Virtual array beamforming of GPS TEC observations of coseismic ionospheric disturbances near the Geomagnetic South Pole triggered by teleseismic megathrusts

Abstract

We identified coseismic ionospheric disturbances (CID) in Antarctica generated by the 2010 Maule and the 2011 Tohoku‐Oki earthquakes analyzing total electron content (TEC) data with a modified beamforming technique. Beamforming in Antarctica, however, is not straightforward due to the effects of array deformation and atmospheric neutral wave‐ionospheric plasma coupling. We take these effects into account and present a method to invert for the seismically generated acoustic wave using TEC observations. The back azimuths, speeds, and waveforms obtained by the beamform are in excellent agreement with the hypothesis that the TEC signals are generated by the passage of Rayleigh waves from the Maule and Tohoku‐Oki earthquakes. The Tohoku‐Oki earthquake is ~12,500 km from Antarctica, making this the farthest observation of CIDs to date using GPS.

Characterizing Coseismic Ionospheric Disturbance for Surface-Rupturing Earthquakes

“…Gõmez et al (2015) neglected amplitude effects and focused only on phase. In a similar manner, we normalize the TEC traces prior to backprojection; however we do not deconvolve the TEC signal to estimate the acoustic signal.…”

“…Due to the fact that the backprojection relies on phase shifts and interference patterns across the array, initial phase of the TEC is important. Gõmez et al (2015) demonstrated that the phase shift caused by the coupling into the ionosphere and the LOS integration cause the TEC phase to vary dramatically from the acoustic wave phase. Thus due to the geometry of the system when TEC is measured, the phase can be different from the acoustic wave.…”

“…Using a plane-wave approximation and far-field simplifications, Gõmez et al (2015) computed the phase shift analytically and then deconvolved the TEC signal to recover the acoustic wave prior to beamforming analysis. Gõmez et al (2015) neglected amplitude effects and focused only on phase.…”

“…One method to do this is to invert for the acoustic wave and was first suggested by Gõmez et al (2015). The TEC is equal to the acoustic wave times a transfer function in the frequency domain.…”

“…In order to analyze the applicability of the analytical model used by Gõmez et al (2015), I also run the model using a simplified 1D divergence that is comparable to the analytic model.…”