*Technical Note* **Multi-Station and Multi-Instrument Observations of F-Region Irregularities in the Taiwan–Philippines Sector**

**Lung-Chih Tsai 1,2,\*, Shin-Yi Su 2, Jun-Xian Lv 1, Terry Bullett <sup>3</sup> and Chao-Han Liu <sup>4</sup>**


**Abstract:** In this study, a multi-station and multi-instrument system, organized and proposed for ionospheric scintillation and equatorial spread-F (ESF) specification and their associated motions in the Taiwan–Philippines sector, is outlined. The issues related to the scintillation and ESF event observed on 26 October 2021, at magnetic quiet conditions are presented and discussed. We first indicate the existence of a plasma bubble in the Taiwan–Philippines sector by using the FormoSat-7/Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (FS7/COSMIC2) GPS/GLONASS radio occultation observations. We verify the latitudinal extent of the tracked plasma bubble using the recorded ionograms from the Vertical Incidence Pulsed Ionospheric Radar located at Hualien, Taiwan. We further discuss the spatial and temporal variabilities of two-dimensional vertical scintillation index *VS*<sup>4</sup> maps based on the simultaneous GPS L1-band signal measurements from 133 ground-based receivers located in Taiwan and the surrounding islands. We also operate two high-sampling, software-defined GPS receivers and characterize the targeted plasma irregularities by carrying out spectrum analyses of the received signal. As a result, the derived plasma irregularities moved eastward and northward. Furthermore, the smaller the irregularity scale, the higher the spectral index and the stronger the scintillation intensity were at lower latitudes on the aimed irregularity feature.

**Keywords:** equatorial and low-latitude ionosphere; ionospheric irregularity; scintillation; radio occultation observation; COSMIC; ionogram; GPS/GNSS
