could only happen when the solar radio wave propagates through a valley where the refractive index is higher than that in the surrounding crests. Therefore, we expect to find that the IPP positions corresponding to any SC being contained within a valley on dTEC map. If so, we would be able to identify the particular TID responsible for the specific SC.

The geographical latitude ($\phi_I$) and longitude ($\lambda_I$) of an IPP can be computed using following formulas given by @Klobuchar1987: $$\begin{aligned}
\begin{gathered}
\beta = 90 - \theta - \arcsin\left[\frac{R_E}{R_E+h}\cos\theta\right],\\
\phi_I = \arcsin(\sin\phi_0\cos\beta + \cos\phi_0\sin\beta\cos A),\\
\lambda_I = \lambda_0 + \arcsin\left(\frac{\sin\beta\sin A}{\cos\phi_0}\right),
\label{Eq1}
\end{gathered}\end{aligned}$$ where $R_E = 6371$ km is the radius of the Earth, and $h$ is the height of the ionosphere surface. These formulas are used to calculate the IPP coordinates for a satellite-receiver pair. In our case, $\phi_0$ and $\lambda_0$ are latitude and longitude of the site of a radio telescope, respectively, while $\theta$ is the solar elevation angle and $A$ is the solar azimuth angle.

It is important to define correctly the height of the ionosphere shell $h$. We set it to be 250 km. The same value was taken for dTEC maps. To assign it, we 