Computation of Topside Ionograms From N (h) Profiles

Model profiles of plasma frequency and electron gyrofrequency are chosen to simulate typical conditions observed in the topside ionosphere by the Alouette 1 sounder (1000 km) and the Alouette 2 sounder near apogee (3000 km). Examples of group refractive index profiles for these models are presented. Applying parabolic‐in‐log X lamination techniques to the plasma frequency profiles, ‘cumulative delays’ are computed to illustrate the relative delays contributed by each lamination to the formation of topside ionograms. Complete ionograms (ordinary, extraordinary, and Z traces) are presented for all the models. Implications of the results to the inverse experimental problem of scaling ionogram data and computing electron concentration profiles are discussed. It is concluded that high‐altitude (sounder height above 1500 km) ionograms cannot be reliably and accurately reduced to electron concentration profiles using standard open‐loop procedures (i.e., N(h) profile computed from scaled extraordinary trace data alone). To process high‐altitude ionograms satisfactorily, closed‐loop procedures, as follows, are required: (1) compute N(h) profile and ordinary and Z traces from extraordinary trace data; (2) compare computed and scaled ordinary and Z traces on a CRT display unit; (3) correct extraordinary trace scaling and iterate computations in (1) and comparisons in (2), until computed and scaled values agree. An operational, man/CRT/computer on‐line system (FILMCLIP: Closed‐Loop Ionogram Processor for the analysis of topside sounder ionogram Films) based on this approach has been developed at Ames Research Center.