building TEC maps. The ionosphere is simplified as a thin layer (shell). The altitude of the shell is generally set equal to the height of the F2 peak (hmF2). The integrated magnitude of electron density within the F2 ionospheric layer is the main contribution to TEC values. Then, the obtained TEC values are given to the so-called ionospheric pierce point (IPP), and mapped on the shell. The IPP represents the point at which the satellite-receiver line-of-sight crosses the ionosphere shell.

Let us substitute a satellite and a receiver for the Sun and a radio telescope, respectively. Therefore, we can localize the place – the IPP position – where the Sun-antenna line-of-sight pierces the ionosphere shell. In this way, within the life time of a particular SC, a set of IPP coordinates can be determined and plotted onto the dTEC maps. This allows us to directly observe the instantaneous dTEC structure and the position at which the wave front of incident radio wave penetrates the ionosphere. For ease, we term the TID wavefront as a crest and the region between two successive wavefronts as a valley. The crests and valleys have high and low magnitudes of electron density, respectively. The focusing 