Fig. 2
Solar radio flux, magnetic condition, and ionospheric TEC variations in August 2018. a From top to bottom are the solar radio flux at 10.7 cm of F10.7 and the magnetic indices of Kp and Dst. b Latitude-time-TEC (LTT) plots along the epicenter longitude. c Time series of the GPS over the epicenter in August 2018. The 5 August 2018 M7.0 earthquake and its followed M6.s earthquakes of 17 August M6.5 and 19 August M6.3/M6.9 are denoted by the vertical red solid and dash lines, respectively. The red curve is the observed GIM TEC. The gray and black curves demote the associated median (\(\tilde{M}\)), upper bound (UB), and lower bound (LB), respectively. The red and blue shaded areas denote positive anomalies of TEC−\(\tilde{M}\) and negative anomalies of \(\tilde{M}\)−TEC, respectively. The blue dashed rectangular denotes the storm signals on 26–28 August 2018. F10.7 generally lies between 67–74 sfu (solar flux unit; 1 sfu = 10−22W m−2 Hz−1) and peaks 74 fsu on 24 August. The Dst index displays that it is relative quiet, especially 1–10 August, and there is an intense magnetic storm with a maximum depression of − 175 nT (Kp 7+) on 26 August. The LTT plots reveal that the EIA (equatorial ionization crest) becomes most intense and moves very poleward on the storm day of 26 August. The most significant positive TEC are observed before those earthquakes on 1, 3, 12, 14, 15, 17, and 18 August 2018