Figure 5: (a) Spatiotemporal variation of TIP along the satellite track of COCO07 and simulated tsunami at 3.5 UT on 26 December 2004. Size and color of the circles on the satellite track represent amplitude of normalized gROT. White diamond is GPS station COCO. Time series of normalized gROT (red), dTEC(blue) and rTEC (green) for COCO07 are given in the right panel; (b), (c) and (d) Time-distance, elevation-distance and azimuth-distance plots color coded with normalized rTEC, dTEC and gROT of COCO07, respectively.
TIP time series of COCO07 show that the amplitudes of TIPs in dTEC are higher than gROT up to 3.5 UT; but, rTEC did not detect any TIP (Fig. 5a). To explore the cause of difference in amplitudes, the TIPs are presented as function of inter-IPP distance, elevation and azimuth (Fig. 5b and 5c) which show the difference between amplitude of dTEC and gROT increases with inter-IPP distance. Further, the difference also varies with both azimuth and elevation (Fig. 5b to 5d). The increase in difference between amplitudes of TIPs detected by the differential method and SPLA with inter-IPP distance shows that the unaccounted inter-IPP distances are aliased as signal in dTEC, particularly, at the distance above 5 km which corresponds to elevation less than 25° and an azimuth of ~225°.
As the elevation, azimuth and inter-IPP distance repeat in time and cause overlaps when plotted as a function of time, we show the perturbations as a function of (i) time vs inter-IPP distance (time-distance plot) (Fig. 5b and 5c, left) and (ii) elevation vs distance (elevation-distance plot) ( Fig. 5b and 5c, middle) and (iii) azimuth vs distance (azimuth-distance plot) (Fig. 5b and 5c, right). The time-distance plots of COCO07 color coded with normalized values of dTEC and gROT (Fig. 5c and 5d) distinguish the difference between TIP amplitudes obtained using the differential method and SPLA, particularly above 5 km of inter-IPP distance at 2-3UT while the elevation angle was varying from 0°-20° (Fig. 5c and 5d). Similarly, the time-distance plots of COCO17 (Fig. 6) show that the signal aliasing occur in dTEC elevation below 15o (∆d > 8km) is absent in gROT upon accounting the inter-IPP distance using SPLA. The differences in amplitude between dTEC and gROT exhibit that the amplitude aliasing in dTEC is caused by the unaccounted inter-IPP distances and show how possibly it could mislead the detection of ionospheric perturbations. Furthermore, the time-distance plots and elevation-distance plots (Fig. 6d) show that the aliases did not occur whenever the satellite is at low elevations, rather, the aliased signals are present only when the time and location of the TIPs coincide with the propagating tsunami (Fig. 6a). This further proves that the signal aliasing in dTEC is prominent only when the ionosphere is heterogeneous and the aliasing increases with ionospheric heterogeneity. Hence, it is important to remove such aliasing by accounting the inter-IPP distance, particularly, while studying the ionospheric perturbations. On the other hand, the residual method completely failed to detect the signal in the case of COCO07, rather the rTEC time series is spurious during the entire span of the observation.