Improvement of the Ionospheric Radio Occultation Retrievals by Means of Accurate Global Ionospheric Maps

Abstract

The Abel inversion method is the classic approach to retrieve electron density profiles from radio occultation measurements, assuming that the electron density is only dependent upon height. In the early 2000s, the total electron content (TEC)‐aided inversion method was introduced, where horizontal gradients in the electron density distribution are taken into account by using information from an external model of total electron content (TEC). We show how the quality of the chosen external ionospheric model influences the radio occultation retrievals. Using a two‐layer TEC model, the 68% percentiles of the global error distributions of retrieved critical frequency and bottomside ionospheric electron content for 2014 improve more than 30%, with regard to the classic Abel inversion results. In a well‐sounded region like Europe, this improvement raises to about 40–45%, while 10% improvement is achieved using two‐layer maps with regard to the retrievals using single‐layer TEC maps. We point out that using the TEC to describe the horizontal gradient incurs an implicit mismodeling that, until now, has not been taken into account. A new technique is presented that, thanks to TEC modeling by means of two layers, can assess the impacts of such mismodeling. From this technique, a method to select the most accurate radio occultation retrievals is applied to demonstrate that the resulting errors in the retrieved critical frequencies for the European region are smaller than 7% in nearly 82% of the cases, very similar to the relative difference between measurements of two nearby ionosondes.

Summer Nighttime Anomalies of Ionospheric Electron Content at Midlatitudes: Comparing Years of Low and High Solar Activities Using Observations and Tidal/Planetary Wave Features

“…The SM is a TEC-aided inversion technique that takes into account the horizontal density gradients by means of external TEC measurements provided by accurate global ionospheric maps (GIMs). The SM improves the accuracy of the ED profiles by more than 30-40% with regard to retrievals from the classic Abel inversion based on the spherical symmetry assumption [29], especially in the bottom-side ionosphere region. In fact, the SM can achieve F2-layer peak electron density values with an accuracy similar to the collocated ionosonde measurements [29].…”

“…The SM improves the accuracy of the ED profiles by more than 30-40% with regard to retrievals from the classic Abel inversion based on the spherical symmetry assumption [29], especially in the bottom-side ionosphere region. In fact, the SM can achieve F2-layer peak electron density values with an accuracy similar to the collocated ionosonde measurements [29]. In the SM, the ED at a given longitude λ, latitude φ and height h, N e (λ, φ, h), is expressed as the product of two functions: a shape function, S(h), representing the vertical variation in ED, and TEC(λ, φ), the external TEC measurements used to describe the horizontal ED variations in the region where RO measurements have been obtained.…”

“…The methodology used to determine the specific contribution of the ionospheric region to the ground TEC at a given location using RO measurements was developed and tested in several previous studies [25,26,28,29]. The main aspects of this method, which follows two steps, are highlighted below.…”

“…In this study, the TEC measurements are derived from the GIMs provided by the International GNSS Service, specifically the rapid GIM products available at https://cddis.nasa.gov/archive/gnss/products/ionex/, accessed on 7 February 2022. Then, using the TEC values from the GIMs, the solution for the S(h) profile is retrieved from each RO following the procedure described in [26,29].…”

“…In addition, the ED profiles used in previous studies were products from the F3C Data Analysis and Archive Center, which were based on the assumption of spherical symmetry in the ED distribution. However, more precise methods of deriving the ED profiles from ROs that do not rely on a spherical symmetry assumption are currently available [26,29].…”