First results from the ionospheric tomography experiment using beacon TEC data obtained by means of a network along a longitude of 136°E over Japan
© 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 2010
Received: 16 July 2009
Accepted: 17 October 2009
Published: 4 March 2010
A chain of newly designed GNU (GNU is not UNIX) Radio Beacon Receivers (GRBR) has recently been established over Japan, primarily for tomographic imaging of the ionosphere over this region. Receivers installed at Shionomisaki (33.45°N, 135.8°E), Shigaraki (34.8°N, 136.1°E), and Fukui (36°N, 136°E) continuously track low earth orbiting satellites (LEOS), mainly OSCAR, Cosmos, and FORMOSAT-3/COSMIC, to obtain simultaneous total electron content (TEC) data from these three locations, which are then used for the tomographic reconstruction of ionospheric electron densities. This is the first GRBR network established for TEC observations, and the first beacon-based tomographic imaging in Japanese longitudes. The first tomographic images revealed the temporal evolution with all of the major features in the ionospheric electron density distribution over Japan. A comparison of the tomographically reconstructed electron densities with the ƒoF2 data from Kokubunji (35°N, 139°E) revealed that there was good agreement between the datasets. These first results show the potential of GRBR and its network for making continuous, unattended ionospheric TEC measurements and for tomographic imaging of the ionosphere.
2. Data and Algorithm
A new digital receiver named the “GNU (GNU is not UNIX) Radio Beacon Receiver (GRBR)” was developed using the open-source hardware called Universal Software Radio Peripheral (USRP) and the open-source software toolkit for the software-defined radio (GNU Radio). The technical details of the receiver have been reported by Yamamoto (2008). The GRBR receivers were installed at Shionomisaki (33.45°N, 135.8°E), Shigaraki (34.8°N, 136.1°E), and Fukui (36°N, 136°E) where they continuously track LEOS, mainly the OSCAR, Cosmos, RADCAL, DMSPF15, and FORMOSAT-3/COSMIC (F3/C) satellites. The OSCAR and Cosmos satellites are at an approximate altitude of 1000–1100 km in a nearly circular polar orbit, whereas the DMSP satellite is at an altitude of approximately 840 km. The F3/C satellites are at an altitude of approximately 750–800 km with an orbital inclination of 72.. Consequently, the TECs between the ground receiver and LEOS are little affected by the plasmaspheric electron content.
3. Results and Discussion
Figure 1(a–h) shows the relative TEC data received at the three stations from different satellite passes on July 10 and 11, 2008. The Ap values on these days were 3 and 6, respectively. The maximum elevations were ±70° for all passes, except those shown in Fig. 1(b–d) which had maximum elevation angles of between 40 and 50° at different locations. Figure 1(a) corresponds to a COSMOS2407 pass from south to north (referred to as the SN pass hereafter) at ∼1151 UT (2051 LT) on July 10, 2008. Small perturbations can be clearly seen in the relative TEC data; these are typical of the mid-latitude summer nighttime over Japan (Saito et al., 1998). In contrast, in the next pass of COSMOS 2407 (from north to south, referred to as NS pass) at ∼2251 UT (0751 LT on the next day, the relative TECs are more smooth (Fig. 1(b)), which corresponds to a typical morning time situation. The equatorial ionization anomaly (EIA) crests are expected to be weaker and closer to the equator at these local times (Sastri, 1990). Figure 1(c) shows the data from an OSCAR 32 SN pass at 0618 UT (1518 LT). From relative TECs shown in this figure, it is evident that the latitudinal gradient is much higher on the southern side than in the northern region. This result is to be expected in the daytime because of the higher electron densities associated with the EIA feature in the low-latitude region. Similarly, Fig. 1(d) shows the data from an OSCAR 32 SN pass at 0806 UT (1706 LT). At this time also, the latitudinal gradient is higher on the southern side than in the northern region because of the presence of EIA. The following datasets were obtained at 0958 UT (1858 LT) (Fig. 1(e)). Figure 1(f) shows the data obtained at 1217 UT (2117 LT), which corresponds to a SN pass of COSMOS 2407. Signatures of wave-like perturbations on the northern side are much stronger at this time than during the previous night. Another interesting feature of Fig. 1(f) is that the relative TEC gradient on the northern side seems to be higher than that on the southern side, which is in contrast to the daytime condition. This feature is persistent in the next satellite pass at 1959 UT (0459 LT, next day) as well (Fig. 1(g)). This was a SN pass of the FM6 satellite of the F3/C constellation. The relative TEC in the southern region can be seen to be much lower than that in the northern region. However, the signatures of the travelling ionospheric disturbances (TIDs) are absent in this data, indicating that the medium scale TID (MSTID) activity has ceased by this time. The next dataset at 2157 UT (0657 LT, next day) shows that the relative TECs in both the southern and northern regions have enhanced after sunrise (Fig. 1(h)). Based on these data, it can be seen that the relative TEC observations themselves can provide significant information on not only the latitudinal distribution of the ionosphere at different local times but also on the presence of features like MSTIDs. These TEC observations can then be used to generate the tomographic images of electron density.
