Modeling the global ionospheric electron densities based on the EOF decomposition of the ionospheric radio occultation observation

Abstract

An attempt is made to model the three-dimensional global structures and temporal variations of the ionospheric electron density (Ne) using the radio occultation (RO) Ne profile data obtained by the COSMIC/FORMOSAT-3, CHAMP and GRACE missions during the period from July 2006 to June 2017. The modeling technique we adopted is based on the empirical orthogonal function (EOF) analysis of the RO Ne dataset which is binned with grids of 2.5° in geomagnetic latitudes and 1/3 hr in geomagnetic local time (equivalent to 5° in geomagnetic longitudes) and 10 km in height. The EOF analysis decomposed the binned (gridded) Ne dataset into a series of eigen modes or basis functions (E i ) representing the variations with geomagnetic latitude, geomagnetic local time as well as height and the associated EOF amplitude coefficients (A i ) representing the variations with seasons as well as solar cycle activity. Our results showed that the EOF components (E i , A i ) obtained by the EOF decomposition have different three-dimensional spatial structures with distinct features attributable to different factors or processes controlling the variations of the ionosphere: the first EOF component represents mainly the global mean structure of Ne and its temporal variation with seasons and solar cycle activity; the second EOF component represents mainly the seasonal control of the solar zenith angle on the ionosphere; the third EOF component is representative of the uplifting or lowering of the peak height where Ne reaches its maximum value; the fourth and fifth EOF components represent respectively the variations of the thickness of the ionosphere in the southern and northern hemispheres. It is found that the obtained eigen series converges quickly, with the first five EOF components contributing as high as 98% of the variances of the Ne dataset. We then modeled the EOF coefficients A i obtained by the EOF decomposition using the harmonic functions representing the annual and semi-annual variations, with the solar cycle dependences being taken into account by including the changes of the harmonic amplitudes with the solar irradiance flux index F10.7. The Ne model is constructed using the A i thus modeled and E i obtained by the EOF decomposition. Comparison between the Ne model output results and the COSMIC-2 observational data showed that the modeled results capture well the global structures of the observational data and the model output results have very high linear correlation coefficients with the observational ones (R > 0.9), justifying the modeling technique used in our present study.