The D-region background at high latitudes

At high latitudes the ion-pair production is most of the time dominated by fluxes of energetic particles. The resulting electron densities exceed the ones expected due to solar control alone; the D-region part of these excess densities is responsible for the absorption measured by a riometer. Hence riometer absorption is a good parameter to describe the disturbed ionosphere. An approach is presented in which a huge number of lower ionosphere electron densities from EISCAT and in-situ measurements from sounding rockets are analysed jointly. The much smaller number of rocket borne measurements constitutes an important extension to lower altitudes and densities. The undisturbed ionosphere deduced from these data sets is compared to a model of the noil-auroral D-region extrapolated to high latitudes. Typical profiles for disturbed conditions are given separately for day and night as a function of riometer absorption. (C) 1999 COSPAR. Published by Elsevier Science Ltd.