Elsevier

Advances in Space Research

Volume 55, Issue 8, 15 April 2015, Pages 2070-2076
Advances in Space Research

Vertical TEC representation by IRI 2012 and IRI Plas models for European midlatitudes

https://doi.org/10.1016/j.asr.2014.07.027Get rights and content

Abstract

Vertical total electron content (vTEC) values computed using IRI-2012 and IRI Plas models have been compared with diurnal GPS vTEC data derived from European mid-latitude GPS station Potsdam. Comparative data-model analysis does not reveal good performance in vTEC representation. It was found that new extension of IRI model – IRI Plas – cannot represent correctly the vTEC variations over European midlatitudes and mainly overestimates GPS vTEC especially for low and moderate solar activity. In order to estimate the source of the data-model discrepancies, the case-study with detailed analysis of the model simulated electron density profiles was done. It was obtained that all models do not represent correctly the topside profile part and tend to overestimate the electron density higher than F2 peak. So, the main problem of the IRI vTEC representation is not situated in the plasmaspheric part, its absence in IRI model or its presence in IRI Plas model, the main source of the resulted discrepancies is still in the IRI topside ionosphere representation.

Introduction

During last several decades dual frequency GPS measuring technique is well proved and widely used for studies of the near-Earth plasma environment (Jakowski, 1996, Davies and Hartmann, 1997, Garner et al., 2008). With the rapid growth of the global and a number of regional ground-based GPS receiver networks, measurements of the total columnar electron content (TEC) along the ray path from a GPS satellite to the receiver can be used as unprecedented large database for the ionosphere monitoring and research. The ionized atmosphere surrounding the Earth at altitudes’ range of about 80–100 km up to 3–5 RE represents a dispersive medium for the GPS signals which propagate through the oxygen-dominated plasma of the ionosphere and the tenuous hydrogen-dominated plasma of the plasmasphere on their way to the ground-based GPS receivers.

As GPS technique presents opportunity to be used in real-time or near real-time ionosphere services for space weather monitoring and the high popularity of GPS TEC observations for the ionosphere research, including generation of global and regional ionospheric maps with high temporal and spatial resolution, there is a great demand in a proper model for GPS TEC specification.

Nowadays the International Reference Ionosphere (IRI) provides one of the better model specifications for the main ionospheric parameters (Bilitza, 2001). However, the IRI model specifies the ionosphere only up to 2000 km, which is a problem because the GPS satellites are located at 20,200 km. While the plasma density above 2000 km is at least two orders of magnitude less than the F-region peak density (e.g. Gallagher et al., 2000), the length above the IRI model (18,200 km) is about two orders of magnitude greater than the thickness of the F layer (Garner et al., 2008). It is necessary to extrapolate the ionosphere to higher altitudes for use with GPS measurements.

The International Standardization Organization, ISO, recommends the IRI model for the specification of ionosphere plasma densities and temperatures and lists several plasmasphere models for extending IRI to plasmaspheric altitudes. IRI Plas model, the International Reference Ionosphere extended to Plasmasphere (Gulyaeva et al., 2002), has been proposed as one of the possible candidate model for the plasmasphere extension of the IRI model (Gulyaeva and Bilitza, 2012).

The present work analyzes results coming from comparison of a long-time series of the GPS vertical TEC data recorded at mid-latitudinal GPS station with simulated vTEC results derived by IRI 2012 and IRI Plas models. As a specific location we chose the GPS station POTS (Potsdam, Germany) as a representative of the European mid-latitude station, besides it is one of the most closely located IGS station to the Juliusruh ionosonde.

Section snippets

GPS vTEC data

The GPS vTEC data are obtained from the GPS receiver of IGS network: POTS (Potsdam, Germany). The station has geographical coordinates (52.4N; 13.1E). The vTEC values were computed from the raw GPS data in RINEX format available at NASA CDDIS archive (CDDIS, 2014) using our algorithm of TEC determination and calibration. The well-known formulas of TEC estimation from the frequency-differenced GPS phase delay were used from (Hofmann-Wellenhof, 2001). The slant TEC (sTEC), defined as the line

Results

As known, IRI model reproduces vTEC estimate as a result of integration of electron density profile within altitudinal limits of 60–2000 km. According to Gallagher et al. (2000) the electron densities in plasmasphere are several orders of magnitude less than in ionosphere and in fact the plasmasphere is often ignored in analysis of GPS vTEC data, assuming that it is the question of several TECU only. Based on such assumptions one can expect that IRI vTEC should be always lower than observed GPS

Discussion and conclusion

So, we found discrepancies between GPS vTEC and model-derived vTEC and we can estimate them for particular geographical location and time, but is it informative enough to make any conclusion about source of the problem? Probably not.

Let us consider one case from the obtained results in more details. We analyze event for 1200 LT of December 2000, when both models represent considerable overestimation over vTEC observations. Fig. 3(a) presents comparison of the electron density profiles (EDPs)

Acknowledgments

We are grateful to International GNSS Service (IGS) for GPS data and products. We acknowledge the Information System and Data Center, GFZ, Potsdam for providing CHAMP data. The Juliusruh ionosonde data are kindly provided to IPS by the Leibniz-Institute of Atmospheric Physics e.V. at University of Rostock (IAP) of Germany. We acknowledge the IRI Working group for providing and evaluating the IRI model FORTRAN code and Dr. Tamara Gulyaeva for IRI Plas code. We thank the Telecommunications/ICT

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