## 3 Results  
The wide distribution of ground-based GNSS networks provides an opportunity for a global overview of the occurrence of ionospheric F-region irregularities. Figure 1 shows the two-dimensional global maps of the ROTI indices as a function of geographical longitude and latitude at 18 UT on 15, 16, and 17 July 2003. The color indicates the ROTI value within a range from 0.0 to 1.0. The spatial resolution of the maps is 0.25 \(\times\) 0.25 degrees. We note that an enhancement of the ROTI appeared from the equator to middle latitudes over Eastern Asia at 18 UT on 16 July 2003 in the middle panel in Figure 1. The local time at 135\({}^{\circ}\) longitude is approximately 03 JST. This means that postmidnight ionospheric F-region irregularities were observed. However, no EPBs signatures were observed over this region at the same moment on 15 and 17 July 2003. Therefore, it is worth further exploring what caused the occurrence of ionospheric F-region irregularities on 16 July 2003, even extending to the midlatitudes where the ionospheric F-region irregularities are rarely detected. Another attractive feature in Figure 1 is that the enhancements of ROTI occurred in the high latitudes in both hemispheres during the three days. The high-latitude ionospheric F-region irregularities are produced by energetic particle precipitation from the Earth's magnetosphere and convective plasma processes [42; 43], which is beyond the scope of this study. Figure 1: Two-dimensional global maps of ROTI indices in geographic coordinates at 18 UT on (**top**) 15, (**middle**) 16, and (**bottom**) 17 July 2003, as calculated from GNSS receivers ([[https://stdb2.ieee.nagoya-u.ac.jp/GPS/GPS-TEC/](https://stdb2.ieee.nagoya-u.ac.jp/GPS/GPS-TEC/)]([https://stdb2.ieee.nagoya-u.ac.jp/GPS/GPS-TEC/](https://stdb2.ieee.nagoya-u.ac.jp/GPS/GPS-TEC/)) (accessed on 20 January 2023)). The color indicates the ROTI value within a range from 0.0 to 1.0. The red vertical lines represent 12 LT. The contours represent the geomagnetic latitude. Figure 2 displays the geophysical conditions that occurred during the period 15-17 July 2003. The solar wind velocity in Figure 2a only increased from ~550 km/s to ~600 km/s. As shown in Figure 2b, the IMF Bz component sharply turned south at ~6 UT on 16 July, remained south for approximately 7 h, then slowly and continuously turned northward over a long period. At ~5 UT on 16 July, the IMF Bz component quickly turned southward for an hour, then northward. We also note that the IMF Bz component turned to the south and lasted for ~3 h at 10 UT on 15 July. In addition, the IMF Bz component also experienced large north-south fluctuations at other times. The increase in the AE index, shown in Figure 2c, indicated that several strong magnetospheric substorms occurred during the period of 15-17 July 2003. In particular, the substorm onset was related to the southward excursion of the IMF Bz component and the maximum value reached was at ~2 UT on 16 July (Figure 2c). At ~4 UT on 16 July 2003, the sudden decrease in the SYM-H index in Figure 2d indicated the start of the main phase of the magnetic storm. Eventually, the SYM-H index reached its minimum value of ~100 nT at ~12 UT on 16 July 2003. After that, this moderately intense magnetic storm experienced a long recovery phase. The value of the Kp index slightly increased to more than 4 during the main phase, as shown in Figure 2e. Taken together, the above parameters in Figure 2 suggest that the PPEF and DDEF were produced on 16 July 2003 during this magnetic storm. Thus, the changed geophysical conditions may have been responsible for the enhancements in the ROTI over Eastern Asia on 16 July 2003 (Figure 1). More investigations are presented in the following text to illustrate the background ionosphere and possible physical processes. Figure 3 displays the SYM-H and IMF Bz indices, and the variation in foF2 observed by a meridional chain of four ionosondes along the longitude of ~135\({}^{\circ}\)E with decreasing latitude during the period 15-17 July 2003.