New Data Source for Studying and Modelling the Topside Ionosphere

Statistics – Computation

Scientific paper

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2447 Modeling And Forecasting, 2467 Plasma Temperature And Density, 2475 Polar Cap Ionosphere, 2481 Topside Ionosphere

Scientific paper

The existing uncertainties about the electron density profiles in the topside ionosphere, i.e., in the height regime from hmF2 to ~2000 km, requires the search for new data sources. Millions of ionograms had been recorded by the ISIS satellites that never were analyzed in terms of electron density profiles. In recent years an effort started to digitize the analog recordings to prepare the ionograms for computerized analysis. To date, approximately 300,000 ISIS-2 topside-sounder ionograms have been digitizd. Computation of electron density profiles from these ionograms requires identifying the echo traces on the ionogram and then applying an inversion algorithm. An automatic topside ionogram scaler with true height algorithm (TOPIST) has been developed that is successfully scaling ~70 % of the ionograms. This paper shows how the digital ionograms are processed and the profiles calculated. The most difficult part of the task is the automatic scaling of the echo traces in the ISIS ionograms to provide R'(f) where R' is the virtual range of the echo at frequency f. Characteristic resonance features seen in the topside ionograms occur at the gyro and plasma frequencies. An elaborate scheme was developed to measure these resonance frequencies in order to determine the local plasma and gyrofrequencies. This information helps in the identification of the O and X traces, and it provides the starting density of the electron density profile from the satellite altitude to hmF2. An 'editing process' is available to manually scale the more difficult ionograms. The electron density data and the TOPIST software will be made available online from NASA's National Space Science Data Center (NSSDC) at http://nssdc.gsfc.nasa.gov/space/isis/isis-status.html. This site provides already access to the digitized ISIS ionogram data and to related software. A search page lets users select data by location, time, and a host of other search criteria. Selected ionogram data can be viewed on CDAWeb (http://rumba.gsfc.nasa.gov/cdaweb/sp_test/) ands there are plans to make all digitized ionograms accessible through CDAWeb. The A/D operation continues with more than 8 GBytes/month corresponding to approximately 15 satellite passes digitized/day. This data restoration effort funded by NASA's Applied Information and System Research Program (AISRP) is producing a unique data base of globally distributed electron densities in the topdside ionosphere over more than one solar cycle.

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