Fig. 3
![Fig. 3](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2Fs40623-015-0257-4/MediaObjects/40623_2015_257_Fig3_HTML.gif)
a 5 min snapshots of vertical TEC anomaly map in the period 08:00–08:35 UT (LT = UT + 9). Simultaneous occurrence of two frontal structures can be seen over the southwestern and central part of Japan, which are labeled as A and B in the snapshot at 08:10 UT, respectively. The enlarged images of the structures A and B at their maximum lengths are shown in b and c, respectively