Localized electrostatic wave packets in the frequency region of lower hybrid waves have been detected by the instruments on the FREJA satellite and also earlier by instrumented rocket payloads. These waves are often associated with local density depletions, indicating that the structures can be interpreted as wave-filled cavities which are strongly elongated along the magnetic field. The basic features of the observations are discussed, providing a survey of the conditions for occurrence and spatial distributions. The basic properties of the individual cavities and associated fluctuating electric fields are discussed as well, and typical densities, widths, and other characteristics are obtained for selected orbits. ¿ 1998 American Geophysical Union