Characterization of the topside ionosphere using Swarm data - the TEGIX and NeGIX indices

Abstract

A topic of current interest in space weather research is the study of ionospheric plasma perturbations that may cause severe amplitude and phase scintillations of trans-ionospheric radio signals. From a practical point of view, the detection and monitoring of small to medium scale irregularities in the ionospheric plasma is also crucial to mitigate their effects on the functionality of modern radio systems for telecommunication, navigation and remote sensing. Ground-based GNSS methods allow monitoring and characterizing the ionosphere, but are limited to regions where good coverage by satellite-receiver links is available. Although with constraints to cover the lower ionospheric layers with even the lowest orbits, space-based observations provide more effective solutions to global monitoring, especially over oceans and regions with limited access. In this context, as part of the Swarm DISC project “Monitoring of Ionospheric Gradients at Swarm (MIGRAS)”, we have profited from the close orbits and synchronization of the ESA EO Swarm satellites Alpha (A) and Charlie (C), and have developed two new products focusing on the monitoring of small- to mid-scale plasma density irregularities with horizontal spatial scales on the order of about 100 km. The first product, the electron density (Ne) Gradient Ionospheric indeX (NeGIX), estimates spatial Ne gradients using in-situ Langmuir probe measurements; and the second product, the Total Electron Content (TEC) Gradient Ionospheric indeX (TEGIX), estimates spatial TEC gradients using GNSS Precise Orbit Determination (POD) data from Swarm. We provide a comprehensive analysis of the capability of these two novel Swarm data products to characterize the perturbation state of the ionosphere at different geographic locations and conditions of geomagnetic activity. Our analysis covers the entire period of available Swarm observations that quantitively describe expected signatures of ionospheric variability. The analysis concentrates on events of perturbed geomagnetic conditions. We also compare the performance of NeGIX and TEGIX with existing ground-based indices (e.g. GIX) and Swarm products (e.g. IPIR).