Earth Planets Space, Vol. 63 (No. 7), pp. 891-896, 2011
Nozomu Nishitani1, Tadahiko Ogawa2, Yuichi Otsuka1, Keisuke Hosokawa3, and Tomoaki Hori1
1Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601, Japan
2National Institute of Information and Communications Technology, Tokyo 184-8795, Japan
3University of Electro-Communications, Tokyo 182-8585, Japan
(Received April 7, 2011; Revised July 7, 2011; Accepted July 8, 2011; Online published September 27, 2011)
Ionospheric responses to the 2011 off the Pacific coast of Tohoku Earthquake are studied using the SuperDARN Hokkaido radar, which is located at (43.5°N, 143.6°E) and which monitors the ionosphere over a wide horizontal area. The radar observed an oscillation of the vertical motion of the ionosphere with a period of about 1 to 2 min. The disturbance propagated northward, away from the epicenter with the velocity of about 6.2, 4.5, 3.9 and 3.5 km/s. The latter three values are basically consistent with the propagation of the Earth's surface waves reported in several previous studies. The propagation velocities decreased with time, which has not been reported in previous studies for this propagation velocity range. The peak-to-peak amplitudes of Doppler velocities of ground/sea scatter echoes observed by the radar were up to 200 m/s, which is considerably larger than previously-reported values using HF Doppler measurements, although they are not extremely large for this historical earthquake (M =9.0). This is the first time that ionospheric data have been obtained with high temporal (8 s) and spatial (22.5 km) resolutions following a giant earthquake, which enables us to discuss the detailed characteristics of the propagation of coseismic ionospheric disturbances.
Key words: Earthquake, ionospheric disturbance, SuperDARN, Rayleigh wave, acoustic wave, ground/sea scatter echoes, Hokkaido radar, high temporal resolution.