and antenna) have single and non-tracking antennas with beam width reaching about several tens of degrees ([@Benz2005]; <http://www.e-callisto.org/>).

Detrended TEC maps
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The TEC data analyzed in this study have been obtained with the GPS receivers in Europe. The data from 1617 permanent GPS receivers on January 8, 2014 have been gathered. Its locations are shown in Figure \[Figure1\]. The measurements of the pseudo-ranges and carrier phases are updated at least every 30 s by each dual-frequency GPS receiver working at $f_\textrm{1}$ = 1575.42 MHz and $f_\textrm{2}$ = 1227.60 MHz. Slant TEC, being integrated electron density along the path between receiver and satellite, is obtained from the carrier phase advance. If only the perturbation component of TEC caused by AGWs is required, as is the case of the present work, it is not necessary to determine the absolute TEC value.

In this study, we obtained the perturbation component of TEC by detrending the vertical TEC time series with 1-h running average for each satellite-receiver pair. This method is appropriate for dense nets of GPS stations and has been extensively used [@Tsugawa2018]. To compute the vertical TEC, the slant TEC was multiplied by the slant factor. The latter is a ratio 