The top panel (a) shows the different NRT-like observables studied together with the satellite elevation. The solid blue line is an example of a reconstructed arc, just like the one in Figure 2c. The orange line instead is the satellite elevation. The bottom panel (b) focuses instead on the detrended version of the four observables presented in panel (a), which share the same colour code. We zoomed into dTEC peaks corresponding to low, medium, and high elevations to better visualize the slight differences between the detrended observables.