Multi-Dimensional Modeling of the Ionosphere (MDION)

Project "MDION" aims at the development of a multi-dimensional integrated model of the ionosphere, by using different space geodetic techniques and applying a combination procedure for computation of global ionosphere models. Geodetic techniques, such as the Global Navigation Satellite Systems (GNSS), satellite altimetry, or FORMOSAT-3/COSMIC allow the observation and modeling of the ionosphere, but each has its specific characteristics which affect the derived ionosphere parameters. The combined model makes best use of the advantages of every particular method, has a more homogeneous global coverage and is more accurate and reliable than the results of each single technique. In the first step models generated from the combination of GNSS and satellite altimetry within the Institute of Geodesy and Geophysics (IGG), Vienna, are integrated with occultation data from Low Earth Orbiter (LEO) satellites such as FORMOSAT-3/COSMIC in order to model ionospheric parameters in terms of the electron density as a function of latitude, longitude, time, and height. Since these LEO missions observe GPS occultation measurements, they have the capability of providing vertical profiles of ionospheric refractivity and would give the opportunity to develop 4D ionosphere models in form of Global Ionosphere Maps (GIM). For further improvement of the results, the models are integrated with external models and data such as the International Reference Ionosphere (IRI), the La Plata Ionospheric Model (LPIM), and the ionosphere data from integrated ionosonde profiles. The International Reference Ionosphere (IRI) has, for many years now, proven to be a valuable resource for modeling the average ionosphere; and as ionosondes are the most abundant and accurate sources of vertical profiles, using these models profiles in the assimilation procedure will be of great benefit for the models developed within project MDION. The integrated combined GIM will be useful for correcting single-frequency measurements carried out by many observation techniques using radio frequencies and for validation and improvement of ionosphere parameters derived by other individual techniques as well as theoretical models. They can also be utilized as information source for the technique-specific instrumental biases, like the GNSS Differential Code Biases (DCB) or satellite altimetry offsets, which are estimated as a by-product. Furthermore, the resolution of the models developed within the project will be increased, and the models will be densified by regional data. In order to accomplish this goal an observation-based variable degree Adjusted Spherical Harmonic (ASHA) model will be developed for near-real time regional ionospheric TEC mapping, primarily over the Austrian permanent GNSS network, and then over the European EUREF permanent network. Generally, the combined models will contribute to various studies of the physics of the upper Earth`s atmosphere and solar terrestrial environment.

Project MDION aimed at the development of a multi-dimensional integrated model of the ionosphere, by using different space geodetic techniques and applying a combination procedure for computation of global ionosphere models. Within the MDION project, ionospheric parameters were modeled in different dimensions. First, Total Electron Content (TEC) was modeled in longitude and latitude (2D) using various space geodetic techniques. Geodetic techniques, such as the Global Navigation Satellite Systems (GNSS), satellite altimetry, or FORMOSAT-3/COSMIC (F/C) allow the observation and modeling of the ionosphere, but each has its specific characteristics which affect the derived ionosphere parameters. The combined model made best use of the advantages of every particular method, had a more homogeneous global coverage and it was shown to be more accurate and reliable than the results of each single technique. Nevertheless, due to the fact that these 2D models provide information about the integral of the whole electron content along the vertical or slant ray path, they are not sensitive to the height variations within the ionosphere. In cases where information about the ionospheric parameters at different altitude is required, e.g. when electron density profiles are required, or when satellite to satellite observations are performed, ionospheric parameters should be modeled in 3D or 4D. In addition, the ionosphere can also include geophysical parameters like the maximum electron density and its corresponding height. Therefore, high resolution modeling of these parameters allows an improved geophysical interpretation. Within the project, to model ionosphere in 3D, electron density was presented as a function of maximum electron density (NmF2), and its corresponding height (hmF2). NmF2 and hmF2 were then modeled in longitude, latitude, and height using two sets of spherical harmonic expansion with degree and order 15. The estimated results were compared to the IRI-2012 model to assess the least-squares estimation procedure and moreover, to validate the developed maps, the results were compared with the F2-peak parameters derived from the F/C data. The comparisons proved that our modeling approach has a great potential to provide accurate and reliable results. Finally, for a 4D model, the temporal variations of the ionospheric peak parameters were taken into account explicitly, and therefrom electron density was modeled in longitude, latitude, height, and time. Due to several developments and modifications of previous conceptual approaches the study accomplished within this project can be considered as a pioneer in the field of modeling the upper atmosphere, using space geodetic techniques.