**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 ±40◦, i.e., approximately seven RO observations per hour in the Taiwan–Philippines sector (between 20◦ ± 15◦N and 120◦ ± 15◦E geographic coordinates). From the FS7/COSMIC2 RO measurements on 26 October 2021, the observed or retrieved limb-viewing SNR, *S*4, and *Ne* 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 *Ne* 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 ~10◦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 ~100 km only and around the F-layer *Ne* peaks. This

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*<sup>4</sup> 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*<sup>4</sup> 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 *fF* of approximately 0.66 Hz and can thus complete the scintillation index *S*<sup>4</sup> 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* <sup>1</sup>−*<sup>p</sup>* of plasma irregularity PSD with frequency. Meanwhile, the derived spectrum break frequencies *fB* are more or less 0.1 Hz, which are all less than the corresponding Fresnel frequencies *fF* 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 *fB* 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 *fs* 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 *foI*, 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 *foI* (*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 *foI* (*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 *Ne* values were approximately the same but the ionospheric *Ne* 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 *Ne*s and *foF2s* but was strong enough to increase the *Ne*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.

**Figure 7.** Time variations of virtual ranges (referring to the left *y*-axis) of ionospheric echoes at different sounding frequencies *fs* of 1.72 (dark blue), 3.08 (light blue), 4.72 (green), 5.65 (orange), and 7.15 MHz (red) from the Hualien ionograms recorded on 26 October 2021. Another temporal profile of scaled *foI* is also shown and referring to the right text. Note that the spread-F features were happened between 13:19~15:04 UT, i.e., 21:19~23:04 LT in Taiwan.

#### **5. Conclusions**

Ionospheric irregularities and scintillations and their associated motions in the Taiwan– Philippine sector have been observed and specified by the FS7/COSMIC2 data, the VIPIR located at Hualien, Taiwan, 133 ground-based GPS receivers located in Taiwan and the surrounding islands, and two high-sampling, software-defined GPS receivers. The integrated system has the potential to provide scintillation intensities, zonal drift measurements, and even three-dimensional irregularity structures. We also suggest 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. In the future, we expect to identify the ionospheric conditions in the Taiwan–Philippines sector that led to the onset of plasma/R–T instabilities and to forecast the growth and the timing/duration of each instability. An examination of these instabilities will form the basis for the forecast of the timing and severity of (GNSS) radio scintillations.

**Author Contributions:** Conceptualization, L.-C.T., S.-Y.S. and C.-H.L.; Data curation, L.-C.T., J.-X.L. and T.B.; Formal analysis, L.-C.T.; Funding acquisition, L.-C.T. and S.-Y.S.; Investigation, L.-C.T., J.-X.L. and T.B.; Methodology, L.-C.T., S.-Y.S. and C.-H.L.; Project administration, L.-C.T. and S.-Y.S.; Resources, L.-C.T., J.-X.L. and T.B.; Software, L.-C.T. and J.-X.L.; Supervision, L.-C.T., S.-Y.S. and C.-H.L.; Validation, L.-C.T.; Visualization, L.-C.T.; Writing—original draft, L.-C.T.; Writing—review & editing, S.-Y.S., T.B. and C.-H.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Office of Naval Research, U.S.A. [grant number N00014- 21-1-2486] and in part by the Ministry of Science and Technology, Taiwan [grant number MOST 110-2119-M-008-003].

**Data Availability Statement:** The FS7/COSMIC2 "ionPhs" data can be downloaded from the COS-MIC Data Analysis and Archive Center (CDAAC, http://cdaac-www.cosmic.ucar.edu/ (accessed on 1 December 2021)) and the Taiwan Analysis Center for COSMIC (TACC, http://tacc.cwb.gov.tw/ cdaac/ (accessed on 1 December 2021)).

**Acknowledgments:** The authors would also like to thank UCAR's CDAAC, NSPO Satellite Operations Control Center (SOCC), and the CWB, Taiwan, for providing FS7/COSMIC2 data and GPS data.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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