of plasma waves and banded fine structure within a density cavity in the terrestrial nightside plasmapause. The observations were obtained by the wideband receiver of the plasma wave instrument on board the Polar spacecraft. We can identify Langmuir/upper hybrid emission and Z-mode emission within two distinct frequency zones of the density cavity source region. These observations appear to be consistent with eigenmode trapping of partially electrostatic waves. The plasma waves appear to be trapped within the density cavity bounded at low frequencies by the L = 0 surface. A second distinct region of plasma waves exists bounded at low frequencies by f = fpe (plasma frequency), but includes Z-mode at higher frequencies up to f = fUH (upper hybrid frequency). Intense fine structure emission bands are observed in both regions of the density cavity. We conclude that density cavities are an important source of plasma waves including continuum and kilometric continuum emission and the fine structure can be a remote diagnostic of density structures within the plasmapause/plasmasphere region.