## 6 Ionospheric Disturbance Responses  
The ionospheric responses to the geomagnetic storm on 23-29 August 2018 are investigated by BDS GEO observations. The diurnal variations of GEO TEC and the corresponding upper bounds and lower bounds are calculated from the GIM model over different locations in Figure 3. The dark blue and light blue dotted lines indicate the corresponding upper bound and lower bound, respectively. The red and purple error areas represent positive and negative disturbance signatures, respectively. The yellow area is the main phase of the storm. As shown in Figure 3, there are no significant disturbance signatures of GEO TEC during quiet time on 23-24 August 2018. The TEC magnitudes during this quiet time are 25 TECU (IISC), 20 TECU (DARW), 21 TECU (SEYG), 15 TECU (MAYG), and 18 TECU (KAT1), respectively. From the beginning of the main phase of the storm on 25 August 2018, the TEC does not immediately respond to the sudden disturbance  
Figure 2: Temporal variations of solar wind velocity, IMF-Bz component, Dst and Kp indexes during 23–29 August 2018. The yellow shadow shows the period of the storm main phase. of the geomagnetic storm. After 6 to 9 h, the magnitude of GEO TEC starts to increase rapidly. The ionospheric TEC from GEO satellites responds severely to the storm. At about 07:00-09:00 UT on 26 August 2018, the magnitudes of GEO TEC from different stations increases to the maximum of 48.2 TECU (IISC), 39.3 TECU (DARW), 38.0 TECU (SEYG), 29.6 TECU (MAYG), and 39.0 TECU (KAT1), respectively. Compared with quiet time, the relative increase from these stations is ranging from 0.8 to 1.16. The red areas show that large positive disturbance signatures of low-latitude ionosphere are detected during the main phase of the storm. The largest positive response is up to about 25.1 TECU at KAT1 station, and the smallest is up to 14.1 TECU at MAYG station. As the recovery phase starting on 26 August 2018, the magnitude of GEO TEC decreases to normal state, and the intensity of positive disturbance signatures gradually weaken, which indicates the local ionosphere observed from these stations returns to quiet state during the recovery phase. Figure 4 shows the variation of critical frequency (\(f_{o}F_{2}\)) and peak height (\(h_{m}F_{2}\)), as well their relative deviations from 23-29 August 2018 at DARWIN and GUAM stations. The peak height of F layer is often used to study the possible driving force of an electrodynamic source to uplift or decrease the ionospheric F layer. In Figure 4, the \(f_{o}F_{2}\) observed from DARWIN and GUAM stations shows an obvious enhancement up to nearly 10 MHz on 26-27 August 2018. The \(Df_{o}F_{2}\) of both two stations even increases to around 0.6-0.8, which indicates that the ionosphere generates a positive disturbance response during the main phase of the geomagnetic storm in August 2018. The \(h_{m}F_{2}\) of DARWIN station has the same trends with \(f_{o}F_{2}\), with an increase to 380 km on 26 August. During the recovery phase of the storm, the \(f_{o}F_{2}\) and \(h_{m}F_{2}\) of two ionosonde stations have no prominent disturbance compared with the averaged quiet days. We discover a special local-area anomaly of the low-latitude ionosphere during the recovery phase of this geomagnetic storm on 27-28 August 2018. Figure 5 shows the variation of GEO TEC and the corresponding disturbance bounds from four stations during 23-29 August 2018. It can be seen that the GEO TEC started to exceed the upper bound at around 02:00 UT on 27 August. The positive disturbance signatures occur, and the duration of signatures is up to about 1.5 days. It is noticeable that the positive response from station CIBG lasted for about 2 days. The magnitude of the GEO TEC series reaches the maximum of 41.1 TECU (PTGG), 34.8 TECU (DJIG), 35 TECU (NKLG), and 48.6 TECU (CIBG), respectively, and the corresponding positive signatures vary from 7.9 TECU to 16.3 TECU during the recovery phase on 27-29 August.