Introduction to the special issue on Exploring the terrestrial and space weather using an operational radio occultation satellite constellation - A FORMOSAT-7/COSMIC-2 Special Issue after 1-year on orbit

Launched on 25 June 2019, the Formosa Satellite Mission 7 and Constellation Observing System for Meteorology, Ionosphere, and Climate and Formosa Satellite Mission 2 (FORMOSAT-7/COSMIC-2, F7/C2) is the first operational global navigation satellite system (GNSS) radio occultation (RO) mission for the weather and space weather forecast that is focusing on mid-latitude and tropical regions. F7/C2 is a bi-lateral satellite constellation mission between Taiwan and the United States. Launched to the low inclination orbit of 24 degrees, the constellation consists of six satellites equipped with GNSS receivers, the advanced Tri-GNSS Radio-occultation System (TGRS), capable of receiving GPS, GLONASS, and Galileo signals. Alongside TGRS, there are two additional space weather-related instruments, the Ion Velocity Meter (or IVM) and the Radio Frequency Beacon (or RF Beacon) provided by U. S. Air Force. IVM observes in-situ ion species, densities, temperatures, and velocities; RF Beacon transmits beacon signals with three frequencies (400, 965, and 2200 MHz) to ground-based receivers for the derivation of the ionosphere electron content (IEC) and SNR of the radio signals. Both IVM and RF Beacon observations provide information on plasma irregularities of the ionosphere.