# Global GNSS-RO Electron Density in the Lower Ionosphere
###### Abstract  
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 (x), it exhibits strong latitudinal variations for the same x 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. GNSS radio occultation; \(D\)/\(E\)-region ionosphere; electron density; total electron content; sporadic-E; satellite constellation; auroral electron precipitation; \(E\)- to \(F\)-transition; global electric circuit +
Footnote †: journal: Remote Sensing  
1577. [[https://doi.org/10.3390/rs14071577](https://doi.org/10.3390/rs14071577)]([https://doi.org/10.3390/rs14071577](https://doi.org/10.3390/rs14071577))  
13 February 2022  
Accepted: 12 March 2022  
Published: 24 March 2022  
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**Copyright** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/](https://creativecommons.org/licenses/by/)]([https://creativecommons.org/licenses/by/](https://creativecommons.org/licenses/by/)))  
**1. Introduction**  
The \(D\)- (60-90 km) and \(E\)-region (90-150 km) ionosphere play important roles in radio wave propagation because of their effective absorption of short-wave signals (e.g., high-frequency, HF, radio) [1] and reflection of long-wave communication and navigation radios (e.g., very low frequency, or VLF) [2]. It overlaps with the neutral atmosphere, known as the mesosphere, and is associated with large variability. The atmospheric temperature decreases with height in the mesosphere (or the \(D\)-region ionosphere), and the atmospheric thermal and density structures can be significantly perturbed by the atmospheric forcings from below (e.g., tides, planetary and gravity waves) [3]. The \(D\)-region is arguably one of the most critical interface layers between the lower atmosphere and the ionosphere. It is the layer where the electrical coupling takes place between the ionosphere and the phenomena such as sprites, elves, and blue jets from tropospheric thunderstorms occur. The \(D\)/\(E\)-region conductivity is important to complete a so-called global electric circuit (GEC), an electrical coupling between the ionosphere and the planetary surface with a DC current driven by tropospheric thunderstorms [4, 5].