Localized electrostatic wave packets in the frequency region of lower hybrid waves have been detected by the instruments on the FREJA satellite. These waves are often associated with local density depletions indicating that the structures can be interpreted as wave filled cavities. The basic features of the observations are discussed. On the basis of simple statistical arguments it is attempted to present some characteristics which have to be accommodated within an ultimate theory describing the observed wave phenomena. An interpretation in terms of collapse of nonlinear lower hybrid waves is discussed in particular. It is argued that such a model seems inapplicable, at least in its simplest form, by providing a timescale and a length scale which are not in agreement with observations. Alternatives to this model are presented. ¿ American Geophysical Union 1996