## 4 Discussion  
As mentioned in the Introduction section, at the times near and/or soon after sunset, an enhanced eastward electric field, i.e., PRE, causes the F-layer to move upward and develop plasma depletions, triggering the Rayleigh-Taylor instability. As a result, the plasma bubbles develop from the bottom side, and the instabilities cause ionospheric irregularities and radio signal scintillations and intrude into the higher altitudinal and latitudinal ionosphere. In this study, a multi-station and multi-instrument system is organized and proposed for ionospheric scintillation and ESF specification in the Taiwan-Philippines sector. The FS7/COSMIC2 program can provide more than 5000 GPS/GLONASS RO observations per day within the region between the geographic latitudes of \(\pm\)40\({}^{\circ}\), i.e., approximately seven RO observations per hour in the Taiwan-Philippines sector (between 20\({}^{\circ}\)\(\pm\) 15\({}^{\circ}\)N and 120\({}^{\circ}\)\(\pm\) 15\({}^{\circ}\)E geographic coordinates). From the FS7/COSMIC2 RO measurements on 26 October 2021, the observed or retrieved limb-viewing SNR, \(S_{4}\), and \(N_{e}\) profiles have been used to identify an ionospheric irregularity and scintillation event that happened from 13:30 to 15:00 UT, i.e., from 21:30 to 23:00 LT in Taiwan, and at magnetic quiet conditions. The results also show that the \(N_{e}\) irregularities and limb-viewing radio intensity scintillations are stronger and distributed into higher altitudes at the southernmost part of the Taiwan-Philippines sector, i.e., near the geomagnetic equator, compared to those near the sector center (geomagnetic latitude \(\sim\)10\({}^{\circ}\)N). Furthermore, as shown in the right panel of Figure 2, the scintillation altitudes of the RO observation #4 and even observations #5 and #6 were distributed at an altitude range of \(\sim\)100 km only and around the F-layer \(N_{e}\) peaks. This  
Figure 6: Power spectrums and their spectral index \(p\) values of the L1-band signal scintillation patches recorded by the Chungli software-defined GPS receiver from GPS satellites #8 (**upper panel**), #21 (**middle panel**), and #27 (**lower panel**). Three data segments are from 13:29 to 13:44 UT, 14:29 to 14:49 UT, and 13:44 to 14:19 UT on 26 October 2021, for GPS satellites #8, #21, and #27 signal observations, respectively. could indicate that the irregularities at higher latitudes are the latitudinal mapping-out facts from lower latitudes, and that there are stronger irregularities around F-layer peaks because of greater conductivities. More evidence of the longitudinal extent of plasma irregularities is also shown by the two-dimensional \(VS_{4}\) maps derived from the CWB GPS data archive. As shown in Figure 5, the significant scintillation event that happened on 26 October 2021, was also observed by the CWB GPS receiving network. The series of \(VS_{4}\) maps shows plasma irregularities distributed with a stronger intensity at lower latitudes and when moving eastward. This indicates that the FS7/COSMIC2 could provide ionospheric irregularity and scintillation observations scanning in different limb-viewing, i.e., near-vertical, directions, and more than one hundred and thirty ground-based GPS receivers operated by the CWB could do it in horizontal directions. We note that the ground-based CWB GPS signal observations have a sampling rate of 1 Hz, which is higher than the possible maximum Fresnel frequencies \(f_{\rm F}\) of approximately 0.66 Hz and can thus complete the scintillation index \(S_{4}\) determination. However, the 1-Hz sampling rate is not enough for irregularity spectral index determination, which needs the rate to be approximately one order larger than its Fresnel frequency. In practice, spectrum analyses applied to the high-sampling, software-defined GPS receiver measurements conform to a power-law variation \(f^{1-P}\) of plasma irregularity PSD with frequency.