Fig. 1

Cumulative distribution function (CDF) of daily TEC occurrence at Tokyo from 1997 to 2018. The occurrence rate, which is the number of days per 100 years, and the occurrence percentage are shown on the left and right axes, respectively. Red, pink, blue, and light blue represent days of HSHG, HSLG, LSHG, and LSLG, respectively. The solid, dotted, and dashed horizontal lines represent occurrence rates of 0.3%, 0.03%, and 0.003%, which correspond to occurrence frequencies of once per year, 10 years, and 100 years, respectively