THE SPATIAL STRUCTURE OF THE DAYSIDE IONOSPHERIC TROUGH

Tomographic imaging provides a powerful technique for obtaining images of the spatial distribution of ionospheric electron density at polar latitudes. The method, which involves monitoring radio transmissions f rom the Navy Navigation Satellite System at a meridional chain of grou nd receivers, has particular potential for complementing temporal meas urements by other observing techniques such as the EISCAT incoherent-s catter radar facility. Tomographic reconstructions are presented here from a two-week campaign in November 1995 that show large-scale struct uring of the polar ionosphere. Measurements by the EISCAT radar confir m the authenticity of the technique and provide additional information of the plasma electron and ion temperatures. The dayside trough, pers istently observed at high latitudes during a geomagnetically quiet per iod but migrating to lower latitudes with increasing activity, is disc ussed in relationship to the pattern of the polar-cap convection.