IMPACT OF IONOSPHERIC HORIZONTAL ASYMMETRY ON ELECTRON DENSITY PROFILES DERIVED BY GNSS RADIO OCCULTATION

The ‘Onion-peeling’ algorithm is a very common technique used to invert Radio Occultation (RO) data in the ionosphere. Because of the implicit assumption of spherical symmetry for the electron density distribution in the ionosphere, the standard Onion-peeling algorithm could give erroneous concentration values in the retrieved electron density profile. In particular, this happens when strong horizontal ionospheric electron density gradients are present, like for example in the Equatorial Ionization Anomaly (EIA) region during high solar activity periods. In this work, using simulated RO TEC data computed by means of the NeQuick2 ionospheric electron density model and ideal RO geometries, we tried to formulate and evaluate an asymmetry level indicator for quasi-horizontal radio occultation observations. This asymmetry index is based on the electron density variation that a ray may experience along its propagation path (satellite to satellite link) in a RO event. Our previous qualitative assessment based on ideal simulations of RO events shows very high correlation between our asymmetry index and Onion-peeling retrieval errors (Shaikh M.M. et al 2013): errors produced by Onion-peeling in the retrieval of NmF2 and VTEC are larger at the geographical locations where our asymmetry index indicates high asymmetry in the ionosphere. In this contribution, an analysis of the asymmetry index has been carried out for the first time using real radio occultation geometries taken from COSMIC mission. This has been done for COSMIC events for which, considering the same RO geometry, simulated Limb-TEC (LTEC) under NeQuick2 background were quite close to the real LTEC observations (providing ‘quasi’ co-located vertical profiles of electron density after inversion). On the basis of the outcomes of our work, for a given geometry of a real RO event and using a suitable ionospheric model, we will try to investigate the possibility to predict ionospheric asymmetry expected for the particular RO geometry considered. We could also try to evaluate, in advance, its impact on the inverted electron density profile, providing an indication of the expected product quality, if standard Onion-peelingalgorithm will be adopted to invert the observables. Results presented in this paper are initial outcomes based on our asymmetry evaluation algorithm.