The Impact of Spherical Symmetry Assumption on Radio Occultation Data Inversion in the Ionosphere: An Assessment Study

‘Onion-peeling’ is a very common technique used to invert Radio Occultation (RO) data in the ionosphere. Because of the implicit 12 assumption of spherical symmetry for the electron density (N(e)) distribution in the ionosphere, the standard Onion-peeling algorithm 13 could give erroneous concentration values in the retrieved electron density profile. In particular, this happens when strong horizontal 14 ionospheric electron density gradients are present, like for example in the Equatorial Ionization Anomaly (EIA) region during high solar 15 activity periods. In this work, using simulated RO Total Electron Content (TEC) data computed by means of the NeQuick2 ionospheric 16 electron density model and ideal RO geometries, we tried to formulate and evaluate an asymmetry level index for quasi-horizontal TEC 17 observations. The asymmetry index is based on the electron density variation that a signal may experience along its path (satellite to 18 satellite link) in a RO event and is strictly dependent on the occultation geometry (e.g. azimuth of the occultation plane). A very good 19 correlation has been found between the asymmetry index and errors related to the inversion products, in particular those concerning the 20 peak electron densityNmF2estimate and the Vertical TEC (VTEC) evaluation