Geolocation of the ionospheric irregularities in the equatorial F layer by back propagation of COSMIC-2 radio occultation signals

Plasma irregularities in the ionosphere induce scintillation of radio signals. Radio occultation (RO) observations of the Global Navigation Satellite Systems (GNSS) signals from low Earth orbit (LEO) allow monitoring of the ionospheric scintillation. Under certain conditions, it is possible to localize (geolocate) plasma irregularities along the line-of-sight between the GNSS and LEO satellites. While several techniques have been considered for the localization, in this study we use the back propagation (BP) of complex RO signals (phase and amplitude) measured at a high rate (HR), 50–100 Hz. Our method is based on a numerical solution of the wave equation, originally proposed for geolocation in 2002, with some modifications. We consider theoretical aspects of the BP technique, including assumptions, approximations and limitations, and perform numerical modeling of radio wave propagation. We investigate geolocation by BP for two regions with aligned and mis-aligned irregularities and explain multi-valued geolocations. We focus on the equatorial F region, consistent with the COSMIC-2 observation sampling and use the IGRF-13 model of the Earth's magnetic field to define the orientation of plasma irregularities. We use our method for processing of COSMIC-2 HR scintillation data collected from the precise orbit determination antennas for 2 years: 2021 and 2023 (years with low and high solar activity). The results, represented by gridded monthly maps of geolocations, show clear seasonal and interannual variations. Additionally, we present comparison of the geolocations obtained independently from L1 and L2 signals for a 2-month period.