The Inversion,quality Verification Of FY-3C Radio Occultation Ionospheric Product And Its Application In Sporadic E Layer Study

As a techonology for probing the Earth's ionosphere and atmosphere,Global Navigation Satellite System(GNSS)Radio Occultation(RO)technology has drawn great attentions in recent years.Using this technology,the ionospheric electron density profiles(EDPs)can be inverted from the phase data observed during RO events.Compared with other ionosphere observation methods,GNSS RO has the advantages of high vertical resolution,global coverage,no platform dependence,no long-term drift,and all-weather capability.It can provide abundant EDPs over the ocean,deserts,snowy plateaus,polar regions and other inaccessible areas,which is of great value for the precise modeling and the mechanism study of the Earth's ionosphere.In this paper,after presenting the basic principle for probing the ionosphere with GNSS RO technology and the general information about Fengyun-3C(FY-3C)RO mission,a series of experiments and analyses were carried out about the inversion and quality assessment of FY-3C RO ionospheric products and about the application of these products in the detection of ionospheric sporadic E(Es)layers.The work and main results of this paper are as follows:(1)The algorithm for inverting the EDPs based on GNSS RO-measured TEC is realized.Two key problems in the inversion procedure,the singularity problem in the Abel integration and the correction for the TEC value between the GNSS and the LEO satellites,is solved by using the "onion method" and by taking adavantage of the observations during non-occultation periods,respectively.Based on this algorithm,the ionospheric EDPs are retrieved from the FY-3C RO and the COSMIC RO observations and are further compared with the official products of the two missions,respectively.The high correlations and the minimal deviations between the inverted results and the official products demonstrate that there is good consistency between the EDPs inverted by us and those provided by the official organizations.(2)The influences of different time and space collocation windows on the comparisons of the ionospheric products derived from the two techniques,GNSS RO and ionosondes,are studied.The results show that the consistency between the RO-derived and the ionosonde-derived ionospheric characteristic parameters(ICPs)decreases gradually with the increase of space and time matching windows.The influence of the increase in the space window on the statistical comparison is much more significant than that of the increase in the time window,and the influence of the variation of the spatial window along the latitude direction is generally greater than that along the longitude direction.We offer suggestions for selecting the appropriate time and space collocation windows under some specific circumstances.(3)The quality of FY-3C RO ionospheric products was evaluated over a long time period,taking the solar elevation angle(SEA)and the azimuth of occultation plane(AOP)as new influencing factors in the evaluation.The results show that the ICPs provided by FY-3C RO mission are of good consistency with COSMIC RO and ionosonde observations.In addition,the degree of the difference between FY-3C ROderived and ionosondes-derived ICPs was significantly positively correlated with the magnitude of SEA,and the RO data quality was the best when AOP was between 60° and 120°.(4)The Es layer detection algorithm based on RO SNR data was implemented,based on which the Es layers are detected using FY-3C RO data.The temporal and spatial distributions of FY-3C RO-derived Es are compared with thosed derived from COSMIC RO observations.The results show that the Es space-time distributions extracted from two RO missions are basically consistent with each other.Significant seasonal variation characteristics are presented in the distribution of Es,and the highest Es occurrence frequency is got in the mid latitudes of summer hemispheres,which is accompanied by apparent downward movements of Es and an increase of multi-layer Es events.The Es occurrence is low over the magnetic equatorial region,and the time period of 3:00-6:00 LT is generally the quiet period of Es.