11:30 AM - 11:45 AM
[PEM11-10] Ionospheric ring current system accounting for ground-level magnetic observations during the 2011 Tohoku earthquake tsunami
Keywords:ionosphere, 2011 Tohoku Earthquake , tsunami, magnetic field, E region, dynamo
Large earthquakes and tsunamis cause acoustic waves reaching the ionosphere and drive many related electromagnetic phenomena. Variations in the total electron content were reported by many papers, which include effects of acoustic waves not only from tsunamis but also from Rayleigh waves (e.g. Tsugawa et al. 2011). The associated ionospheric dynamo in the E region generated observable magnetic variation at the ground level, as implied by numerical simulations of Kherani et al. (2013). However, the form of the electric current system due to the E-region dynamo has never been revealed from the perspective of geomagnetic observations at the ground level.
To reveal the current system in the ionosphere during the 2011 Tohoku earthquake event, we analyzed 1-min sampling magnetic data at 20 on-land stations on the Japan Arc and at one seafloor magnetometer ~200 km off the Tohoku. We, first, subtracted the effects of non-seismic magnetic variation from the magnetic data, using inter-station magnetic transfer functions between the stations of interest and a reference site in China, Changchun. We further subtracted magnetic variation due to the oceanic electric currents induced by the tsunami propagation by the time-domain three-dimensional simulation of Minami et al. (2017). In the rotated coordinate system where the horizontal axes consist of radial and transverse ones in terms of the tsunami centre, we found a clear concentric propagation of magnetic variation with phase velocities of ~3.6 km/sec in the period band of 3 to 10 min and ~0.9 km/sec in the period band of 15 to 40 min. The former corresponds to the speed of Rayleigh wave, while the latter does to the speed of the acoustic wave in the ionosphere. The polarities of the radial and downward components of the radially propagating magnetic variation imply the presence of anti-clockwise electric currents in the ionosphere in the both period bands. We tried to express the corresponding electric current system in the ionosphere by fitting a steady ring current model without induction to the magnetic data. Our preliminary ring current model successfully explains the concentric propagating features and the long-lasting offset in the radial and downward components of the magnetic variation at most of the observation sites. In the presentation, we will report our analysis of the geomagnetic data at ground level and describe their concentric propagation during the 2011 Tohoku earthquake tsunami event. Furthermore, we will discuss possible ionospheric ring current system accounting for the observed magnetic variation at the ground level.
To reveal the current system in the ionosphere during the 2011 Tohoku earthquake event, we analyzed 1-min sampling magnetic data at 20 on-land stations on the Japan Arc and at one seafloor magnetometer ~200 km off the Tohoku. We, first, subtracted the effects of non-seismic magnetic variation from the magnetic data, using inter-station magnetic transfer functions between the stations of interest and a reference site in China, Changchun. We further subtracted magnetic variation due to the oceanic electric currents induced by the tsunami propagation by the time-domain three-dimensional simulation of Minami et al. (2017). In the rotated coordinate system where the horizontal axes consist of radial and transverse ones in terms of the tsunami centre, we found a clear concentric propagation of magnetic variation with phase velocities of ~3.6 km/sec in the period band of 3 to 10 min and ~0.9 km/sec in the period band of 15 to 40 min. The former corresponds to the speed of Rayleigh wave, while the latter does to the speed of the acoustic wave in the ionosphere. The polarities of the radial and downward components of the radially propagating magnetic variation imply the presence of anti-clockwise electric currents in the ionosphere in the both period bands. We tried to express the corresponding electric current system in the ionosphere by fitting a steady ring current model without induction to the magnetic data. Our preliminary ring current model successfully explains the concentric propagating features and the long-lasting offset in the radial and downward components of the magnetic variation at most of the observation sites. In the presentation, we will report our analysis of the geomagnetic data at ground level and describe their concentric propagation during the 2011 Tohoku earthquake tsunami event. Furthermore, we will discuss possible ionospheric ring current system accounting for the observed magnetic variation at the ground level.