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.