Lower hybrid cavities (LHC), i.e., small-scale density depletions coinciding with enhanced lower hybrid waves, are frequent in the upper ionosphere. It is found that they exist in all regions the Freja satellite has scanned, as long as lower hybrid waves are present. They are found most frequently at magnetic latitudes between 55¿ to 65¿, preferably on the morning side. In the auroral oval, the LHCs do not occur as densely as at lower latitudes but they usually form larger structures.¿ 1997 American Geophysical Union