Freja multiprobe observations of electrostatic solitary structures

Various electrostatic solitary structures are observed by the Freja satellite in the upper ionosphere in the auroral zone. The use of multiple probes makes the determination of their propagation characteristics possible. Some of them are possibly similar to the solitary waves and weak double layers previously observed in the magnetosphere, but the Freja examples move much slower. Their size is less than 100 meters. Another type is the lower hybrid cavities, density depletions with a lower hybrid wave packet inside. The depletions are about 5%, and they have little or no speed relative to the ambient plasma in the direction perpendicular to the geomagnetic field. A third type is related to the lower hybrid cavities, but inside the density depletion there is a solitary electrostatic structure, as well as lower hybrid waves.