Density depletions and current singularities observed by Freja

High time resolution data collected by the Freja spacecraft in the topside ionosphere (h similar to 1500 km) reveal that the "low-frequency turbulence" observed commonly below the ion gyrofrequency consists of a large number of coherent plasma structures which represent intense field-aligned currents (j(parallel to) similar to 30 - 300 mu Am-2) accompanied by plasma density depletions (cavities). The electromagnetic fluctuations are identified with inertial electron Alfven waves (IEAW) at perpendicular wavelengths less than or similar to 6 km. The coherent structures with singularly strong j(parallel to) are consistent with being the resonance cones of IEAW generated by localized sources at h greater than or equal to 1 R-E. The density cavities are observed to be directly related to the magnetic field gradient, i.e., intensity of field-aligned current. Using multiresolution wavelet analysis we have investigated also the wave impedance and the polarization of the electromagnetic structures at different scales. The polarization records show phase discontinuities at small-scale electromagnetic structures which is consistent with the hypothesis that they represent resonance cones or interference patterns of IEAW.