Fig. 1

a Variation in \(f_\mathrm{o}F_{2}\) on October 19, 2012. The black horizontal line corresponds to the threshold, i.e., 4 MHz. The shorter horizontal lines represent the averaged values for each hour. b Variation in one standard deviation (SD) of \(f_\mathrm{o}F_{2}\) for each hour. The black horizontal line corresponds to the threshold, i.e., 0.5 MHz. c Variation in the solar zenith angle (SZA). The black horizontal line corresponds to the threshold, i.e., 96°. The shorter horizontal lines represent the minimum values for each hour. d Variation in the number of \(f_\mathrm{o}F_{2}\) data for each hour. The black horizontal line corresponds to the threshold, i.e., 5. Each value, which satisfied each threshold, is shown in blue, otherwise in gray. When all the four thresholds were satisfied at the same time, the values at that time are marked in red. Note that the time is written in UT (\(=\) LT − 1 h (for winter time), at Tromsø)