REGIONAL IONOSPHERE CHARACTERIZATION THROUGH VERTICAL AND OBLIQUE RADIO-SOUNDING DATA ASSIMILATION

Abstract

The knowledge of the physical state of the ionosphere is important in many technological applications involving radio signals, such as HF frequency radio communications, over-the-horizon radar techniques and Global Navigation Satellite Systems (GNSS). For this reason, the study of phenomena of solar origin that produce variations in the magnetic fields and the plasma in the space around the Earth (Space Weather) has acquired increasing importance. In the attempt to mitigate its harmful effects on technological systems, many efforts are being directed to the development of prediction models of short-term disturbances of the ionosphere, which make use of data assimilation techniques. For the assimilation in these models, there is a growing interest in collecting real-time ionospheric data. Despite the development of numerous methods of investigation, which make use both of satellite and ground-based instruments, the vertical radio-sounding technique is still the most used one, thanks to the worldwide network of modern ionosondes, which can provide vertical ionograms in a digital format, and the use of software for their automatic interpretation (autoscaling). Another possibility is to direct efforts to allow also the oblique radio-sounding technique to be used for ionospheric monitoring purposes, through the autoscaling of oblique ionograms. Some advantages of this technique are the possibility to obtain information on the ionosphere in large geographical areas, corresponding to the radio propagation channel between the transmitter and the receiver, and to provide them on areas where ionosondes for vertical radio soundings cannot be installed, as over oceans and desert areas. This work is focused on the development of a regional 3D ionospheric model able to ingest vertical and oblique radio-sounding data, and on achieving improvements on oblique ionograms autoscaling. The Regional Assimilative Three-dimensional Ionospheric Model (RATIM) has been firstly developed to ingest vertical plasma frequency profiles fp(h) and tested over the Italian area, using data from the ionospheric stations of Rome (41.8º N, 12.5º E), Gibilmanna (37.9º N, 14.0º E) and San Vito dei Normanni (40.6º N, 18.0º E). It is constructed based on empirical values for a set of ionospheric parameters over the considered region, some of which can vary in order to fit the actual ionospheric condition through an assimilation procedure. The procedure consists in minimizing the root-mean-square deviations (RMSDs) between the observed and modeled values of fp(h) at the points where observations are available. The interest has been to obtain and verify appropriate capability to adapt to any ionospheric condition. For this reason, the model has been tested in quiet and disturbed conditions, and during day-time and night time hours in different seasons, having selected two different periods during June-July 2013, and February 2014. A further test has been performed during the March 20, 2015 partial solar eclipse over Italy, improving the model performance by adding new empirical formulations for the ionospheric critical frequencies during solar eclipse conditions. The model has been then applied to the Japanese-South Korean region, where oblique radio-soundings are systematically performed. The data used are vertical ionograms recorded at Jeju (South Korea, 33.4° N, 126.3° E) and Icheon (South Korea, 37.1° N, 127.5° E), and oblique ionograms recorded between Kokubunji (Japan, 35.7° N, 139.5° E) and Icheon, from which autoscaled Maximum Usable Frequency (MUF) values are ingested. Assimilating data from oblique radio-soundings is in fact of particular interest. Furthermore, the combined ingestion of vertical and oblique radio-soundings data is often possible when oblique radio-soundings are performed, thanks to the availability of vertical data at the receiver and the transmitter. As the quality of Kokubunji-Icheon test-mode oblique ionograms is still poor, improvements on autoscaled MUF values reliability have been obtained before applying the assimilation procedure. Several filters have been applied to the ionograms to reduce the noise before autoscaling the MUF values by the Oblique Ionogram Automatic Scaling Algorithm (OIASA). Eventually, a procedure made by the combined use of OIASA and Autoscala programs has been applied to reduce the percentage of wrong MUF estimates. This procedure makes use of the conversion of oblique ionograms in vertical equivalent ones to be processed by Autoscala, and it has shown good results when applied to a set of high-quality oblique ionograms recorded in Australia during 2015. A procedure for the MUF assimilation in RATIM has been then proposed and applied to MUF values autoscaled by OIASA from the Kokubunji-Icheon oblique ionograms. Applying an ionospheric ray-tracing technique based on the ray equation, simulated values of the ground range for Kokubunji-Icheon radio-link have been obtained from the MUF provided by OIASA. A simplified ionosphere between the transmitter and the receiver, obtained starting from RATIM ionospheric representation, has been assumed. A comparison between the real and simulated ground range values is performed for each combination of the RATIM free parameters tested during the fp(h) ingestion, introducing a further condition to the fp(h) RMSD minimization. When applied to ionosonde data recorded in oblique radio-sounding areas, the system proposed could be used for the estimation of the regional electron 3D density distribution, making use of information from both vertical and oblique ionospheric radio-soundings.