Fig. 1

Vertical TEC anomaly maps showing Es patches that appeared in three different latitude regions, a Wakkanai ~45 N, b Kokubunji ~35 N, and c Yamagawa ~30 N. Frontal structures are commonly seen although their lengths range from 100−500 km. A typical example of frontal structure movement perpendicular to the elongation, north-northeastward in this case, is shown in (d). Local time (LT) in Japan is ahead of the universal time (UT) by 9 h