## 6 Ionospheric Disturbance Responses
Compared with the main phase of the storm, there is a long-term TEC enhancement and the duration of positive response in the recovery phase is almost 2-3 times that of the main phase. The magnitude of positive response during the recovery phase is smaller than that of the main phase. The bottom subgraph of Figure 5 represents the variation of solar wind velocity and IMF-Bz on 23-29 August 2018. It shows that there is an intensive increment on solar wind velocity, along with a moderate perturbation of IMF-Bz during the recovery phase of the storm. Figure 3: Diurnal variations of Geostationary Earth Orbit (GEO) total electron content (TEC) and corresponding upper bounds and lower bounds calculated from the global ionospheric map (GIM) model over different stations during 23–29 August 2018. The red and purple areas represent positive signatures and negative signatures, respectively. The yellow vertical bar is the duration of the main phase. The VTEC is vertical TEC, and the TECU is TEC Units. Figure 4: The \(f_{o}F_{2}\) and \(h_{m}F_{2}\) (red lines) and corresponding averaged quiet values (blue lines) at DARWIN and GUAM stations. The green and purple lines represent \(Df_{o}F_{2}\) and \(Dh_{m}F_{2}\), respectively. The yellow bars represent the duration of the main phase. The results from Figures 3-5 show that a prominent positive disturbance response of the ionosphere is detected during the main phase of the storm on 26 August 2018. The magnitudes of positive signatures over all the selected stations vary from 14.1-25.1 TECU, which shows that BDS-GEO satellites can effectively detect the fine variations of ionospheric TEC. Meanwhile, the \(f_{o}F_{2}\) and \(h_{m}F_{2}\) parameters of the F2 layer also display an increment during the main phase. GEO TEC observed from four stations generate a persistent enhancement along with long-duration positive disturbance signatures on 27-29 August, which is distinct from normal performance during the recovery phase of other storms. In addition, the unique phenomenon does not cover all low-latitude areas in this paper, but exhibits a local-area characteristic, compared with similar studies on this geomagnetic storm [37; 55]. Therefore, we have selected the SANYA and LEARMONTH ionosonde  
Figure 5: Diurnal variations of GEO TEC and the corresponding disturbance bounds from four stations during 23–29 August 2018. The bottom figure shows the variation of solar wind velocity (blue line) and IMF-Bz (red line), respectively. The yellow bar is the disturbance duration during the recovery phase. stations that are close to the CIBG station in the longitude direction to further investigate the performance of the ionospheric F layer during the recovery phase of the storm on 27-29 August 2018. Figure 6 shows the variation of \(f_{o}F_{2}\) and \(h_{m}F_{2}\) parameters at SANYA and LEARMONTH stations on 23-29 August. It shows that a persistently enhanced peak height up to 400 km of the F2 layer on 27-29 August is observed from two ionosonde stations. The previous studies show that the sudden increment in the layer height of the ionosphere is likely to cause the Rayleigh-Taylor instability and the development of spread-F irregularities [40]. Combined with ionosonde results, we think that the uplift of F layer height is one of the possible factors contributing to the local-area positive disturbance signatures over low-latitude regions during the recovery phase of this storm. Figure 6: The \(f_{o}F_{2}\) and \(h_{m}F_{2}\) (red lines) and corresponding averaged quiet values (blue lines) at SANYA and LEARMONTH stations. The green and purple lines represent \(Df_{o}F_{2}\) and \(Dh_{m}F_{2}\), respectively. The yellow bars represent disturbance duration during the recovery phase. Figure 7 presents the variations of the electron density Ne observed from the Swarm A and C satellites on 25-27 August 2018. The Ne is regarded as the regular level reference to analyze the disturbed variation of Ne from Swarm A and C during the storm time on 25 August.