STOCHASTIC INVERSION IN IONOSPHERIC RADIOTOMOGRAPHY

The stochastic inversion method in ionospheric radiotomography is revi ewed with a special emphasis on regularization used in the inversion p rocess. Regularization is used both for preventing vigorous point-to-p oint oscillations and for controlling the peak altitude and thickness of the inversion result. The latter usually means importing a priori i nformation on the layer height and thickness to the solver. In this pa per it is pointed out that due to the curvature of the Earth and of th e ionosphere, the measurements contain some information on the ionosph eric altitude and the profile shape even in the case of a purely horiz ontally stratified layer. If this information could be used in choosin g an appropriate regularization, no additional information would be ne eded. Simulation tests are presented which indicate that the altitude of a horizontally stratified layer can be determined with a reasonable accuracy without any a priori information. An attempt is also made to use the data for determining the shape of a proper regularization pro file. Although some success is achieved in this effort, it is conclude d that available a priori information, for example, ionosonde or incoh erent scatter measurements, should be used in choosing the regularizat ion profile. The ideas are tested with true data obtained from differe nce Doppler measurements carried out in Scandinavia, and the results a re compared with simultaneous observations made by the European incohe rent scatter radar. The comparison shows a reasonable agreement, altho ugh clear discrepancies also occur, for instance, in the shape of the bottomside profile.