Properties and Theory of Lower-Hybrid Density Cavities

Lower-hybrid cavities are wave-filled, cylindrical density cavities aligned with the geomagnetic field. They have relative density depletions of several to tens of percent, diameters of order 20-50 m, and are associated with ion heating transverse to the geomagnetic field. Several aspects of these structures remain unexplained, including the cause of the density depletion and the reason for their relatively narrow distribution of diameters. We present statistical properties of several hundred cavities observed on the OEDIPUS-C and GEODESIC sounding rockets, flown into the nightside auroral ionosphere through plasma densities varying over two orders of magnitude. The average cavity chord lengths are observed not to depend on density, demonstrating that cavity sizes are not determined by electron inertial length, for example. A subset of cavities also exhibit slight density increases or ``shoulders'' at the their perimeters. Density cavities with these features can be explained by tracing ion trajectories in the presence of ion heating localized on the scale of an ion gyroradius. The dependence of cavity depth and shape on heating intensity and scale size is predicted using Monte Carlo and semi-analytical descriptions of heated ion motion near cavities.