# Advances in Ionospheric Space Weather by Using FORMOSAT-7/COSMIC-2 GNSS Radio Occultations
## 1 Introduction  
Radio occultation (RO) technique detects the changes to a radio signal when it passes through the atmosphere of a planet, and extracts information about the atmosphere based on the detected changes [1]. Though initially used for probing planetary atmospheres, the RO technique has been recognized as one of the effective diagnostic tools for remotely sensing the state of the atmosphere and ionosphere, especially as the global positioning system (GPS) signals were made accessible to the scientific community. The changesin the relative positions between a GPS satellite and a low-Earth orbit (LEO) satellite allow us to determine the vertical profiles of temperature and water vapor pressure in the atmosphere as well as the electron density in the ionosphere. In 1995, the first low-Earth orbit satellite, MicroLab-1, receiving GPS signals demonstrated the application of limb sounding to probe the Earth's atmosphere and ionosphere [2; 3; 4]. Though the method has the potential of providing uninterrupted data across the globe, until the launch of the FORMOSAT-3 Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC or F3/C), RO was limited to only a handful of attempts. The F3/C mission, a joint Taiwan-United States (US) space program, with six identical micro-satellites placed into circular orbits with 72 deg inclination at ~800 km altitude, revolutionized ionospheric research, providing three-dimensional (3D) global electron density profiles, unaffected by measurement location or observing conditions [5]. The electron density profiles are retrieved from the GPS signals received by the GPS occultation experiment (GOX), the primary payload of F3/C. Interested readers may refer to [PERSON] et al. [6] for the details of deriving electron density profiles in the ionosphere as well as temperature, pressure, and water content profiles in the atmosphere by the RO technique based on the time delay and the bending angle of the GPS signals. Launched in the beginning of the second quarter of 2006, the F3/C satellites offered unprecedented amounts of ionospheric soundings, yielding about 2000-3000 RO events per day, providing researchers worldwide with the opportunity to examine 3D ionospheric characteristics in greater detail, unlike what was possible before. The following years witnessed a large influx of quality research articles appearing in international journals, making use of the rare ionospheric sounding measurements from a unique space-based observing platform and portraying 3D electron density features and their temporal evolutions for the first time from actual measurements. There were several commendable efforts to validate the F3/C measurements by comparing with ionosonde and incoherent scatter radar observations [7; 8; 9; 10; 11; 12], other in situ measurements [13; 14], and model predictions [7; 15; 16; 17], and to develop global models of ionospheric parameters based on F3/C measurements [15; 16; 17; 18; 19]. The results also led to several investigations to mitigate the impact of spherical symmetry assumption on the Abel inversion retrieval of F3/C RO electron density profiles [20; 21; 22]. Amidst such validation studies, the F3/C observations were applied to numerous research applications such as electron density responses to 27-day solar rotation [23], extracting plasmaspheric electron content [24], ionospheric irregularities from the fluctuations of signal-to-noise ratio (SNR) and phase data [25; 26; 27; 28; 29; 30; 31; 32; 33; 34], retrieving topside scale heights [35] and equivalent ionospheric slab thickness [36; 37], diurnal, seasonal, and longitudinal variations of electron density distribution [38; 39; 40; 41; 42; 43; 44; 45; 46], new aspects of middle-latitude trough [47] as well as mid-latitude summer nighttime anomalies [48; 49; 50], ionospheric perturbations by seismic activities, severe weather systems, solar eclipses, and geomagnetic disturbances [51; 52; 53; 54; 55; 56; 57; 58], long-term ionospheric variations [59], etc. [PERSON] et al.