A near real-time phenomenological model of ionospheric electron density based on GPS radio occultation data

A numerical and phenomenological model of global ionospheric electron density (N(e)) is investigated. The three-dimensional N(e) model has been named the Taiwan Ionospheric Model (TWIM) and constructed from monthly weighted and hourly vertical N(e) profiles retrieved from FormoSat3/COSMIC GPS radio occultation measurements. The TWIM exhibits vertically fitted Chapman layers, with distinct F2, F1, E, and D layers, and surface spherical harmonics approaches for the fitted Chapman layer parameters including peak density, peak density height, and scale height. These results are useful in investigation of N(e)ar-Earth space and large-scale N(e) distribution with diurnal and seasonal variations, along with geographic features such as the equatorial anomaly (EA). This paper also investigates the diurnal and seasonal variations of EA within different ionospheric layers and specifically attempts to account for the latitudinal and longitudinal structures caused by atmospheric tides.