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Ionospheric total electron content and WAAS in the Indian zone
Figure 2 shows the variation of Slant TEC with time. The observed values are translated from equivalent Vertical to Slant TEC by simple multiplication with the geometrical factor Secc. For the model values, the ionization along the path of ETS-II are integrated in the altitude range 90-1600km for layers of thickness 20km and the result is referred to as the Path Integrated (PI) Slant TEC. It is observed that the difference between the actual observations and the model computations still persists, but to a smaller extent. The distinction between the simple Secc multiplied Slant values and the Path Integrated Slant values, both obtained from PIM, are well illustrated in Figure3. It is observed that although the basic nature of variation of electron content is the same in both the cases, the Path Integrated values offer a better alternative as they overestimate the actual observations to a less extent. Figure 4 shows the relation between the measured seasonal mean equivalent Vertical TEC averaged over the local time interval of 11-16hrs and the corresponding sunspot number during the period 1978-1985. The
values in summer and winter follow a trend similar to the sunspot number. During the equinoxes the satellite was eclipsed and there were large data gaps. Although the minimum value of electron content remains almost the same irrespective of solar activity, the maximum ranges from 114 TEC units during 1979 to 29 TEC units in 1985.
 Figure 2.  Figure 3.  Figure 4. Conclusions A comparison of the electron content measured from the plane polarized beacon transmission from the Japanese geostationary satellite ETS-II with the values generated from the Parameterized Ionospheric Model (PIM1.6) do not yield satisfactory results for the equatorial region, particularly during the afternoon hours of equinoctial months in high solar activity years. PIM, which is a fast global ionospheric and plasmaspheric model, produces electron density profiles between 90 and 25000km altitude, corresponding critical frequencies and heights for the ionospheric E and F2 regions and TEC for specified geophysical conditions. This model which predominantly uses data from the American and European longitude sector as its data base exhibits significant deviations from the observed data even during the summer months of 1979, a high solar activity year. The presence of large spatial gradients of TEC in the equatorial region coupled with its variability under different geophysical conditions renders simple geometrical conversion of equivalent Vertical TEC to Slant values unreliable. Even with the Path Integrated Slant TEC values, there are wide differences between the actually observed electron content and the model computations. The result reflects the fact that climatic models like PIM are unable to properly represent the dynamic nature of the equatorial plasma transport processes and do not account for the variability of the sharp spatial gradients of ionization in this region. It is also observed from ground-based measurements that while the ratio of maximum TEC varies by a factor of 4 in the equatorial region, its variation in the mid-latitudes is greater. PIM uses some input parameters, like electrodynamic drift obtained from observations made in the western hemisphere. The validity of these parameters around the present location in the equatorial region, particularly the Indian longitude sector, is to be further examined. References
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