Observation of the ionospheric storm of October 11, 2008 using FORMOSAT-3/COSMIC data
The electron density profiles retrieved from the COSMIC radio occultation measurements were examined in order to estimate the possibility of its use as additional data source to study changes in electron density distribution occurred during ionospheric storms. The ionosphere behaviour during moderate geomagnetic storm which occurred on October 11, 2008 was analysed. The short-duration positive effect was revealed distinctly in GPS TEC and ionosonde measurements. For the European mid-latitude region it reached the factor of 2 or more relative to the undisturbed conditions. COSMIC data were analyzed and their validity was tested by comparison with ground-based measurements. It was shown the good agreement between independent measurements both in quiet and disturbed conditions. Analysis of COSMIC-derived electron density profiles revealed changes of the bottom-side and topside parts of the ionosphere.
Key wordsRadio occultation COSMIC GPS ionosphere
The radio occultation (RO) technique using GPS signals collected by Low Earth Orbit (LEO) satellites has been proven to be a promising technique to retrieve accurate profiles of the ionospheric electron density with high vertical resolution on a global scale. This technique was firstly applied at MicroLab 1 satellite, also known as OrbView 1, launched on April 3, 1995. That mission payload had an atmospheric monitoring instrument (GPS/MET). The analysis of GPS/MET sensor data had shown that the signals from the GPS satellite constellation intended for precise navigation and positioning can also be used to provide important atmospheric data. Besides the main goal of weather monitoring, it has also been possible to use GPS/MET sensor for sounding of the ionosphere from the orbit altitude to the Earth’ surface and retrieve one-dimensional profiles of ionospheric electron density from measurements of ray-path bending angle or TEC (Hajj and Romans, 1998). Following the successful GPS/MET experiment, similar satellite missions, such as CHAMP (Germany), SAC-C (Argentina), GRACE (US and Germany), and TerraSAR-X (Germany), were carried out. A number of relevant publications were based on these mission measurements and problems of data processing and validation (e.g., Schreiner et al., 1999; Hernandez-Pajares et al., 2000; Jakowski et al., 2002; Colomb et al., 2004; Garcia-Fernandez et al, 2005).
However, the existing satellites performing radio occultation experiments are mainly solo-satellite missions, and therefore, a long observation time is required to complete global coverage. FormoSat-3 (Taiwan’s Formosa Satellite Mission #3) / COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) is a joint scientific mission of Taiwan and the U.S.A., which was launched on April 15, 2006. The mission placed six small micro-satellites into six different orbits at 700∼800 kilometers above the earth surface. The orbit inclination is 72°. Each microsatellite has a GPS Occultation Experiment (GOX) payload to operate the ionospheric RO measurements. Depending on the state of the constellation COSMIC has been producing between 1,500–2,500 good soundings of the ionosphere and atmosphere per day, uniformly distributed around the globe. This number of RO is much greater than ever before. The total number of the ionospheric occultations for years 2006–2009 is over 2,000,000 (more than 50,000 profiles per month). The previous missions were able to produce only 4,000–6,000 RO profiles per month (only several hundred per day). So, COSMIC data can make a positive and considerable impact on global ionosphere studies providing essential information about vertical distribution of the electron density and particularly over regions that are not accessible with ground-based instruments such as ionosondes and dual frequency GPS stations.
2.1 COSMIC RO data
Since May 2006 the retrieved Ne profiles have been available from the Taiwan Analysis Center for COSMIC (TACC, http://tacc.cwb.gov.tw/en/) and the COSMIC Data Analysis and Archive Center (CDAAC, http://cosmic-io.cosmic.ucar.edu/cdaac/). Generally COSMIC can perform over 1,500–2,500 RO measurements per day, and for more than 70% of the RO measurements it is possible to successfully retrieve the Ne profiles, one of the most important products for space weather and ionospheric science. At CDAAC, the ionospheric profiles are retrieved by the Abel inversion from TEC along LEO-GPS rays. Detailed description of CDAAC data processing and Ne profile retrieval method can be found in Kuo et al. (2004), Syndergaard et al. (2006). We used the second level data provided by CDAAC—‘ionprf’ files containing information about ionospheric electron density profiles. COSMIC sounding points during a 24-hour period have a rather good global distribution. For this study, occultations with tangent points of the signal ray path within the limits of the European region were selected. The path of the tangent point during the oc-cultation is named the occultation trace. Depending on the relative COSMIC-GPS satellite constellation the data coverage changes permanently. Usually the total number of oc-cultation traces in the European region is about 35–50 per day. According to the information provided by CDAAC a small part of the COSMIC electron density profiles are affected by cycle slips in the GPS phase data. In some cases this results in obviously distorted profiles, whereas in other cases the errors due to cycle slips are more subtle. For this study we focused on the manual checking and painstaking screening of initial data. The detected distorted RO profiles were removed from analysis.
