IS THERE LOWER-HYBRID WAVE COLLAPSE AT AURORAL LATITUDES - THEORY VERSUS OBSERVATIONS

Observations of lower hybrid wave packets in density depletions at alt itudes of 500 to 13,000 km are compared with predictions from the theo ry of lower hybrid wave nucleation and collapse. It is shown that a wi dely accepted interpretation of these cavities as collapsing lower hyb rid wave packets localized in self-consistent density wells produced b y their ponderomotive force is only partly consistent with observation s. Observed potential 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 f orce balance, which predicts them to be positive and negative equally often; moreover, they are typically 100 times greater in magnitude tha n this theory predicts. It is concluded that the electric-field and le ngth-scale data are consistent with the occurrence of lower hybrid col lapse, but that the lower hybrid waves either accumulate in preexistin g density depressions, or produce the depressions ponderomotively via multiple collapses at a single site, or by means other than their pond eromotive 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 oppo site limit.