## 4 Discussion
Meanwhile, the derived spectrum break frequencies \(f_{B}\) are more or less 0.1 Hz, which are all less than the corresponding Fresnel frequencies \(f_{\rm F}\) of approximately 0.2 Hz for a frozen ionosphere. This indicates that the targeted plasma irregularities moved northward too and had positive velocity components along the IPP-tracing directions to decrease the relative radio-scanning speed and the experimental Fresnel frequencies obtained by the derived \(f_{B}\) values. In sum, integrating the observations from the FS7/COSMIC2, the CWB GPS receiving network, and two software-defined GPS receivers located in Taiwan, the experimental results show that the targeted plasma irregularities moved eastward and northward. Furthermore, the smaller the irregularity scale, the higher the spectral index and the stronger the scintillation intensity at lower latitudes on the aimed irregularity feature. As described, ESF features usually accompany equatorial plasma bubbles and can also be observed and scaled from ionograms, as shown in Figure 2. Figure 7 shows the time variations of the virtual ranges of ionospheric echoes at different sounding frequencies \(f\)s of 1.72, 3.08, 4.72, 5.65, and 7.15 MHz from the Hualien ionograms recorded on 26 October 2021. Figure 7 also shows the corresponding temporal profile of scaled _fol_, which is approximately equal to _foF2_ without spread-F and/or sporadic E features but higher than _foF2_ with spread-F features. We note that the sunset time of the day was approximately 18:30 LT, i.e., 10:30 UT, at Hualien, Taiwan, and at a 300 km altitude. As shown in Figure 7, the spread-F features happened and were observed between 13:19-15:04 UT, i.e., 21:19-23:04 LT. Furthermore, after 15:04 UT, i.e., the end of the ESF event, the _fol (foF2)_ values decreased, and the virtual heights of the fixed-frequency ionospheric echoes increased as usual facts of nighttime ionograms. Before 13:19 UT, i.e., the start of the ESF event, the _fol (foF2)_ values show more or less a flat portion, but the virtual heights of fixed-frequency ionospheric echoes decreased except for those at a sounding frequency of 1.72 MHz, which are actually one-hop and two-hop sporadic E echoes and thus almost invariant in virtual heights. This indicates that after sunset and before the ESF event, the peak ionospheric \(N_{e}\) values were approximately the same but the ionospheric \(N_{e}\) values at the bottom side ionosphere were increased. This could be due to a strong PRE, i.e., eastward electric field enhancement that happened near or after sunset on the day and produced an equatorial fountain effect. As a result, the equatorial ionosphere moved upward, developed steep density gradients and large-scale plasma depletions in the bottom side F-region and became unstable, triggering the R-T instability. Meanwhile, ionospheric plasma moved out along the geomagnetic field line and from the magnetic equator to higher latitudes. In this study, such an equatorial fountain effect was not strong enough to increase the peak \(N_{e}\)s and _foF2s_ but was strong enough to increase the \(N_{e}\)s at the bottom side ionosphere in the low-latitude region, e.g., the Taiwan area. We found that a post-sunset decrement of the virtual heights of fixed-frequency ionospheric echoes could be a good precursor for post-sunset scintillation and ESF events.