Comparison of Electron Density Profiles from CHAMP Data with NeQuick Model

Norbert Jakowski1, Konstantin Tsybulya1, Sandro M Radicella2, Marta Cueto3,Miguel Herraiz3

1 Deutsches Zentrum für Luft und Raumfahrt, e.V. (DLR), Institut für Kommunikation und Navigation, Kalkhorstweg 53, Neustrelitz, Germany, Norbert.Jakowski@dlr.de

2 Aeronomy and Radiopropagation Laboratory, Abdus Salam ICTP, 34100 Trieste, Italy. 3 Department of Physics of the Earth, Astronomy and Astrophysics I, Universidad Com-

plutense de Madrid, Avenida Complutense s/n 28040, Madrid, Spain.

Summary. Vertical electron density profiles (EDP) derived from ionospheric radio occultation (IRO) measurements onboard the German CHAMP satellite are calculated by the Institute of Communications and Navigation of DLR on a regular basis since 11 April 2001. In order to validate the ionospheric radio occultation data obtained from this satellite, any systematic comparison with independent measurements but also with results from ionospheric models should help to get a better feeling about the quality of the data. On the other hand, if the IRO data quality is sufficient, the data may contribute to evaluate the accuracy of the ionospheric models. In this study we compare CHAMP EDP data derived in 2002/03 with the NeQuick model, which is one of the three electron density models developed at the Abdus Salam ICTP in Trieste (Italy) and the Institute for Meteorology and Geophysics in Graz (Austria). We discuss results of this comparison, showing changes in bias and deviation with latitude and local time. The best agreement between both types of data was found above a height of 300 km.

Keywords: Ionosphere, radio occultation, NeQuick, electron density, validation

1 Introduction

Low Earth Orbiting satellites carrying a dual frequency GPS receiver onboard of-fer a unique chance to monitor the actual state of the global ionosphere on a con-tinuous basis. No other profiling technique unifies profiling through the entire F2-layer with global coverage.

The German CHAMP satellite provided first ionospheric radio occultation (IRO) measurements on 11 April 2001. Preliminary IRO retrieval and validation results are discussed in Jakowski et al. (2002). In this paper we discuss a compari-son of up to about 50000 CHAMP measurements with corresponding model val-ues derived from the ionosperic model NeQuick referenced in the following as NeQ.

The NeQ model is one of the three electron density models developed at the Abdus Salam ICTP in Trieste and the Institute for Meteorology and Geophysics in

484 Norbert Jakowski et al.

Graz (Hochegger et al., 2000; Radicella and Leitinger, 2001). All these models are based on the DGR “profiler” concept further developed by Radicella and Zhang (1995) and provide electron density as a function of solar activity (10.7 cm flux), month, Universal Time, geographic latitude, longitude and height. The “quick cal-culation” model NeQ uses a simple formulation for the topside F-layer, which is essentially a semi-Epstein layer with a thickness parameter that increases linearly with height. In its general form the model is driven by the ITU-R (former CCIR) global coefficients that describe the f0F2 and M3000 ionospheric characteristics. The version of the NeQ model used in this study was the one available on Febru-ary 4, 2003.

In order to come up with significant conclusions on the comparison between the profiles retrieved by the IRO technique and the NeQ model, statistical studies were performed taking into account about 50000 electron density profiles. Mean values of electron density and plasma frequency differences between NeQ and IRO profiles (called biases) at fixed heights have been calculated from 95–400 km height with a resolution of 5 km. Because the relationship between plasma fre-quency (fp) and electron density (Ne) is not linear, both were calculated sepa-rately.

The dispersion of these differences (RMS) have also been calculated to indicate the variability between the model and the observations.

2 Data basis

The CHAMP data are automatically processed in the DLR/Institute of Communi-cations and Navigation by an operational data processing system (Wehrenpfennig et al., 2001). The computed data products are made available to the international science community via the Information and Science Data Center (ISDC) of GFZ Potsdam.

For retrieving vertical electron density profiles, a tomographic approach is es-tablished that uses spherically layered voxels with constant electron density (Ja-kowski, 1999, Jakowski et al., 2002). The IRO measurements onboard CHAMP provide more than 150 vertical EDPs per day (see Jakowski et al., this issue). The database for this comparative study (about 50000 profiles, period of May 2002 – April 2003) has been processed automatically without additional checking.

The CHAMP EDP retrievals have been validated from the CHAMP orbit down to the E-layer altitude by comparison with vertical sounding data. Jakowski et al. (this issue) report corresponding results for the European region, which indicate a systematic positive bias of the IRO data in the order of less than 0.6 MHz plasma frequency (fp) or 8x1010 m-3 electron density (Ne) respectively and a standard de-viation of less than 1.0 MHz (1.3x1011 m-3) throughout the entire profile.

When comparing and discussing the IRO data with NeQ values in the following section, these validation results can be used as a valuable reference.

Comparison of Electron Density Profiles from CHAMP Data with NeQuick Model 485

3 Results and Discussion

Keeping in mind the validation results of IRO derived EDP for mid-latitudes, the comparison with NeQ data might provide valuable information on the quality of the NeQ model if deviations exceed the measured bias and the standard deviation range.

