HR: 08:30h
AN: G51D-03    [Abstracts]
TI: Strong Ionospheric Disturbances Observed by a Dense GPS Array After Large Earthquakes: Case Study of the 2003 Tokachi-oki Earthquake and its Geophysical Mechanism
AU: * Heki, K
EM: heki@ep.sci.hokudai.ac.jp
AF: Div. Earth Planet. Sci., Hokkaido Univ., N10 W8, Kita-ku, Sapporo, HOKKAIDO, 060-0810 Japan
AU: Ping, J
EM: jsping@miz.nao.ac.jp
AF: National Astron. Obs., 2-12 Hoshigaoka, Mizusawa, IWATE, 023-0861 Japan
AB: Ionospheric disturbances have been detected after, e.g. Northridge (Calais and Minster, 1995) and Denali (Ducic et al., 2003) earthquakes. Similar signals observed after the 2003 Tokachi-Oki Earthquake, the largest earthquake in Japan after the completion of GEONET, a nationwide array composed of over 1000 CGPS stations. We followed a standard procedure: applying a band-pass filter for the ionospheric combination of the L1 and L2 phase signals and calculating subioonospheric points (SIP) assuming thin ionosphere at the height of 350 km. Owing to the high density of SIP, many interesting features are observed and several important parameters were constrained, e.g. (1) apparent propagation speed, (2) directivity of disturbance signals, (3) decay during propagation, etc. As for (1), the observed speed of about 1 km/sec is significantly smaller than the Rayleigh Wave velocity, significantly faster than Travelling Ionospheric Disturbances (TID), but is consistent with the sound velocity at the ionospheric heights. The acoustic wave generated by sudden vertical movement of the Earth's surface first propagate upward. Then it will be refracted by height-dependent velocity structure resulting in horizontally propagating wave through the ionosphere. The observed TEC variation, with a wavelength of a few hundred km, may reflect electron density oscillation caused by the passage of such an acoustic wave. Regarding (2), there was a clear indication that the wave does not propagate northward. As first suggested by Calais et al. (1998), such a blocking is considered to be due to interaction between the geomagnetic field and the movement of charged particles comprising the ionosphere associated with the acoustic wave propagation. The model predicts that there will be no southward propagation of ionospheric disturbances caused by earthquakes in southern hemisphere mid-latitudes, which needs be confirmed by future earthquakes. The point (3) enabled the authors to define the empirical equation to calculate "Ionospheric disturbance magnitude" using the focal distance and disturbance amplitudes. Because the ionospheric disturbance monitoring does not require precise orbit information, such magnitudes could be determined near real time. This may help us, e.g. issue early warning message of Tsunami.
DE: 6984 Waves in plasma
DE: 7215 Earthquake parameters
DE: 2435 Ionospheric disturbances
DE: 1223 Ocean/Earth/atmosphere interactions (3339)
DE: 1243 Space geodetic surveys
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting