The Retrieval And Comparative Validiation Of COSMIC Radio Occultation Electron Density Profiles

GNSS radio occultation is a newly-developed technique to probe ionosphere,which has unique advantages compared with other conventional ionosphere study technique.By limb-observing,GNSS radio occultation can obtain electron density profiles from ground to LEO orbit height,which can cover globally,have high vertical resolution,24-h observation and no systemical bias.Under the spherical symmetry assumption,electron density profile could be obtained by Abel inversion from TEC series.However,this method would introduce significant error in E layer height where ionosphere horizontal gradient exists.Based on the basic ionosphere theory,this paper introduces the method and process inverting into ionosphere electron density profile and validate the COSMIC retrieval results.Generally speaking,the main objectives of this article contain:(1)The impact of solar activity on Abel inversion.Solar radiation plays an importrant role in electron ionozition.Based on the EGOPS software simulation and by setting background ionosphere as NeUoG model,we can obtain the excess phase delay data through forward modeling.Thus,the electron density profile could be retrieved using Abel transform inversion.Finally,the inversion errors are obtained by subtracting the model truth.For signle occultation event,the inversion error under different solar activity levels are analyzed.The result shows that the Abel inversion error increase with solar activity.The inversion occurs large error under the F2 layer peak height and even exceed 200%at E layer under solar maximum period.(2).The statistical validation between COSMIC-retrieved F2 layer critical frequency foF2 and ionosonde observation.Three ionosonde stations aligned on the same longitude located over Southeast Asia area are used to validate simitanneous COSMIC observations.The result shows that COSMIC retrieved results have largest error on the ionosonde station located at northern EIA crest.The differences are smaller at ionosonde station located near the southern EIA crest.And the COSMIC observation has the best agreement with ionosonde station located near the geomagnetic equator.This is mainly because of the inhomogeneous distribution of electron density in EIA crest area,which cause large inversion error of 'onion-peeling' algorithm that based on spherical symmetry assumption.Besides,we also conduct the study of the difference under different solar activity,seasonal difference and diurnal variation of two techniques.(3)Comparison between COSMIC retrieved peak parameters(NmF2 and hmF2)with IRI-2016 model predictions.We studied the global distribution characteristics in NmF2 and hmF2 obtained from COSMIC and IRI-2016 model during 1300-1500LT in solar maximum year 2014 and solar minimum year 2008.COSMIC can capature longitudal variation,hemispherical asymmetry and winter anomaly well.Comparing with COSMIC,in 2008 daytime(1300-1500LT),IRI model usually overestimate NmF2 and underestimate hmF2 over EIA area.The difference between IRI model and COSMIC peak parameters varies with local time and geomagnetic latitude.COSMIC radio occulataion data during 2008-2014 are used to study the performance of IRI-2016 model and different options to calculate NmF2 and hmF2 in IRI-2016 over China.The result shows that the performance is better in high solar activity year than low solar activity year.Besides,the correlations between hmF2 predictions and COSMIC observtions are worse than that of NmF2.Both IRI_CCIR and IRI_URSI models overestimate NmF2 at noontime,which is more evident during low solar activity year.Both CCIR hmF2 and AMTB hmF2 prediction are higher than COSMIC observations regardless of seasons during solar low year.Shubin model predictions are closely near the COSMIC observations.However,the deviations of AMTB model predictions are larger,especially in winter.(4)The impact of geomagnetic storms on the precision of COSMIC-retrieved electron density profiles.Peak parameters from 11 ionosonde stations located over geomagnetic low latitudes during kp>3+ are used to compare with that from COSMIC.The result shows that the standard deviation of relative difference in NmF2 during geomagnetic storm is 24.93%,the averaged absolute hmF2 is 20.1702km.Besides,the precision degrades with the storm level.However,generally speaking,the accuracy of COSMIC electron density profiles are good enough to be used for specific research.