Impact of Ionospheric Horizontal Asymmetry on Electron Density Profiles Derived by GNSS Radio Occultation

摘要

The ‘Onion-peeling’ algorithm is a very commontechnique used to invert Radio Occultation (RO) data inthe ionosphere. Because of the implicit assumption ofspherical symmetry for the electron density distributionin the ionosphere, the standard Onion-peeling algorithmcould give erroneous concentration values in theretrieved electron density profile. In particular, thishappens when strong horizontal ionospheric electrondensity gradients are present, like for example in theEquatorial Ionization Anomaly (EIA) region during highsolar activity periods. In this work, using simulated ROTEC data computed by means of the NeQuick2ionospheric electron density model and ideal ROgeometries, we tried to formulate and evaluate anasymmetry level indicator for quasi-horizontal radiooccultation observations. This asymmetry index is basedon the electron density variation that a ray mayexperience along its propagation path (satellite tosatellite link) in a RO event.Our previous qualitative assessment based on idealsimulations of RO events shows very high correlationbetween our asymmetry index and Onion-peelingretrieval errors (Shaikh M.M. et al 2013): errors producedby Onion-peeling in the retrieval of NmF2 and VTEC arelarger at the geographical locations where ourasymmetry index indicates high asymmetry in theionosphere. In this contribution, an analysis of theasymmetry index has been carried out for the first timeusing real radio occultation geometries taken fromCOSMIC mission. This has been done for COSMIC eventsfor which, considering the same RO geometry, simulatedLimb-TEC (LTEC) under NeQuick2 background were quiteclose to the real LTEC observations (providing ‘quasi’ colocatedvertical profiles of electron density afterinversion). On the basis of the outcomes of our work, fora given geometry of a real RO event and using a suitableionospheric model, we will try to investigate thepossibility to predict ionospheric asymmetry expected forthe particular RO geometry considered. We could also tryto evaluate, in advance, its impact on the invertedelectron density profile, providing an indication of theexpected product quality, if standard Onion-peeling algorithm will be adopted to invert the observables.Results presented in this paper are initial outcomesbased on our asymmetry evaluation algorithm.