Wavelet-based detection and identification of atmospheric-induced ionospheric disturbances in GNSS signals
Abstract
Acoustic-Gravity Waves (AGWs) in the neutral atmosphere are known to induce disturbances in the distribution of electrons in the ionosphere. Ionospheric disturbances can be observed as small variations in the Integrated Electron Content (IEC), which are measurable through phase advance in trans-ionospheric Global Navigation Satellite System (GNSS) signals. Substantial previous studies on the GPS-derived ionospheric disturbances have been presented for understanding the interactions between ionospheric perturbations and solid Earth's events such as volcano eruptions, earthquakes and tsunamis. In this research, we proposed a wavlet-based method for detecting, identifying and classifying ionospheric disturbances, from IEC time series. A generalized wavelet coherence defined for an ensemble of multiple IEC time series was used for detecting and isolating the potential disturbances within sub-areas measurements of a large and dense GNSS network. The findings in time-frequency space (or phase space) corresponding to each strong coherence structures were applied to the reconstructions (or filtering) of the identified disturbance signal. The speed and direction of each identified disturbance was estimated by cross-correlating pairs of each reconstructed signal in each sub-area. The proposed method is, then, applied to study ionospheric disturbances observed following two North Korean underground nuclear tests and three earthquake events producing tsunamis. The propagation direction and speed of the identified disturbances were then compared with ground motion observations, tide gauge measurements, tsunami simulations by MOST (Method of Splitting Tsunami) model as well as earth geomagnetic field. MOST is a suite of numerical simulation codes capable of simulating three processes of tsunami evolution: earthquake, transoceanic propagation, and inundation of dry land. A week of data centered on the event dates of three earthquakes and tsunamis as well as two underground test events were used for this study. On the study of two UGTs, we observed two different types of TIDs with two dominant wave frequency bands% 0.0027-0.0083Hz and . The short period ionospheric disturbances propagates with speeds up to 600m/s, and the propagation speeds of long period disturbances are in the range from 61 - 273 m/s. For earthquake and tsunami cases, this analysis is demonstrated on data from 1235 stations in the Japanese GEONET GPS network, 48 stations in Hawaii, 47 stations in Southern America, 174 stations in New Zealand and 253 stations in the west coast of the United States. Four different classes of ionospheric disturbances were identified from the measurements of the above GNSS networks for the tsunami and earthquake events. The short-period (2 - 6 minutes) disturbances with speeds up to 3 km/s were observed in the near-fields, and the long-period (8-22 minutes) disturbances with speeds (195 - 354m/s) were identified in both near- and far-fields. In addition, we have conducted the a study of comparative observations of traveling ionospheric disturbances and other geophysics measurements that were observed on the day of the 2011 March 11 Tohoku-Oki Earthquake and Tsunami event. Through use of the wavelet coherence analysis, we are able to find major wave trains, present in the data collected from these networks, with two dominant frequency bands propagating in four different speed ranges.
Degree
Ph.D.
Advisors
Garrison, Purdue University.
Subject Area
Aerospace engineering|Electrical engineering
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