Figure 15: Top tow rows are time series of TIPs obtained using differential method (blue) and its normalized periodograms (red). The bottom two rows are variation of inter-IPP distance of the corresponding satellite-receiver pairs (blue) and its normalized periodograms (red). To compare the frequencies of the inter-IPP distance and TIP, the frequencies within range of acoustic gravity component are only shown here.
We, further, tested differences in the frequency of perturbations obtained by adopting the differential, residual and SPLA methods using CWT analysis (Grinsted et al., 2004). Results reveal the significant variation in frequency between the residual method and SPLA. We present two typical cases here, in which one is similar to the second scenario demonstrated in theoretical study (COCO17) and another one (COCO7) is a clear detection of TIP by gROT when the residual method finds the entire time series noisy (Fig. 16).
Time series of COCO17 show that in gROT and dTEC a strong TIP appears between 7 to 8 UT and a weak TIP appears between 11 to 14 UT along with two observational breaks. However, in rTEC the weak TIP along with breaks (Fig. 6), similar to the second scenario in the theoretical study (Fig. 2), amplified greater than the strong TIP observed in gROT and dTEC between 7 to 8 UT. The corresponding CWT also shows that the artifacts in rTEC caused by the discontinuities in observation aliases as a strong signal within the frequency range of acoustic gravity wave induced TIPs. On the other hand, the frequencies of TIPs detected by gROT are clear and well confined within the expected range of 0.5 to 5 mHz (33 to 3.3 minutes). Furthermore, the CWT of gROT detects TIPs of small amplitude at 14 to 15 UT as shown in the theoretical case and distinctly detects the discontinuities in observations without any aliasing compared to dTEC.
In the case of COCO07, the residual method completely failed to detect the TIP which is present at the beginning of dTEC and gROT time series (Fig. 17). This is due to the misfit of 10th order polynomial used to represent the trend of vTEC, particularly at the beginning and end of the observation (Fig. 17). This reveals the reason for the failure of the residual method to detect the perturbations occurring at the beginning and end of the observations. Further, the TIP time series of dTEC and gROT, and their corresponding CWTs show the difference between dTEC and gROT is due to the aliasing in dTEC (Fig. 16). This further emphasis the significance of removing the aliases and selection of appropriate method to detect the ionospheric perturbations. Furthermore, CWTs of these observations establish resolving ability of SPLA and its capability of detecting ionospheric perturbations without aliasing under all conditions.