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.