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 a sharp N_e vertical gradient (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 take advantage of improved spatiotemporal coverage from recent SmallSat/CubeSat constellations. The new algorithm removes F-region contributions in the excess phase profile with a linear function fit to the data, before it is input for the D/E-region N_e retrieval. The N_e morphology from GNSS-RO revealed many interesting features in the diurnal, seasonal, solar-cycle, and magnetic-field-dependent variations. While the D/E-region N_e is a strong function of solar zenith angle (c), it exhibits strong latitudinal variations for the same c with a distribution asymmetric about noon. Large longitudinal variations are observed along the same magnetic field pitch angle. The summer-midlatitude N_e and sporadic-E (E_s) distributions resemble closely each other. The distribution of auroral electron precipitation correlates better with the pitch angle from the magnetosphere than from 100 km. In addition to the D/E-region N_e, a new TEC retrieval technique was developed using 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.