7.1 Impact on phase
Phase and time of occurrence of the ionospheric perturbations are the required parameters to determine velocity of the perturbations. Velocity of a ionospheric perturbation is one of the important characteristics as it directly affects the ray tracing which eventually lead to error in locating the source of the perturbation. To quantify the impact of aliases and artifacts on the phase, we carried out a cross-correlation analysis and identified the difference in phase and time of occurrence between the perturbations obtained using the three methods. The gROT of each receiver-PRN pair was correlated with rTEC and dTEC, and the phase differences were converted into time differences. The results show that occurrence time of TIP in rTEC is different from gROT. Among the 44 TIP time series, in 16 cases the time of occurrence of TIP in rTEC is delayed compared to gROT, in 17 cases rTEC detects TIPs prior to gROT and at 11 cases rTEC did not identify the TIPs (Table 1). The delay varies from 2.5 to 6.0 minutes and the advancement is ranging from 2.0 to 5.5 minutes (Fig. 14 and Table 1). The difference in time of occurrence between rTEC and gROT may be due to either aliasing or the subjective selection of the high-order polynomial to represent the trend of GPS TEC. Such differences in time of occurrence will consequently affect the characteristic velocity estimates of the tsunami induced ionospheric perturbations. However, there is no phase difference found between dTEC and gROT.
Similar to tsunami case, the difference in time of occurrence between rTEC and gROT of CIPs are varying from 7.64 minutes of advancement to 2.55 minutes of delay (Fig.14,). Further, rTEC failed to detect 50% of CIPs (18 cases out of 36) compared to dTEC and gROT (Table 2). This difference in time of occurrence show that the signal detection using residual method by employing uniform order of polynomial across all station-PRN pair mislead the detection. These results emphasize the importance of considering such methodology related differences while studying the physics of ionospheric perturbations.