Ionospheric ripples excited by superimposed wave fronts associated with Rayleigh waves in the thermosphere

Coseismic ionospheric disturbances (CIDs) associated with the 2011 Tohoku earthquake off the Pacific coast (M_w 9.0, Tohoku EQ) were examined using total electron content and seismic wave data. A faster CID propagated at ~3.0 km/s only in the west-southwest, while a slower CID propagated concentrically at 1.2 km/s or slower from the tsunami source area. Taking the propagation speed and oscillation cycle into account, the faster CID was associated with a Rayleigh wave, but the slower CID was associated with an acoustic or gravity wave. The north-south asymmetry of the CID associated with the Rayleigh wave suggests that the Rayleigh wave did not act as a point source of the acoustic wave because a point source propagating in all directions must produce symmetric CID in all directions. Therefore, a superimposed wave front of acoustic waves was excited by the Rayleigh wave and produced the north-south asymmetry of the faster CID due to the magnetic inclination effect, which is different from a well-known north-south asymmetry of CID excited at the epicenter. On the other hand, above and south of the tsunami source area, the CID with a period of 4 min was excited by a point source located at the tsunami source area because atmospheric waves propagating from a point source produce north-south asymmetry in the resulting CID.