A comparison of a model using the FORMOSAT-3/COSMIC data with the IRI model
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 18 August 2010
Accepted: 8 October 2011
Published: 27 July 2012
In this study, an empirical model constructed using data of FORMOSAT3/COSMIC (F3/C) from 29 June, 2006, to 17 October, 2009, retrieves altitude profiles of electron density (Ne). The model derives global Ne profiles from 150 to 590 km altitude as functions of the solar EUV flux, day of year, local time and location under geomagnetically quiet conditions (Kp < 4). Ne profiles derived by the model are further compared with those of the International Reference Ionosphere (IRI). Results show that the F2 peak altitude hmF2 and the electron density NmF2, as well as the electron density above, derived by the model are lower than those of the IRI model. The F3/C model reproduces observations of F3/C well at 410-km altitude while the IRI model overestimates them. The overestimation of the IRI model becomes large with decrease of EUV flux. It is found that the topside vertical scale height of the F3/C model shows high values not only magnetic dip equator but also middle latitude. The results differ significantly from those of IRI, but agree with those observed by topside sounders, Alouette and ISIS satellites.
The International Reference Ionosphere (IRI) has been developed since 1978 (Rawer et al., 1978) and is established as the most standard and reliable ionospheric empirical model. Since a large amount of ionosonde data has been used, IRI derives a relatively accurate electron density (Ne) profile below the F2 peak. However, the IRI model might still have some shortcomings in the topside ionosphere, because very limited satellite data are included. Bilitza (2004) and Bilitza et al. (2006) based on Alouette/ISIS topside sounder observations reported that the IRI model overestimates Ne above the F2 peak height. Furthermore, Kakinami et al. (2008) found an in-situ Ne observation at a 600-km altitude with the Hinotori satellite which differed from the IRI Ne. This shortcoming also results in a difference between the Total Electron Content (TEC) reproduction and real observations, because the TEC is calculated using an integration of the Ne profile. Meanwhile, the IRI model overestimates the TEC in the equatorial region (Bilitza and Williamson, 2000) during high solar activity, while the IRI model overestimates and underestimates the TEC over Taiwan (24°N 120E) during low and high solar activity, respectively (Kakinami et al., 2009).
Six FORMOSAT-3/COSMIC (F3/C) micro satellites which constituted a global positioning system (GPS) occultation experiment (GOX) payload were launched on 14 April, 2006, and put into a low Earth orbit of 800-km altitude with a 72° inclination. An average of 1800 electron density profiles was obtained in a day globally. Lei et al. (2007) reported that Ne profiles measured with F3/C are consistent with NeF2 and hmF2 obtained with incoherent scatter radars at Millstone Hill and Jicamarca. An Ne profile obtained using GOX has an advantage in its coverage of observations, compared with the peak density and peak altitude of the ionosphere obtained with ground-based observations, because it was able to cover ocean and desert areas, where there are usually no receivers. Taking advantage of this feature, we have constructed an empirical model of the Ne profile measured with F3/C, which reproduces the Ne profile globally. In this paper, we describe the methodology in constructing the model and compare the empirical model based on F3/C observations with the IRI model.
2. Methodology of the Construction of an Empirical Model
The α are calculated in each 3-dimensional bin. Finally, the modeled Ne is obtained by linearly interpolating between the bins.
3. Results and Discussion
We have constructed an empirical model based on Ne profiles observed with F3/C under geomagnetically quiet conditions (Kp < 4). The F3/C model derives global Ne profiles between altitudes of 150 and 590 km as functions of the solar EUV flux, day of year, local time and location. The Ne above the F2 peak, and the F2 peak altitude, derived by the F3/C model is lower than those derived by the IRI. The F3/C model reproduces a longitudinal structure better than the IRI model. The F3/C model also derives VSH which show good agreement with measurements obtained from topside sounders onboard the Alouette and ISIS satellites. Since the F3/C model has a big advantage of greater data coverage, it helps us to understand a variety of upper ionospheric phenomenon. As a result, the F3/C model contributes an improvement over the IRI model.
This work was partially supported by the Earth Observation Research Center, Japan Aerospace Exploration Agency (Y. K.) and the National Science Council project NSC 98-2116-M-008-006-MY3 grant of the National Central University (Y. K. and J. Y. L.).
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