Global GNSS-RO Electron Density in the Lower Ionosphere

Lack of instrument sensitivity to low electron density (N-e) concentration makes it difficult to measure sharp N-e vertical gradients (four orders of magnitude over 30 km) in the D/E-region. A robust algorithm is developed to retrieve global D/E-region N-e from the high-rate GNSS radio occultation (RO) data, to improve spatiotemporal coverage using recent SmallSat/CubeSat constellations. The new algorithm removes F-region contributions in the RO excess phase profile by fitting a linear function to the data below the D-region. The new GNSS-RO observations reveal many interesting features in the diurnal, seasonal, solar-cycle, and magnetic-field-dependent variations in the N-e morphology. While the D/E-region N-e is a function of solar zenith angle (chi), it exhibits strong latitudinal variations for the same chi with a distribution asymmetric about noon. In addition, large longitudinal variations are observed along the same magnetic field pitch angle. The summer midlatitude N-e and sporadic E (E-s) show a distribution similar to each other. The distribution of auroral electron precipitation correlates better with the pitch angle from the magnetosphere than from one at 100 km. Finally, a new TEC retrieval technique is developed for the high-rate RO data with a top reaching at least 120 km. For better characterization of the E- to F-transition in N-e and more accurate TEC retrievals, it is recommended to have all GNSS-RO acquisition routinely up to 220 km.