Search

Several models for estimating sporadic-E intensity from Global Navigation Satellite System (GNSS) radio occultation (RO) observation have previously been developed using a single perturbation or intensity parameter, such as phase-based total electron content (TEC) or the amplitude-based S4 index. Here, we outline two new models that use a combination of phase and amplitude parameters for the L1 and L2 signals. These models show a significant improvement over the baseline models used for comparison. Furthermore, the GNSS-RO parameters are compared with several different ionosonde intensity parameters including the direct foEs and fbEs measurements along with the metallic-ion based fo^Es and fb^Es parameters which account for the background E-region density. Interestingly, the phase-based o^ scintillation index shows the strongest correlation to foEs and fbEs while amplitude-based S4 shows the strongest correlation to fo^Es and fb^Es. While the metallic-ion based fo^Es and fb^Es parameters are physically ideal for GNSS-RO observations, we show difficulties in practical implementation due to the reliance on a background E-region density estimate using a model such as the International Reference Ionosphere (IRI). Ultimately, we provide two improved sporadic-E intensity models that can be used for future GNSS-RO based studies along with a recommendation to compare against the ionosonde-based foEs parameter. [ABSTRACT FROM AUTHOR]

Global Navigation Satellite System (GNSS) Radio Occultation (RO) has shown great promise for monitoring sporadic-E layers. However, extracting sporadic- E information from RO signals remains a difficult task due to the many unknown parameters such as length, intensity, vertical thickness, and small-scale structure or turbulence. To further our understanding of sporadic-E turbulence, we investigate the power spectra of sporadic-E layers during Kelvin-Helmholtz billow formation. Additionally, RO signals traversing the billows are simulated to analyze the impact on both amplitude and phase. From this, we find that the horizontal power spectrum is generally steeper in sporadic-E layers without billow formation, and the spectrum flattens as small-scale structures develop. Additionally, the typical "U"-shaped RO amplitude profiles produced by sporadic- E layers become asymmetric and less defined as the billows form and progress, showing that a single sporadic-E layer can produce a variety of RO signatures as it evolves over time. Ultimately, these results provide valuable insight for both modeling RO signals through sporadic-E layers and inverting RO data to extract information about the layers. [ABSTRACT FROM AUTHOR]

GNSS-LEO radio links from Precise Orbital Determination (POD) and Radio Occultation (RO) antennas have been used increasingly in characterizing the global 3D distribution and variability of ionospheric electron density (Ne). In this study, we developed an optimal estimation (OE) method to retrieve Ne profiles from the slant total electron content (hTEC) measurements acquired by the GNSS-POD links at negative elevation angles (ε < 0°). Although both OE and onion-peeling (OP) methods use the Abel weighting function in the Ne inversion, they are significantly different in terms of performance in the lower ionosphere. The new OE results can overcome the large Ne oscillations, sometimes negative values, seen in the OP retrievals in the E-region ionosphere. In the companion paper in this Special Issue, the HmF2 and NmF2 from the OE retrieval are validated against ground-based ionosondes and radar observations, showing generally good agreements in NmF2 from all sites. Nighttime hmF2 measurements tend to agree better than the daytime when the ionosonde heights tend to be slightly lower. The OE algorithm has been applied to all GNSS-POD data acquired from the COSMIC-1 (2006–2019), COSMIC-2 (2019–present), and Spire (2019–present) constellations, showing a consistent ionospheric Ne morphology. The unprecedented spatiotemporal sampling of the ionosphere from these constellations now allows a detailed analysis of the frequency–wavenumber spectra for the Ne variability at different heights. In the lower ionosphere (~150 km), we found significant spectral power in DE1, DW6, DW4, SW5, and SE4 wave components, in addition to well-known DW1, SW2, and DE3 waves. In the upper ionosphere (~450 km), additional wave components are still present, including DE4, DW4, DW6, SE4, and SW4. The co-existence of eastward- and westward-propagating wave4 components implies the presence of a stationary wave4 (SPW4), as suggested by other earlier studies. Further improvements to the OE method are proposed, including a tomographic inversion technique that leverages the asymmetric sampling about the tangent point associated with GNSS-LEO links. [ABSTRACT FROM AUTHOR]