Coseismic disturbances in ionosphere have been observed after recent thrust earthquakes off the Pacific coast of Japan, with a dense Global Positioning System array. Positive pulses in electron content, as large as several times 10 15 electrons/m 2 and as long as 4-5 minutes, emerge ~10 minutes after earthquakes and propagate horizontally with apparent velocity of ~1 km/sec, close to the sound velocity at the ionospheric height. They may have been excited by coseismic uplifts of the sea surface initially as upward propagating compressive pulses of atmosphere, which were gradually refracted to propagate horizontally in ionosphere. The propagation has strong north-south asymmetry, and interaction between the movements of charged particles in acoustic waves and geomagnetic fields might have selectively attenuated the northward propagating disturbances.

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