IMPROVED BACKGROUND REPRESENTATION, IONOSONDE INPUT AND INDEPENDENT VERIFICATION IN EXPERIMENTAL IONOSPHERIC TOMOGRAPHY

The lack of near-horizontal ray paths through the ionosphere poses ser ious problems for the reconstruction of vertical profiles in ionospher ic tomography. Additional information from ionosondes on the bottomsid e profile has been incorporated into the reconstruction process to pro vide an improved image, but representation of the topside by means of ionospheric models is limited in certain circumstances. An alternative approach to reproduction of the topside profile is described here. Ob servations by means of incoherent scatter radars, both in the UK and i n Scandinavia, have provided a large database of topside electron dens ities. These profiles have been normalised and averaged to produce a r ange of topside shapes. The topsides have been attached to bottomside profiles, obtained using information from ionograms, to create a set o f model ionospheres for use in the representation of the background io nosphere in the reconstruction process. The success of the method is d emonstrated by means of tomographic images that show improved agreemen t with independent co-ordinated measurements made by the EISCAT radar in the SP-UK-TOMO mode. Limitations to the use of ionograms in the for mation of the bottomside profile are also discussed.