Fig. 1 shows the bias and standard deviation between NeQ and IRO data for all measurements under consideration. The centered line in each panel represents the mean bias, surrounded by two lines indicating 1σ deviation. The comparison indi-cates quite good agreement in the F2 layer altitude range above 300 km height. Ignoring the E-layer, the maximum deviation (underestimation) of –1 MHz fp (1.4x1011 m-3 Ne) is observed in the 200–250 km height range. To study the latitu-dinal and local time dependence of the difference, the data set has been divided into three parts representing high (⏐φ⏐ > 60°), medium (30 ⏐φ⏐ 60°) and low latitude (⏐φ⏐ < 30°) ranges. These data sets are subdivided in 4 groups corre-sponding to different local times (night, morning, day, evening) as it is shown in Fig. 2. Ignoring the E-layer, the mean deviation of the plasma frequency fp (elec-tron density Ne) is generally less than 1.2 MHz (1x1011 m-3) at high latitudes and less than 1.8 MHz (3x1011 m-3) at mid latitudes under daytime conditions in the 220–230 km height range. At low latitudes the bias reaches about 1.0 MHz (2.5x1011 m-3) that is most pronounced at all day-time conditions in the altitude range 200–280 km.

Fig. 1. Comparison of CHAMP IRO data with corresponding NeQ EDP, according to the difference NeQ–IRO (May 2002 – April 2003, 53905 profiles). Left – electron density Ne, right – plasma frequency fp.

486 Norbert Jakowski et al.

Fig. 2a, b. EDP comparison (NeQ–IRO) at high (upper panel) and mid (lower panel) latitu-de sectors (May 2002 – April 2003).

Table 1. Maximum electron density and plasma frequency biases, RMS values and corre-sponding heights for high, mid and low latitudes (according to NeQ – IRO).

Electron Density Plasma Frequency

Latitude Bias(x1011 m-3)

RMS(x1011 m-3)

Height(km)

Bias(MHz)

RMS(MHz)

Height(km)

High < 1.5 < 2.5 240 < 1.0 < 1.5 230 Mid < 3.5 < 4.0 240 < 1.6 < 2.0 240 Low < 2.5 < 6.0 270 < 1.0 < 2.3 260

Comparison of Electron Density Profiles from CHAMP Data with NeQuick Model 487

Fig. 2c. Same as 2a, b, but for low latitude.

It is difficult to comment the low latitude results because entire IRO derived Ne profiles have not yet been validated in a systematic way for this latitude range. We are aware of the fact that the spherical symmetry assumption in IRO retrievals is violated in particular in the crest region due to strong horizontal gradients of the electron density distribution and therefore errors cannot be excluded. However, it has to be mentioned that the biases are smaller than those obtained for mid-latitudes. The enhanced absolute dispersion is surely due to the high ionisation at low latitudes.

Considering the fact that the bias is only slightly positive (< 0.4 MHz) through-out the entire profile at least in mid-latitudes (Jakowski et al., this issue), the un-derestimation of the electron density in the height range of about 200–300 km is a clear finding. Due to the persistence of this feature at all latitude ranges we as-sume that NeQ generally underestimates the ionospheric ionisation in this height range under high solar activity. In order to check the consistency of the results, computations have been made separately for the Northern and Southern hemi-spheres. Comparing bias and dispersion values of corresponding latitude ranges, no substantial difference can be found. Excluding the E-layer range, the percent-age accuracy (∆fp/fp) is generally better than 24%. Although the ∆fp/fp may ex-ceed 100% in the E-layer range, it is surprising that the agreement is best at low latitudes (deviation < 16 % ) where the ionospheric conditions for retrieving the EDPs from IRO measurements are not optimal.

The maximum average deviations of the electron density and plasma frequency are summarized in Table 1 together with the corresponding altitudes. In any case, further improvements, e.g. the improvement of the topside ionosphere model, are planned to enable more accurate observations.

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4 Summary and Conclusions

We have reported preliminary results obtained by analyzing GPS radio occulta-tions through the ionosphere measured onboard the CHAMP satellite in the period May 2002–April 2003. The retrieval results obtained by applying a model assisted retrieval technique are promising. Because the radio occultation technique is rather new, a systematic validation of data products in particular with ionosonde and incoherent scatter data is going on. Generally speaking, the comparison with ionospheric models is helpful for improving their performance. The comparison of IRO data with corresponding NeQ data indicates a systematic underestimation of the electron density in the 200–300 km height range of NeQ compared with the IRO data. Future versions of NeQ (here we used the version of February 4, 2003) should take into account this fact. It should be underlined that the IRO vertical density profiles analyzed here are the direct output of the automatically working IRO processing system without further reviewing. Although the number of IRO retrieval outliers is rather small (< 1%), their removal would certainly improve the agreement between the NeQ model data and the CHAMP IRO measurements.

Acknowledgments. The authors are very grateful to all colleagues from the international CHAMP team who keep CHAMP in operation.

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