2.2 DIAS data
Ionosondes are typically used as diagnostic tools important for calibrating other more complicated observation methods such as incoherent scatter radar, satellite beacon tomography and radio occultation. Vertical sounding measurements by ground based ionosondes provide a precise determination of the bottom side electron density profile, but they are incapable to deliver information about the topside part of electron density profile (above the height of F2 layer peak). Therefore, the complete electron density profile in the ionosonde measurements consists of a measured bottomside and a modeled topside part. In order to validate the reliability of COSMIC data the ionograms, foF2 values and electron density profiles provided by the European Digital Upper Atmosphere Server (DIAS) were used. The DIAS server (http://www.iono.noa.gr/DIAS) operates since May 2005 and delivers such products as realtime and archive ionograms from all DIAS ionosonde stations, frequency plots and maps of the ionosphere over Europe based on the foF2, M(3000)F2, MUF and electron density parameters (Belehaki et al., 2005). This server collects information from the ionosondes located in Rome, Pruhonice, Juliusruh, Athens, Chilton, Ebre and El Arenosillo. The DIAS ionosonde network is equipped with the DPS-4 ionosondes (except for Ebre and El Arenosillo equipped with DGS-256) produced by the University of Lowell, Massachusetts, United States. On these stations the ARTIST (Automated Real Time Ionogram Scaler with True height) software for ionogram automatic scaling and real-time NHPC with true height inversion are used. The ARTIST system is a very skilled algorithm characterized by high percentage of reliable autoscaled data (Reinisch and Huang, 2001; Reinisch et al., 2005; McNamara, 2006; Galkin et al., 2008). Real-time ionosonde ionograms are scaled by autoscaling program and sent to the server. However, serious autoscaling errors can occur and it is recommended to check the ionosonde data against the ionogram of the same sounding for accuracy. In order to eliminate possible errors introduced by autoscaling we have done manual verification of all involved ionograms using DIAS database.
2.3 IGS data
The permanent GPS network provides regular monitoring of the ionosphere on a global scale with high resolution TEC measurements. The TEC data obtained from GPS observations of the IGS (International GNSS Service) network were used to study the ionosphere storm-induced effects in the European region. The methodology described by Baran et al. (1997) was used to restore the daily variation in TEC using the measurements of all satellite passes over the station within the 24-hour interval. In these calculations the ionosphere is approximated as a thin layer located at a fixed height (h = 400 km). The slant TEC (along the ray) and vertical TEC with first-order approximation can be related by the geometrical factor: VTEC = TEC × cos z′, where z′ is the zenith angle of the satellite on the layer height. Using this procedure an absolute TEC variation over single station is calculated.
Also in the present investigation, the IGS Global Ionospheric Maps (GIM) of TEC in the IONEX format were used. IONEX data are accessible at the ftp server: ftp://cddis.gsfc.nasa.gov/pub/gps/products/ionex. The GIMs are generated routinely by the IGS community with resolution of 5° longitude and 2.5° latitude and temporal interval of 2 hours; one TEC unit (TECU) is equal 1016 electrons/m2. In this investigation the final IGS combined GIMs produced by the Geodynamics Research Laboratory of the University of Warmia and Mazury in Olsztyn (GRL/UWM) were used.
In order to estimate the shape of the electron density profiles, the electron density values were normalized to the peak electron density. Figure 6(b) shows the normalized profiles for these days. Figures 6(c) and 6(d) present the same comparison for the ionosonde-derived Ne profiles, which are given as a benchmark data. One can see the increase of F2 layer height (∼60 km), but in general, the shape of the profiles near F2 region is a very similar for both quiet and disturbed days. These results indicate that the ratio of topside to the bottom-side values of electron content remained practically the same.
