Fig. 2

STEC changes observed at the GNSS station 0729 (square in the map) over 2.4–3.2 UT, February 11, 2011. An explosive eruption of the Shin-Moe volcano (star in the map) occurred at 2:36 UT (solid vertical line), and small ionospheric observations are seen to occur in signals with GPS satellites 4, 10, and 13 around 10 min after the eruption (dashed vertical line). In the map, we show the trajectory of SIPs with solid circles and red stars indicating the 3:00 UT and 2:36 UT, respectively. The same data on the previous and the next days are shown in Additional file 1: Fig. S3