Ionosphere comparison study of VGOS and global maps of Total Electron Content

Ionosphere comparison study of VGOS and global maps of Total Electron Content.

Nataliya Zubko, Minghui Xu, Niko Kareinen, Tuomas Savolainen, Markku Poutanen.

 

Total Electron Content (TEC) of the ionosphere is an important characteristic whose accurate estimation is needed in various application which are based on measurements in radio wave band. There is a number of Global TEC models designed to describe conditions of ionosphere in different parts of the world. We have conducted a comparative study of the two selected TEC global maps with the results from the observations of the VLBI Global Observing System (VGOS). VGOS network has been established recently and it is continuously growing. The estimated differential TEC (dTEC) from VGOS data has high precision with the formal error of dTEC is of about 0.01-0.2 TECU. It can be used in evaluation of the TEC global maps, as well as an additional data source for the further improvement of the TEC map models.

VLBI measures radio waves emitted by Active Galactic Nuclei (AGN) using a network of radio telescopes distributed around the globe. The measured signal propagates through different parts of ionosphere having different local properties, since distances between radio telescopes spans the range from 400 km to more than 10,000 km. To account for the ionosphere delay effect, geodetic VLBI estimates dTEC for each observing baseline that is formed by a pair of radio telescopes.

Precision of the estimated dTEC with VGOS has been improved considerably compared to the traditional geodetic S/X VLBI observations. One needs to note that VGOS dTEC still have both instrumental (slowly varying) and source structure systematic contributions that will need to be decoupled from the ionosphere measurement. We have compared VGOS ionosphere product with the dTEC calculated using global ionosphere TEC maps. For analysis, we selected two TEC global models, CODE GIM and Neustrelitz TEC Model Global (NTCM-GL). The comparison was performed for the VGOS observations made in 2019, when the solar activity was at about its minimum. The comparison shows a good agreement between VGOS dTEC and dTEC obtained using global TEC maps. However, it also reveals shortages of the global TEC models in some locations. The VGOS data can be considered as an additional information source and, hence, they can be used for the further improvement of the global TEC models.