Electron density structures in the polar cap imaged by ionospheric tomography

Tomographic images are presented of electron density structures obtained from measurements on Svalbard during an experimental campaign in December 1994. Observations near magnetic noon reveal enhanced densities in the cusp region, aligned along the near-vertical geomagnetic field, which may relate to particle precipitation or to the tongue of ionisation extending into the polar cap. Large enhancements in density possibly linked to patches are found in the convective flow of the central polar cap. After magnetic midnight, very low densities are seen in the polar-hole of the dawn cell, where the convecting plasma is in perpetual darkness. Ionisation profiles obtained from the tomographic reconstructions are compared to those of the EISCAT GIVEME model.