Observations of lower hybrid wave packets in density depletions at altitudes of 500 to 13,000 km are compared with predictions from the theory of lower hybrid wave nucleation and collapse. It is shown that a widely accepted interpretation of these cavities as collapsing lower hybrid wave packets localized in self-consistent density wells produced by their ponderomotive force is only partly consistent with observations. Observed potentials match or exceed those required for collapse and most observed length scales and aspect ratios lie in the theoretically predicted range. However, associated density perturbations appear to be almost always negative, contrary to theory based on ponderomotive force balance, which predicts them to be positive and negative equally often; moreover, they are typically 100 times greater in magnitude than this theory predicts. It is concluded that the electric-field and length-scale data are consistent with the occurrence of lower hybrid collapse, but that the lower hybrid waves either accumulate in preexisting density depressions, or produce the depressions ponderomotively via multiple collapses at a single site, or by means other than their ponderomotive force, as suggested by some previous authors. This work also suggests a possible resolution of conflicting results relating to the dominance of electron or ion heating by collapsing lower hybrid wave packets, with electron heating dominating for large ratios of electron temperature to ion temperature and ion heating prevailing in the opposite limit. ¿ American Geophysical Union 1996