## 3 Results  
This section presents the multi-station and multi-instrument observations of a scintillation event that occurred in the Taiwan-Philippines sector on 26 October 2021, which was at magnetic quiet conditions, referring to Kp indexes between 0+ and 2+. Figure 1 shows the geographical geometry of the F-layer irregularity observations obtained during 13:00-15:30 UT, i.e., 21:00-23:30 LT in Taiwan, on 26 October 2021, by the FS7/COSMIC2 GPS/GLONASS RO sounding experiment. Six (#1 to #6 observations) out of fourteen RO observations were recorded and classified as scintillation observations according to their maximum limb-viewing L1-band \(S_{4}\) values larger than 0.1 [14]. Their recording times, which are the FS7/COSMIC2 LEO satellite orbiting times at the peak altitudes of RO observations, are used to identify the scintillation event period, at least from 13:54 to 14:59 UT. For each RO scintillation observation, a set of RO limb-viewing links at a 15 s sampling rate (between 150 and 450 km altitudes), which connects the occultation points to their conjugate points, is shown in coded colors of L1-band \(S_{4}\) values to present the possible projection area of ionospheric irregularities, and the trace of perigee points (or tangent points) is also shown. We note that the first three (#1, #2, and #3) RO observations are located near the magnetic equator and experience strong scintillations. The later three (#4, #5, and #6) RO observations are located in the northeast directions of the observations #1, #2, and #3 and could be their latitudinal mapping-out cases; thus, they experience weaker scintillations. In contrast, as shown in Figure 1, the peak tangent point positions of the other eight RO observations without definable scintillations are located outside of the aimed scintillation area covering observations #1 to #6. Figure 1: Geographical geometry of the F-layer irregularity observations during 13:00–15:30 UT on 26 October 2021, by the FS7/COSMIC2 GPS/GLONASS RO sounding experiment. Six out of fourteen RO observations were obtained and classified as the scintillation observations #1 to #6. Sets of limb-viewing links (at a de-sampling rate of 15 s and altitudes between 150 to 450 km) connecting the occultation points to their conjugate points are shown in coded colors by L1-band \(S_{4}\) values to present the possible projection area of ionospheric irregularities. The traces of limb-viewing tangent points are also shown in black. The peak locations of the other eight RO observations without scintillation are shown by green squares, the location of the Hualien VIPIR position is shown by a black square, and two magnetic latitude lines at dip 0\({}^{\circ}\) and 17\({}^{\circ}\) are shown in brown. ionosphere is 1 Hz and less than the possible Fresnel frequencies of plasma irregularities, and thus the derived \(S_{4}\) values should be underestimated at undersampling conditions. As shown in Figure 2, strong and moderate fluctuations occur in the L1-band SNR amplitudes of observations #1 and #4, respectively, from the occulting-side observations but not from the calibrating-side observations. It indicates that the amplitude fluctuations are induced by the ionospheric irregularities, which are located along the limb-viewing links between the occultation points and their conjugate points, as shown in Figure 1. We note that the strong amplitude fluctuations, i.e., large undersampling \(S_{4}\) values, as well as the \(N_{e}\) fluctuations from observation #1, were distributed over a wider altitude region up to the LEO orbiting altitude than those from observation #4, and the peak scintillations and \(N_{e}\) irregularities happened around the F-layer electron density peaks. This is consistent with the observations from the theoretic descriptions in the Introduction section. Equatorial spread-F (ESF) features usually accompany equatorial plasma bubbles and have been observed by numerous authors using ionosonde.