The shape and evolution of lower hybrid density cavities observed by FREJA

A large number of narrow density cavities (50 m wide in the direction perpendicular to the magnetic field) are observed by the FREJA satellite. These cavities are correlated with electric field enhancements in the frequency range of Lower Hybrid waves. By closer inspection of individual cavities it is shown that they have a Gaussian shape across the magnetic field. It is also observed that the narrowest cavities are the shallowest ones and the depths of individual cavities are only weakly related to the amplitudes of the corresponding electric field fluctuations. The observed phenomenon is unlikely to be explained by a collapse theory for lower-hybrid waves.