Ionospheric perturbations associated with 26th December 2004 Indian ocean tsunami: A detailed investigation through Indian Geodetic GNSS network observations

Ionospheric perturbations associated with Indian ocean tsunami triggered by the 26th December 2004 was reported by Liu et el. (2006), Lognonne (2006), Das gupta et al. (2006) and many authors numerically modelled this ionospheric perturbations [e.g. Occhipinti et al. (2006)]. All those previous reports and observations are from few IGS stations in this region other than Das gupta et al. (2006). Das gupta et al.(2006) reported the TEC enhancement associated with the earthquake from observations of few Indian GPS stations. However, no detailed analysis using Indian GPS stations segregating the ionospheric perturbations associated with tsunami is reported so far. Observations from the regional network and detailed analysis will help to to refine the numerical models as well as to understand the physics of tsunamigenic ionospheri perturbations. In this paper, We present a detailed investigation on perturbations in the ionospheric Total Electron Content (TEC) induced by the gravity waves generated by open ocean Tsunami triggered by the 26th December 2004 Sumatra-Andaman earthquake using observations from Indian Geodetic GNSS network. Indian geodetic GNSS network consists of more than twenty continuous mode geodetic GNSS stations spread across Indian subcontinent. GPS code and phase observations from all these stations and IGS stations in this region have been used to estimate TEC and geometry-free differential TEC (dTEC) representing ionospheric perturbation using the software IONODETECT developed at CSIR 4PI. The non-tsunamigenic perturbations have been filtered out from the geometry-free dTEC using a bandpass filter ranging 0.3 to 3.3mHz following Lognonne et al (2006). We also show the applicability of data from low elevation observations which are usually discarded to avoid noise due to multi-paths (multipath-free perturbation signals are ensured through multi-PRN multi-receiver combination). Using low elevation observations, for the first time, we present perturbations over Arabian sea associated with diffracted tsunami waves around the peninsular India, which preceded tsunami arrival at tide gauges in the west coast, in addition to observations over the Indian ocean and Bay of Bengal. The earliest detection of ionospheric perturbation at sub-ionospheric pierce points (SIP) associated with tsunami is from PRN 28 at 2-3 UTC over Indian ocean - roughly an hour before tsunami reaches the east Indian coast. Similar perturbations observed from other PRNs at various SIP and its propagations are discussed in detail. The perturbations observed with respect to distance from source of the earthquake correlate well with the direction and speed of the tsunami. The results are also compared with tsunami simulation and tide gauge data which indicates the cause of perturbations are gravity waves generated by tsunami. These results further strengthen the confidence on possibilities of early warning of devastating tsunamis using ground based GPS observations.