of the ionosphere thickness (250 km) for the zenith path to the signal path length between 200 and 450 km of altitude. In the slant-to-vertical TEC conversion routine the TEC data from satellites at zenith angles $\leq55^\circ$ have been used.

The two-dimensional dTEC maps over Europe have a latitude-longitude resolution of $0.15^\circ\times0.15^\circ$. Each pixel represents the average obtained from dTEC values of satellite-receiver pairs whose signal paths cross the pixel at the altitude of 250 km. To get denser filling in each dTEC map, we applied spatially smoothing within $5\times5$/$\cos(\phi)$ pixels in latitude and longitude, where $\phi$ is latitude in degrees [see details in @Tsugawa2007]. Consequently, dTEC distributions are mapped to a geographical map at an altitude of 250 km with spatial resolution of $0.75^\circ\times0.75^\circ$/$\cos(\phi)$ ($\thicksim80$ km $ \times$ 80 km). The dTEC values have been obtained in an area from $20^\circ$ W to $50^\circ$ E in longitude and from $30^\circ$ N to $75^\circ$ N in latitude.

![Distribution of GPS stations over Europe on January 8, 2014.[]{data-label="Figure1"}](Figure1.png){width="50.00000%"}

The TID-SC pair identification methodology
------------------------------------------

The goal of our study is to pinpoint a “parental” TID that causes a SC. To implement this, we adopt the method which is generally used when 