Figure 2(b) shows the reconstruction using the data shown in Fig. 1(b). The densities are now much higher than in the previous case, and the electron densities in the lower altitudes (100–150 km) are higher than those obtained in the nighttime. The enhanced electron densities in the northern region have disappeared, and there is now greater electron density on the southern side than in the northern region, which is to be expected at this local time (0800 LT). The tomographic images shown in Fig. 2(c, d) correspond to the data obtained from the subsequent passes at 0618 UT (1518 LT) and 0806 UT (1706 LT), respectively. These images show a clear enhancement in electron densities in the southern region relative to the northern region. The maximum electron density in the southern region is observed in the image obtained at these times, which agrees with the daytime EIA behavior in the low-latitude region (Sastri, 1990). The EIA feature has been successfully imaged using tomographic methods (e.g., Andreeva et al., 2000; Franke et al., 2003; Materassi et al., 2003; Yizengaw et al., 2007). It must be mentioned here that what we are seeing is not the crest of the EIA (which would be further south of the imaged region), but the edge of the EIA-associated enhancement, because of which the southern region has more electron density than the northern region. It can also be seen that unlike the nighttime, the electron density structure is rather smooth without any major perturbations.
The maximum electron density (Nmax) values obtained from the tomographic reconstructions were compared with those derived from the ionosonde located at Kokubunji (35°N, 139°E) (Fig. 3). In generating the tomographic images, we did not use the bottomside profile from the ionosonde as the initial profile and, therefore, the images reconstructed using TEC data are completely independent of the ionosonde data. A few cases in which the solution of the inversion did not converge within a difference of 40% in ƒoF2, when compared with the ionosonde data were totally discarded as ‘unreliable’ reconstructions. This often occurs due to limitations in the data, either in quality or coverage. After discarding such images, we obtained a total of 73 images, which were used further. The correlation was found to be ∼0.9 and was significant at >95%. It must be realized that one cannot expect an exact agreement between the maximum electron density and the ionosonde signals because the peak density from tomograms is averaged over a pixel of vertical size 50 km, whereas the ionosonde signals may be scattered from a particular height.
A comparison of such estimates with the tomographic observations yielded a correlation with R = 0.3 for the entire dataset, and a slightly better correlation with R = 0.5 when the nighttime values alone were considered for comparison (not illustrated). The discrepancy between these estimate arises from two factors: (1) the tomography gives the hmF2 with a resolution of 25 km, whereas the ionosonde estimates are ‘point measurements’; (2) the estimate of hmF2 using Eq. (4) is only an approximation to the actual hmF2, especially when the values of ƒ o E are not reliable. Similarly, the tomographic estimates of hmF2 largely depend on the hmF2 value given as the initial guess. In short, both estimates are limited in terms of accuracy. It must also be noted here that Kokubunji is about ∼300 km away from the chain and, horizontally, this is greater than the wavelength of MSTIDs. Depending on the azimuth and elevation angle of the satellite pass, the distance between the satellite observation and Kokubunji would vary. These are the limitations inherent in doing such comparisons. Nevertheless, these comparisons serve as an easy tool to validate the tomographic reconstructions.
The first ground-based GRBR network established for tomography experiments is now being used for the first time to monitor the ionospheric TEC variability. Beacon tomographic imaging of the ionosphere is being performed for the first time from Japanese longitudes. Here, we present the initial results from the tomography experiments. The images reveal the temporal evolution as well as prominent features of the mid-latitude ionosphere, such as TIDs and MSNA. A comparison with the ƒoF2 values obtained from ionosonde data were used to validate the tomographic reconstruction. Selected examples of tomographic images shown here illustrate that tomographic reconstruction can reveal the temporal evolution and variability of the mid-latitude ionosphere. Data from the GRBR network are currently being used to study the structures of the ionosphere around Japan. The GRBR and its network have proven to be quite useful for the continuous, unattended ionospheric TEC measurements and for the tomographic imaging of the ionosphere. The results also reiterate the usefulness of the-GRBR network for other regions as well, especially for the routine observations and the potential of tomographic imaging to provide information on ionospheric variability over a wide spatial region.
The work of ST is supported by the Japan Society for the Promotion of Science (JSPS) foundation. The authors thank Dr. Akinori Saito for his support in the installation of the GRBR network, and NICT, Japan, for the ionosonde data.
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