## 3 Results
These plasma blobs on both tracks also identified at similar latitudes in the Southern Hemisphere as suggested in Figure 7, indicating a possible linkage of the extension of EPBs [51; 52]. Figure 8a shows a series of weak plasma depletions on tracks a5 and a6, corresponding to the density irregularity structure recorded near the magnetic equator in Figure 7a. We inferred that these weak plasma depletions were the remnant of the EPBs initiated during sunset hours. Some of plasma density enhancements shown in Figure 8c were also identified as ionospheric irregularity structures in Figure 7c. The detailed generation mechanism of plasma density enhancement is beyond the scope of this study. To study the behavior and evolution of plasma depletion, Figure 9 shows the variation in the 5-s averaged plasma vertical and zonal drifts for the corresponding ROCAST-1 tracks as shown in Figure 8b. The UT time is drawn in the bottom right corner of each subfigure. Many wave-like variations exist in plasma drift especially in zonal drift on some tracks, such as b1-b4, which is due to the influence of solar photons on RPA after sunrise [53]. In addition, some outliers were also occasionally recorded by ROCSAT-1. Figure 1:3 ROCSAT-1 observations of in situ ion density (Ni) variation as a function of geographic longitude over the Eastern Asian sector during 13–24 UT on (**a**) 15, (**b**) 16, and (**c**) 17 July 2003. The corresponding labels are the same as those used in Figure 1:4 Universal time (UT) information is given in the upper right corner of each subfigure. Therefore, we need to try our best to recognize the feature of drift inside plasma depletion and reveal some disturbance effects caused by magnetic storm. Figure 9a shows that the vertical drifts inside plasma depletions were upward with the magnitude of over 100 m/s on tracks b5-b7 in the postmidnight sector. This upward drift indicated the continuous growth and extension of plasma depletion, suggesting that plasma depletion is still alive during that postmidnight period. Another important feature in Figure 9a is that the vertical drift has a weak upward velocity in the ambient ionosphere on most tracks, as opposed to the downward plasma velocity during quiet-time periods. The upward drift moves plasma from a lower altitude to a higher altitude, causing an uplift of the ionospheric F region [51; 53; 54]. Since plasma resides at higher altitudes where the recombination rate is low, the enhancements in foF2 and in situ ion density were observed by ionosondes and ROCSAT-1, respectively. Importantly, owing to the lower recombination rate at high altitudes, the strong uplift of the ionosphere is more conducive to EPBs generation due to the high RT instability growth rate. Moreover, such ionospheric conditions also become more favorable to the existence of long-lasting plasma bubbles because it should take a long time for the photoionization process to produce enough new plasma particles to refill the low-density region at high altitudes [51; 54]. During the quiet-time period, the nighttime zonal drifts were eastward, and reversed its direction westward at ~05 LT as measured by the C/NOFS satellite [55]. However, the ambient zonal drifts were westward in the midnight-morning sector observed during this storm as shown in Figure 9b. In particular, zonal drifts inside plasma depletion on tracks b5 and b6 were strongly westward. The above results suggested that plasma drifts in the ambient ionosphere were strongly affected by the magnetic storm. Figure 1:5 ROCSAT-1 observations of 5-s averaged plasma (**a**) vertical and (**b**) zonal drift variation as a function of geographic longitude over the Eastern Asian sector during 13–24 UT on 16 July 2003. The orbits are the same as those used in Figure 7b. Universal time (UT) information is given in the lower right corner of each subfigure. Figure 10 presents the CHAMP descending tracks and the corresponding in situ electron density over Eastern Asia during the period 11-15 UT on 15-17 July 2003.