OPTICAL TOMOGRAPHY OF THE AURORA AND EISCAT

Tomographic reconstruction of the three-dimensional auroral are emissi on is used to obtain vertical and horizontal distributions of the opti cal auroral emission. Under the given experimental conditions with a v ery limited angular range and a small number of observers, algebraic r econstruction methods generally yield better results than transform te chniques. Different algebraic reconstruction methods are tested with a n auroral are model and the best results are obtained with an iterativ e least-square method adapted from emission-computed tomography. The o bservation geometry used during a campaign in Norway in 1995 is tested with the are model and root-mean-square errors, to be expected under the given geometrical conditions, are calculated. Although optimum geo metry was not used, root-mean-square errors of less than 2% for the im ages and of the order of 30% for the distribution could be obtained. T he method is applied to images from real observations. The corresponde nce of original pictures and projections of the reconstructed volume i s discussed, and emission profiles along magnetic field lines through the three-dimensionally reconstructed are are calibrated into electron density profiles with additional EISCAT measurements. Including a bac kground profile and the temporal changes of the electron density due t o recombination, good agreement can be obtained between measured profi les and the time-sequence of calculated profiles. These profiles are u sed to estimate the conductivity distribution in the vicinity of the E ISCAT site. While the radar can only probe the ionosphere along the ra dar beam, the three-dimensional tomography enables conductivity estima tes in a large area around the radar site.