In the results presented above only two parameters were compared—the peak electron density (consequently, the critical frequency) and the peak height of the F-region. In order to enlarge this comparison, the ground-based GPS TEC data were added to the analysis. To compare GPS TEC with RO and ionosonde’s data these electron density profiles were integrated. In addition, the data integration up to the height of F2 layer maximum was carried out. The left panels in Fig. 5 demonstrate the results of these calculations. Each bar graph illustrates GPS TEC, COSMIC and ionosonde IEC (ionospheric electron content). The grey crosshatched bar histogram indicates the electron content up to the height of peak electron density. Hence, it is possible to compare the bottom-side and the topside parts of electron density profiles.
In general, bottom parts of COSMIC and ionosonde data are in a better agreement than topside ones. Practically for all cases the understated values of electron density in the topside part of the ionosonde profiles were observed in comparison with RO profiles. It can be explained by the fact that the topside ionosonde profile is obtained by fitting a model to the peak electron density value, the COSMIC one is obtained from experimental data and can make an important contribution to the investigation of the topside part of the ionosphere. Also, it is necessary to note that GPS TEC values are greater than COSMIC and ionosonde-derived ones as GPS TEC contains IEC and PEC (plasma-spheric electron content). This procedure can be useful to estimate the contribution of PEC on TEC.
Both TEC and IEC increased significantly for the moment of 13 UT (Fig. 5). GPS TEC increased by a factor of 2.2 in comparison with quiet day value (from 8.6 to 19 TECU). COSMIC-derived IEC increased by a factor of 2.7 and increase of ionosonde-derived IEC reached the factor of 3.7. It is interesting to estimate the proportion of topside and bottom-side parts to the IEC values. For the considered moment of time (13 UT) the ratio between different parts were calculated. It was found out that for quiet day the contribution of the topside part into COSMIC-derived IEC value was about 68% and the bottom-side part provided 32%. For disturbed conditions these values were 72% and 28%, respectively. For the ionosonde-derived values of IEC the topside part was about 66% of IEC for quiet day and 68% for disturbed day, bottom-side part—34% and 32%, respectively. Examining the matter it was found out that in spite of the significant storm-induced enhancement of IEC, the proportion between topside and bottom-side parts remained practically invariable and these parts were increased proportionally by the same factor.
The day-side enhancement in electron density was observed during geomagnetic disturbance on October 11, 2008. In European region the strong short-term positive effect reached factor of 2 and more relative to the undisturbed conditions. This effect was revealed distinctly both in GPS TEC and ionosonde measurements. COSMIC RO data were analyzed and their validity was tested by comparison with ground-based measurements. The results show the good agreement between independent measurements during quiet and disturbed conditions. Analysis of COSMIC-derived Ne profiles over European region demonstrated that for the given ionospheric storm RO was able to provide well-comparable and reliable characteristics of Ne profile such as NmF2 and hmF2. Unfortunately, the number of Ne profiles provided by COSMIC mission is still insufficient to study the daily behaviour of the ionosphere over any specified point. Therefore, COSMIC measurements can be effectively used only as additional data source for analysis and reconstructing of electron density distribution in the ionosphere.
COSMIC RO measurements were used in the analysis of the ionosphere behaviour during geomagnetic disturbance. It was shown that there is a possibility to complement the ionosphere studies based on the standard ground-based GPS measurements with the information about the vertical electron density distribution retrieved from COSMIC RO measurements. The distinctive feature of this moderate geomagnetic storm was the short duration of the ionospheric effect. This positive effect was revealed distinctly in RO electron density profiles and products based on these data— ionospheric electron content and global maps of electron density. The quality of RO profiles was estimated by comparison with the data provided by the ionospheric sounding stations of the European network DIAS. COSMIC RO measurements have several perspective advantages in comparison with the traditional ground-based measurements— global distribution of the occultation events and probing of the topside part of the ionosphere. In addition, RO technique based on LEO constellation data gives very valuable information for the ionosphere diagnostics in the areas where ground-based stations do not exist.
We acknowledge the Taiwan’s National Space Organization (NSPO) and the University Corporation for Atmospheric Research (UCAR) for providing the COSMIC Data. We are grateful to European Digital Upper Atmosphere Server (DIAS) for providing the ionosondes’ products and to International GNSS Service (IGS) for GPS Data.
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