A comparison of FORMOSAT-3/COSMIC radio occultation and ionosonde measurements in sporadic E detection over mid- and low-latitude regions

Abstract

The investigation of sporadic E or Es layers typically relies on ground-based or satellite data. This study compares the Es layers recorded in ionograms with those detected using GNSS L1 signal-to-noise ratio data from FORMOSAT-3/COSMIC radio occultation at mid and low latitudes. GPS radio occultation measurements of Es layers, during an 11-year time span of 2007–2017, within a 2° latitude × 5° longitude grid around each ionosonde site are compared to the Es recordings of the ionosonde. By comparing multi-year radio occultation data with recordings from six ionosonde stations at mid and low latitudes, it was discovered that at least 20% of the Es layer detection results between each ionosonde and its crossing GPS radio occultation measurements did not agree. The results show that the agreement between the two methods in Es detection is highly dependent on the season and local time. This study suggests that Es layer recordings from ground-based ionosonde observations have the best agreement with the Es layers detected by radio occultation data during daytime and local summers. The difference in the Es detection mechanisms between the two methods can explain the inconsistency between Es events measured by these two methods. The detection of Es layers in ionograms relies on the high plasma concentration in the E region, whereas signal scintillations caused by a large vertical gradient of the plasma density in the E region are considered a sign of Es occurrence in satellite techniques.

Improved models for estimating sporadic-E intensity from GNSS radio occultation measurements

“…In addition, both models perform better for daytime measurements with slight overpredictions of weak nighttime intensities. The increased daytime performance is consistent with a recent study by Sobhkhiz-Miandehi et al (2023), which found a stronger agreement between ionosonde and RO measurements during the day with low solar zenith angles.…”

“…In addition, small-scale structures and turbulence caused by Kelvin-Helmholtz instabilities (Bernhardt, 2002;Hysell et al, 2016) can create significant changes in GNSS-RO observations [also discussed in Emmons et al (2022)]. Finally, a recent comparison of ionosonde with RO observations found the best agreement during the day of local summer and also noted that certain ionosonde sites systematically showed stronger agreement with RO E s observations (Sobhkhiz-Miandehi et al, 2023). This site-to-site and temporal variation is the result of site-dependent ionosonde hardware, background ionosphere fluctuations, and perhaps a geographic and temporal dependence of E s spatial characteristics and small-scale structures, which should be examined in more detail in future studies.…”

A Simplified Method of True Height Analysis to Estimate the Real Height of Sporadic E Layers

“…The use of h′Es instead of hEs has been applied in many sporadic E studies. Among others, these include: (a) several Es diurnal and seasonal variability investigations (e.g., see a recent paper by Haldoupis et al (2023) and more references therein), (b) "Height Time Intensity" (HTI) studies of tidal periodicities and altitude descent of sporadic E layers (e.g., see Haldoupis et al, 2006; and a review paper by Haldoupis (2012)), (c) comparison studies of Es altitudes measured independently by satellite GPS (Global Positioning System) radio occultation (RO) methods and ionosondes (e.g., see Arras, 2010;Ellis et al, 2024;Gan et al, 2022;Resende et al, 2018;Sobhkhiz-Miandehi et al, 2023, and several more references), and (d) the computation of foμEs, which is an ionogram-based parameter for measuring Es metal ion densities and thus the associated layer intensities (Ellis et al, 2024;Emmons et al, 2023;Haldoupis, 2019;Haldoupis et al, 2020). Therefore, the convenient adoption of hEs being equal to h′Es should have caused inaccuracies in past studies.…”