Validation results of the Swarm ionospheric indices NeGIX and TEGIX

Abstract

The successful completion of the Swarm DISC project “Monitoring of Ionospheric Gradients at Swarm (MIGRAS)” has led to the development of two new data products – the electron density gradient ionosphere index (NeGIX) and the total electron content gradient ionosphere index (TEGIX) –, for characterizing the perturbation degree of the topside ionosphere. NeGIX estimates spatial electron density gradients using L1 Langmuir probe measurements, and TEGIX estimates spatial TEC gradients using L2 GNSS Precise Orbit Determination (POD) data of Swarm. Unlike existing Swarm ionospheric indices based on single satellite measurements (e.g. the ionospheric bubble index IBI, or the ionospheric plasma irregularities index IPIR), the two new products benefit from the combination of spatially- and temporally-related measurements from Swarm satellites Alpha (A) and Charlie (C) to estimate ionospheric gradients in spatial scales of up to 200 km. This provides a better insight into ionospheric irregularities and structures not only along the satellites’ orbits, but also in the zonal direction. In this work, we present the results of a validation study of the Swarm data products NeGIX and TEGIX based on more than 10 years of Swarm data availability. The long-term data analysis enables us to examine the applicability of the newly-developed indices to characterize time- (seasonal, diurnal) and spatial-dependent ionospheric features such as the equatorial crests, mid-latitude trough, or the South Atlantic Anomaly. On the other hand, analysis of concrete events of severe geomagnetic activity, such as the St. Patrick’s Day storm of 2015 or the geomagnetic storm on May 10th and 11th of 2024, allows us to evaluate their applicability to serve as reliable proxies for the identification of perturbations that can threat the availability and functionality of radio systems for telecommunication, navigation and remote sensing. We compare the performance of the MIGRAS products versus other existing ionospheric indices (e.g. the ground-based GIX, IBI or IPIR) that describe small- to mid-scale irregularities, such as ionospheric plasma bubbles. Beyond the scientific and applicable purposes of this work, our validation study aims also at endorsing the operational availability of NeGIX and TEGIX among the unique and broad spectrum of Swarm data services for space weather research.