Structure and dynamics of the earth's polar ionosphere: Recent results inferred from incoherent scatter sounders

For 20 years, a large part of ionospheric research has been devoted to high latitudes and in particular to the range 60-70° where an oval of auroras permanently encircles each pole. The auroral light emissions are accompanied by the production of ionization, electric currents and fields. Indeed, the auroral latitudes play a dominant role in the ionospheric electrodynamics because electric fields and currents reach their largest intensities there. Observations from low-altitude satellites and from ground-based facilities have contributed to the analysis and modelling of the structure and dynamics of the auroral ionosphere. The results illustrated here are inferred from observations of the European Japanese incoherent scatter radars (EISCAT) based in North Scandinavia. Recently, the field of view of the EISCAT facilities has been extended toward the pole with two radars built in 1996 and 2000 at Spitzbergen (78°N): the EISCAT Svalbard radars. Other ground-based instruments (magnetometers, photometers, etc) have also been deployed at the same location. At first sight, the ionization production in the polar ionosphere is expected to be weak because of the reduced solar illumination. The first observations reveal, in contrast, the presence of intense and variable structures, which are still under investigation. To develop our understanding of these events, we discuss the theoretical results given by the particle penetration from solar origin, and of its effects into the dayside polar ionosphere.