A comparison of reconstruction techniques used in ionospheric tomography

This paper compares the performance of three different reconstruction algor ithms used for tomographic imaging of the ionosphere. The test cases chosen make use of experimental electron content observations obtained from a mer idional chain of receivers in Scandinavia at times when the European incohe rent scatter radar facility provided independent measurements of electron d ensity for verification of the reconstructions. The examples include ionosp heres with layers peaking at both anomalously high and very low altitudes, in addition to a smooth horizontally stratified layer. The results expose t he limitations of an algorithm based on use of a standard ionospheric model but demonstrate that with appropriate choice of background a priori profil es, reliable images are obtained from both discrete inverse theory and quad ratic programing. For these two latter algorithms the resultant reconstruct ions are essentially independent of the algorithm used. The study highlight s the critical role of the range of the background input to the algorithms. A density value at a single grid cell provides sufficient additional a pri ori information to constrain the imaging of a horizontally stratified ionos phere.