[1] In this paper, presented for the first time the three-dimensional global morphology and seasonal variations of scintillation index (S4 index) measured from the signal-to-noise ratio (SNR) intensity fluctuations of L1 channel of GPS radio occultation (RO) signals using FORMOSAT-3/COSMIC (in short, F3/C) satellites for a low solar activity year 2008. The S4 index, which confined around AE30 magnetic latitudes, is found to start around post-sunset hours (1900 MLT, magnetic local time) and often persists till post-midnight hours (0300 MLT) between 150 and 350 km altitudes during equinox and northern winter seasons while no activity is observed during southern winter season. However, high latitudes are characterized with no scintillation activity beyond 150 km during any season, which implying that in the solar minimum period the drives of instabilities in the auroral, cusp and polar cap regions, namely the gradient drift and velocity shear, are absent. The S4 index at F region altitudes during magnetically quiet times is more intense and extends to higher latitudes than that observed during disturbed time consistent with earlier studies. The equatorial S4 index appears below the peak of F2 layer (hmF2) during most of the seasons although the associated intensities and the time of maximum occurrences are relatively higher and earlier during vernal equinox followed by autumn equinox. This equinoctial asymmetry could be primarily attributed to the asymmetries in eastward drift velocities, thermospheric meridional winds and plasma densities. Further, the global maps of S4 index at E region altitudes (between 75 and 125 km) show strong seasonal variations with highest activity during northern and southern summer solstice in the middle latitudes while it appears on both sides of magnetic equator with less or no activity at and around the equator during equinox seasons. The absence of S4 index along the equator can be understood in terms of the vanishing vertical component of the magnetic field lines that can inhibit the vertical movement and layered deposition of ionized particles of thin irregular electron density layers such as Es-layers. Keeping in view the importance of these valuable database, we would like to emphasize that the F3/C GPS RO technique can be used to study the ionospheric irregularities at GHz frequency globally directly from the high-rate L1 data, which reiterating its importance as a powerful tool to explore the terrestrial ionosphere on a global scale.
Abstract. The GPS data provides an effective way to estimate the total electron content (TEC) from the differential time delay of L1 and L2 transmissions from the GPS. The spacing of the constellation of GPS satellites in orbits are such that a minimum of four GPS satellites are observed at any given point in time from any location on the ground. Since these satellites are in different parts of the sky and the electron content in the ionosphere varies both spatially and temporally, the ionospheric pierce point (IPP) altitude or the assumed altitude of the centroid of mass of the ionosphere plays an important role in converting the vertical TEC from the measured slant TEC and vice versa. In this paper efforts are made to examine the validity of the IPP altitude of 350 km in the Indian zone comprising of the ever-changing and dynamic ionosphere from the equator to the ionization anomaly crest region and beyond, using the simultaneous ionosonde data from four different locations in India. From this data it is found that the peak electron density height (hpF2) varies from about 275 to 575 km at the equatorial region, and varies marginally from 300 to 350 km at and beyond the anomaly crest regions. Determination of the effective altitude of the IPP employing the inverse method suggested by Birch et al. (2002) did not yield any consistent altitude in particular for low elevation angles, but varied from a few hundred to one thousand kilometers and beyond in the Indian region. However, the vertical TEC computed from the measured GPS slant TEC for different IPP altitudes ranging from 250 to 750 km in the Indian region has revealed that the TEC does not change significantly with the IPP altitude, as long as the elevation angle of the satellite is greater than 50 degrees. However, in the case of satellites with lower elevation angles (<50°), there is a significant departure in the TEC computed using different IPP altitudes from both methods. Therefore, the IPP altitude of 350 km may be taken as valid even in the Indian sector but only in the cases of satellite passes with elevation angles greater than 50°.
[1] We compare ionospheric parameters including total electron content (TEC), peak density, and height of the F2 layer (NmF2 and hmF2) between FORMOSAT-3/COSMIC (F3/C) GPS radio occultation (RO) technique retrieved and International Reference Ionosphere (IRI-2001) model predicted during different seasons in a low solar activity (LSA) year 2007. The comparison of topside TEC (tTEC, obtained by integrating electron density from hmF2 to 800 km) between IRI and F3/C shows that the IRI overestimates tTEC during both equinox seasons at around ±15°magnetic latitudes during daytime, especially in March equinox while underestimation of tTEC predicted by IRI is a dominant feature for both solstices. Further, a common finding is that the IRI overestimates tTEC from evening to prenoon hours irrespective of season at around ±20°magnetic latitudes, which is most likely due to the underestimation of hmF2 (by around 30-40 km), a key parameter on which the build of electron density profile depends in the model and inaccurate representation of the real profile by the topside electron density profiler model in IRI. The global distributions and seasonal variations of NmF2 show clear semiannual and annual asymmetry features during daytime and such features also reflected in IRI predicted peak densities with few exceptions. This high degree of agreement between tTEC comparison and the characteristic features of NmF2 global distributions indicates that the majority of contribution for tTEC has come from the F2 region. Further, it is also presented the global distributions of topside vertical scale heights (VSH) computed using electron density profiles retrieved from F3/C and predicted by IRI model during different seasons in year 2007. An important finding is that the topside VSH of F3/C profile data is meticulously following the geomagnetic equator during daytime irrespective of season and tends to increase toward higher latitudes. An appreciable latitudinal difference is found in the season averaged scale heights that derived with F3/C data during daytime while completely opposite results are found for IRI predicted scale heights. The discrepancies of topside VSH between the F3/C derived and predicted by IRI indicating that the shape of the topside electron density profile in the IRI model should desperately be revised accordingly such that it more closely resembles the real situation.Citation: Potula, B. S., Y.-H. Chu, G. Uma, H.-P. Hsia, and K.-H. Wu (2011), A global comparative study on the ionospheric measurements between COSMIC radio occultation technique and IRI model,
Abstract.A study carried out on the occurrence of post midnight spread-F events at a low-latitude station, Waltair (17.7 • N, 83.3 • E), India revealed that its occurrence is maximum in the summer solstice months of the low solar activity period and decreases with an increase in the sunspot activity. The F-region virtual height variations show that 80% of these spread-F cases are associated with an increase in the Fregion altitude. It is suggested with the support of the night airglow 6300 A zenith intensity data obtained with co-located ground-based night airglow photometer and electron temperature data from the Indian SROSS C2 satellite that the seasonal variation of the occurrence and probable onset times of the post midnight spread-F depend on the characteristics of the highly variable semipermanent equatorial Midnight Temperature Maximum (MTM).
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