The observation of seafloor crustal deformation is very important to understand plate motions, nucleation processes and mechanisms of great interplate earthquakes as well as the activities of submarine volcanoes. We have been developing an observation system for seafloor crustal deformation. This system consists of two main components; (1) kinematic GPS positioning of an observation vessel and (2) accurate acoustic measurements of distances between a transponder attached on the side of the vessel (onboard station) and one located on the ocean bottom (seafloor station). In this study, we performed numerical simulations to estimate measurement errors with acoustic positioning assuming acoustic velocities in the sea water and the distribution of observation points around the single seafloor station. We found that the position of the seafloor station which we can obtain by analyzing travel-time data might have around 18-cm discrepancy with respect to its “true” position. Colombo et al. (2001) reported that the position of the vessel can be determined with about 10-cm error by kinematic GPS positioning. These results indicate that the system should be able to detect seafloor crustal deformation much larger than 28 cm, including pre-, co-, and post-seismic slips due to the large earthquakes at subduction zones, slow and silent earthquakes, etc. Therefore, we emphasize the importance of continuous observations with a nationwide geodetic observational network for seafloor crustal deformation.

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Latest Miocene-Pleistocene synrift sediments at Ocean Drilling Program sites 1109, 1115, and 1118 (Leg 180), located on the hanging wall margin north of the Moresby fault in theWoodlark Basin, showed variations in magnetic parameters carried by magnetite and maghemite related to sedimentation process in the basin. At sites 1109 and 1115, an increase in the sedimentation rate at 3.8 Ma was accompanied by the deposition of sediments with low ferrimagnetic mineral concentrations. An increase in the ferrimagnetic mineral concentrations occurred between 3.4 Ma and 3.2 Ma at the three sites. The onset age of the change became younger with distance from the subsidence center of the basin near the Moresby fault: 3.4 Ma at Site 1118, 3.3 Ma at Site 1109, and 3.2 Ma at Site 1115, which implies a northward onlapping of sediments with high ferrimagnetic mineral concentration. Sediments with finer-grained ferrimagnetic minerals were deposited between 2.3 and 2.0 Ma at sites 1118 and 1109 and later, 2.8 Ma at Site 1115 during a period of a low sedimentation rate. The upper parts of sites 1109 and 1115 had a diamagnetic contribution, which is attributed to relatively high concentrations of diamagnetic pelagic materials at a low sedimentation rate associated with the low frequency of turbidites.

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Magnetotelluric data from 323 sites in the Western Canada Basin have provided impedance tensors from which invariants based on Mohr circle analysis are calculated. From thresholds determined from an initial examination of the data, the invariant responses are used to explore the dimensionalities of subsurface structures. This leads to a classification of the electrical structure of the region with respect to dimensionality wherein zones with sites of similar nature are delineated, and demonstrates that the invariants can be used as a dimensionality indicator to infer regional geoelectrical complexity. It is observed that the dimensionality appears to increase with period and this result is consistent with the response from sounding a zone of increasing areal extent and may indicate more laterally complex structure at depth, but may also be due to fundamental limitations of galvanic distortion analysis.

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An imaging riometer (IRIS) was installed newly in the southern area of Brazil in order to investigate precipitation of energetic electrons into the South Atlantic Magnetic Anomaly (SAMA). An unusual ionospheric absorption event was observed in the nighttime (-20 h LT) near the maximum depression (D_st - -164 nT) and the following positive excursion during the strong geomagnetic storm on September 22-23, 1999. The unusual absorption that has short time-duration of 30-40 min shows two characteristic features: One feature is a sheet structure of the absorption appearing at the high-latitude part of the IRIS field-of-view, showing an eastward drift from the western to the eastern parts and subsequent retreat to the western part. Another feature is a meridionally elongated structure with a narrow longitudinal width (100-150 km) appearing from the zenith to the low-latitude part of the IRIS fieldof-view, enhanced simultaneously with the sheet absorption, and is subsequently changed to a localized structure. These features likely characterize precipitation of energetic electrons into the SAMA ionosphere, associated with substorm occurrences during the strong geomagnetic storm. From the eastward drift (-250 m/s) of the sheet absorption, precipitating electrons are estimated to be -20 keV energies, assuming plasmaspheric electric fields of 1.8 mV/m. However, no ionospheric effect due to the precipitating electrons was definitely detected by the ionosonde measurements at Cachoeira Paulista, separated eastward by about 1000 km from the IRIS station.

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In previous our papers (Sekiya and Ishitsu, 2000 and 2001), hydrodynamic stability of the dust layer in the solar nebula is investigated. However, these papers neglected the rotational effects, that is, the Coriolis and tidal forces. These forces may stabilize the shear instability of the dust layer. In this paper, the linear stability analysis with the Coriolis and without tidal force is done in order to elucidate the effects of the Coriolis force. Our results indicate that the growth rates of the instabilities are similar between the cases with and without the Coriolis force. However, we found a new type of instability which resembles the Lindblad resonance. This instability only emerges if the growth rate is similar to or smaller than the Keplerian angular frequency. The energy source of the instability is different from that of the shear instability.

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Magnetospheric substorms frequently have oscillating auroral phenomena associated with them, and which may be related to field line resonances. In this paper we present an analysis of photometer data which displays such oscillations in luminosity. We use data from the Gillam station in the CANOPUS array for April 1, 2000. Clear pulsations in magnetometer and photometer data are observed at 2.2 mHz. The latitudinal distribution of the photometer pulsations indicates that the auroral luminosity variations, caused by precipitating electrons and protons, are modulated by large scale global compressional modes (field line resonances). The proton and electron auroras were found to oscillate essentially out of phase. As well, the variation of phase across the peak in the luminosity resonance follows the pattern expected for the coupling of resonant Alfvén modes by normal compressional modes in the magnetotail.

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A multiple Pi2 event was observed in the midnight sector at the low-latitude ground magnetometer station. Meanwhile, particle fluxes in the midnight sector were measured during the multiple Pi2 event by the AMPTE/CCE spacecrafts in the L range from 4.8 to 5.8 and by geosynchronous satellite, S/C1984-129, at the dawn sector. We present an examination of the magnetic field changes and the associated particle behavior during the multiple Pi2 event. We argue that the temporal intensification of the dusk-to-dawn currents near the geosynchronous altitudes may trigger the Pi2 pulsations. The plasmas injected in the inner magnetosphere are possible candidate for driving the dusk-to-dawn currents.

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