Abstract:
GPS-derived Total Electron Content (TEC) is an integrated quantity; hence it is difficult to relate
the detection of ionospheric perturbations in TEC to a precise altitude. As TEC is weighted by the
maximum ionospheric density, the corresponding altitude (hmF2) is, generally, assumed as the
perturbation detection altitude. To investigate the validity of this assumption in detail, we conduct an
accurate analysis of the GPS-TEC measured early ionospheric signatures related to the vertical surface
displacement of the Mw 7.4 Sanriku-Oki earthquake (Sanriku-Oki Tohoku foreshock). Using 3D acoustic
ray tracing model to describe the evolution of the propagating seismo-acoustic wave in space and time,
we demonstrate how to infer the detection altitude of these early signatures in TEC. We determine that
the signatures can be detected at altitudes up to ~130 km below the hmF2. This peculiar behaviour is
attributed to the satellite line of sight (LOS) geometry and station location with respect to the source,
which allows one to sound the co-seismic ionospheric signatures directly above the rupture area. We
show that the early onset times correspond to crossing of the LOS with the acoustic wavefront at lower
ionospheric altitudes. To support the proposed approach, we further reconstruct the seismo-acoustic
induced ionospheric signatures for a moving satellite in the presence of a geomagnetic field. Both the
3D acoustic ray tracing model and the synthetic waveforms from the 3D coupled model substantiate
the observed onset time of the ionospheric signatures. Moreover, our simple 3D acoustic ray tracing
approach allows one to extend this analysis to azimuths different than that of the station-source line.
