## 3 Results
Figure 7 displays the multiple satellite ascending tracks of ROCSAT-1 and the corresponding density irregularity structure detected over Eastern Asia during the period 13-24 UT on 15, 16, and 17 July. The tracks were marked with the corresponding labels. The purple stars along the ROCSAT-1 orbits represent each of the identified density irregularity structures (the criterion of \(\sigma\geq\) 0.3%), determined by the calculation from the method proposed by [PERSON] et al. [1]. The red dots represent the geographic location of the four ionosondes (WK545, TO536, YG431 and OK426) used in this study. As seen in Figure 7b, from the magnetic equator to midlatitudes, a large number of density irregularity structures were detected by ROCSAT-1 on tracks b1-b7, indicating a strong disturbance in the nighttime ionosphere over Eastern Asia. The observation of density irregularity structure in ROCSAT-1 tracks over Japan can be associated with spread F observed in ionograms during the same period, as shown in Figure 4. Significantly, ROCSAT-1 detected a few density irregularity structures on 15 and 17 July as shown in Figure 7a,c, which are consistent with the observations in Figure 1a,c. On 15 July, Figure 7a shows that the density irregularity structure mainly occurred near the magnetic equator of tracks a5 and a6. On 17 July, Figure 7c displays that the density irregularity structures were mainly scattered on track c7. Thus, combined with the results of Figure 7a,c, the observation in Figure 7b indicated that magnetic storms should perform an important role in the formation of the density irregularity structure over Eastern Asia. In order to more directly identify plasma density depletion, Figure 8 further shows the variation in in situ ion density measured by ROCSAT-1 as a function of geographic longitude with its tracks corresponding to the marks in Figure 1:0 The UT time is drawn in the upper right corner of each subfigure. Figure 8 shows that compared with the plasma density on 15 and 17 July, the overall plasma density on 16 July was obviously enhanced, consistent with the enhancements of foF2 observed during the magnetic storm in Figure 1:1 In addition, Figure 8a,c show relatively stable changes in plasma density. However, Figure 8b shows that ROCSAT-1 encountered clear plasma density depletion on tracks b1-b7, proving the reliability of the density irregularity structure identified in Figure 7b. It should be noted that this plasma depletion mainly occurred in the midnight-moring sector. No signatures of plasma depletion were measured on track b8, which may be due to the time required for EPBs to grow from the magnetic equator to  
Figure 1:2 ROCSAT-1 ascending tracks marked with different labels over the Eastern Asian sector during 13–24 UT on (**a**) 15, (**b**) 16, and (**c**) 17 July 2003. The purple stars along the ROCSAT-1 orbits represent each of the identified density irregularity structures (the criterion of \(\sigma\geq\) 0.3%) obtained from the method proposed by [PERSON] et al. [1]. The red dots represent the geographic location of the four ionosondes (WK545, TO536, YG431, and OK426 used in this study). The blue lines indicate the geomagnetic equator. southern midlatitudes. We note in Figure 8b that the eastern edge of plasma density depletion moved westward as time passed, especially from tracks b5 to b1. We checked the westward drift motion of plasma depletion on the zonal drift parameters recorded by ROCAST-1. After 22 UT, only small magnitudes of plasma density depletions were detected around the latitude of Japan on tracks b1 and b2. This is because after dawn, the photoionization process can produce new plasma particles to fill plasma depletion at high altitudes. As shown in Figure 8b, other ionospheric irregularity structures, i.e., plasma blobs, appear in the form of plasma density enhancement around the longitude of \(\sim\)135\({}^{\circ}\)/\(\sim\)160\({}^{\circ}\) on tracks b6/b7, respectively.