## 4 Discussion  
Using multi-instrumental observations, this study reports an interesting case study of midlatitude plasma depletions over Eastern Asia that occurred during a moderate magnetic storm on 16 July 2003. These depletions are widely distributed in latitudes, even extending  
Figure 10: (**a**) CHAMP descending tracks marked with the corresponding right subgraph labels over the Eastern Asian sector during 11-15 UT on 15 (red), 16 (blue), and 17 (black) July 2003. 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. The corresponding in situ electron density (Ne) variations as a function of geographic latitude are shown in right panels (**b–g**), respectively. The arrows indicate the descending orbits. to midlatitudes over Eastern Asia, as detected by the in situ plasma density measured by ROCSAT-1 and CHAMP. Correspondingly, SSFs are recorded in the ionograms at three lower latitude stations over Japan on 16 July 2003, as shown in Figure 4. Generally, SSFs are thought to be related to ionospheric scintillation [45; 46]. Thus, it is reasonable to infer that the enhancement of ROTI observed in Figure 1 is associated with SSF and depletions during this magnetic storm. Although no obvious signatures of plasma depletions were detected by satellites and GPS on 15 and 17 July 2003, FSF was still observed in the ionograms at multiple stations over Japan. In addition, it can be seen from Figure 4 that the types of spread F are mainly FSF at higher latitude station WK545, which is significantly different from that at the three lower latitude stations of TO536, YG431, and OK426. This implies that these two types of midlatitude spread F observed by ionosonde are possibly produced by the different physical mechanisms in our case. The midlatitude plasma depletions observed during this magnetic storm are mainly centered in the midnight-morning sector, accompanied by an increase in plasma density. We note that the IMF Bz turned northward during this period, indicating an eastward penetration electric field caused by the over-shielding electric field in the nighttime ionosphere [22]. Simultaneously, the significant westward drift of plasma was confirmed by the observations of ROCSAT-1 in Figure 9b. Therefore, the expected DDEF could be produced by the thermospheric disturbance westward winds during the recovery phase of the storm [22; 23; 24]. DDEF is usually eastward (westward) in the nighttime (daytime) ionosphere. These superimposed eastward electric fields moved the F region plasma upward as observed by ROCSAT-1 in Figure 9a, which is conducive to the growth of the RT instability, and hence, results in favorable conditions for the generation and latitudinal extension of EPBs [7; 19; 20; 50]. As a result of the extension of EPBs along the magnetic field lines, plasma blobs that occurred at low latitudes in the Southern Hemisphere were identified by ROCSAT-1 on tracks b6/b7. Eventually, midlatitude SSF related to the extension of EPBs was recorded in ionograms over Japan during this storm. In addition, we also note in Figure 4 that the lower the latitude of the ionosonde station is, the earlier the time of SSF appears. This characteristic suggests that the midlatitude SSF that occurred during this storm is not related to the enhanced activity of traveling ionospheric disturbances caused by Joule heating at auroral regions [19; 20]. In addition, the enhanced eastward electric field may transport plasma from the equatorial region to higher latitudes [19; 30], causing the enhancement of plasma density during this storm as observed by satellites and ionosondes. As for FSF observed in ionograms, it may be induced by other physical processes. As shown in Figure 5, the signatures of the Es layer can be seen almost every ionogram. At the same time, a high occurrence of FSF was recorded by the four ionosondes in Japan.