We, for the first time, report the evidence of incessant excitation of the Earth's free oscillations, mainly the fundamental spheroidal modes in a frequency range from 0.3 to 5 mHz, based on the three year record of a superconducting gravimeter at Syowa Station, East Antarctica. The frequency-time spectrogram of this record is striped by more than 30 lines at nGal level parallel to the time axis, mostly corresponding to the fundamental spheroidal modes. This spectrogram is characterized by relatively efficient excitation of gravest fundamental modes, enhancement of signal intensities in the austral winter and amplification of signal in the frequency band from 3 to 4 mHz. Assuming that earthquakes are only the sources for the free oscillations, we calculate the synthetic spectrograms, which have not shown such a series of parallel lines as observed. The result of this synthetic test and characteristics of the observed spectrogram suggest that the mode signals we found are not of earthquake origin. We tentatively suggest atmospheric or oceanic origin for this newly discovered phenomenon of the solid Earth.

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Shape of magnetic hysteresis loops of the Chinese loess-paleosol sequence is variable with low-field susceptibility and is weakly constricted in samples with intermediate susceptibility (-1.0 × 10-6 m3/kg). The analyses of the hysteresis loops show that both low- (ferrimagnetic) and high-coercivity (antiferromagnetic) components are present and the ferrimagnetic component dominate the magnetic characteristics. The ratio of ferrimagnetic over antiferromagnetic minerals (S ratio) and the superparamagnetic fraction increase with increasing susceptibility. Neither simple two-component mixtures of ferrimagnetic and antiferromagnetic minerals nor of single-domain and superparamagnetic grains fully explain the constricted hysteresis loops. We interpret the variation of the loop shape with susceptibility in the following way. When the ratio of the ferrimagnetic to antiferromagnetic contribution is relatively low (low susceptibility), the broad loop is controlled by lithogenic ferrimagnetic and antiferromagnetic minerals. For samples with intermediate susceptibility values, constricted shape originates from an addition of a broad loop from the lithogenic fraction and a narrow loop from a pedogenic fraction with high superparamagnetic content. Then with further susceptibility increase, the constricted shape almost disappears and the loop is dominated by the pedogenic fraction. The variation of hysteresis loop shape with susceptibility can be a useful indicator of the degree of pedogenesis for loess-paleosol samples.

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Deconvolution of magnetic remanence which has been continuously measured along a long-core sample is improved by eliminating the disturbance of severe intensity variation confined to a narrow layer. The deconvolution is constrained by the smoothness of the magnetization measured by the L₂-norm of second order difference. The previous model uses a single smoothness parameter for the entire sample and the optimum smoothness is obtained by minimizing ABIC (Akaike's Bayesian Information Criterion), while an additional parameter is introduced in the modified scheme for a layer where intensity fluctuation is strong. The modified deconvolution scheme was applied to 5-mm interval magnetic remanence data for two sections of ODP Hole 767B. We assumed another smoothness for the uppermost 1.5 cm of Section 767B-6H1 where the coring disturbance is severe. For Section 767B-6H2, the Australasian microtektite layer (108-113 cm), where the magnetization intensity shows a strong peak, is expressed by a different smoothness parameter from other part. In both cases the minimum ABIC was lowered and the calculated error was reduced, demonstrating the improvement of the model by incorporating additional information other than magnetic remanence data in the prior distribution of the data.

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We have carried out a paleomagnetic study on well-dated Early Miocene volcanic rocks from the Ninohe area in northern NE Japan. Dacitic welded tuffs ranging from 24 to 21 Ma possess westerly paleomagnetic directions with normal polarity (formation-mean: D/I = 294.5°/44.2° with α95 = 8.3°, 8 sites), while the andesite flows formed at 17 Ma exhibit southerly directions with reversed polarity (D/I = 186.6°/-61.9°, 2 sites). A positive conglomerate test assures the stability of high-temperature components of remanent magnetization. Our results demonstrate that northern NE Japan rotated counter-clockwise through more than 60° between 21 and 17 Ma, most likely in association with the opening of the Japan Sea. The rotation of northern NE Japan thus preceded the about 15 Ma rapid clockwise rotation of SW Japan, and was synchronous with the possible pre-16 Ma southward translation of it. In comparison with published data, we further suggest that intra-arc block rotations occurred in the back-arc region of NE Japan during and after the arc rotation.

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The Japan Sea Cable (JASC) was retired from telecommunications services and its ownership transferred to the scientific community in February, 1996. For the first stage of its scientific reuse program, a Japan-Russia joint project has been initiated to monitor electrical potential differences across the Japan Sea to study the electrical conductivity distribution in the Earth and the water transport across the cable. This paper describes preliminary investigations by forward modelling study to explore possible results from the voltage observation, before analyzing real data. On the resistivity structure, modelling has suggested that voltage data is most sensitive to the conductance of resistive lithosphere, especially at longer periods. Water transport modelling has shown that the heterogeneity of sub-bottom resistivity does not greatly influence the cable voltage, and that water transport of 5-6 Sv in the Japan Sea can generate voltage differences of 70-80 mV across the JASC. A preliminary observation was found to be roughly consistent with this estimation.

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An application of Ampère's law to the MAGSAT orbit and data enabled us to study the net space current (total intensity I) flowing through the plane enclosed by the satellite orbit, and its dependence on the ground magnetic disturbance revealed in the Kp- or AE-index. Even on magnetically quiet days, MAGSAT often (or sometimes persistently) detected minor or moderate disturbances in the polar regions, in particular inside the auroral oval, without increasing Kp- or AE-values. Such disturbances are attributable to field-aligned currents into or out of the ionosphere, which produce a magnetic field (of toroidal nature) detectable above the ionosphere associated with a weak magnetic field on the ground, resulting in an occasional poor correlation of I with Kp- or AE-indices. The calculated I-values on quiet days are shown to be the order of 105 A, with a small-range UT variation. During magnetic storms or substorms the I-values become one order of magnitude greater, and the net space current is always antisunward, with its intensity roughly proportional to the AE-index values. The antisunward space current under the MAGSAT level is the Pedersen current in the ionosphere, and it constitutes the return current of the westward partial ring current (flowing at a distance of several earth radii in the dusk-side magnetosphere) along with the field-aligned currents between the partial ring current and the high-latitude ionosphere in the dayside and nightside hemispheres. The antisunward ionospheric current under the MAGSAT level contributes to a noticeable enhancement in the dawn-dusk asymmetry of the H-decrease at MAGSAT level in comparison with the ground data at the developing stage of magnetic storms or substorms.

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Regular variations of the F₂-layer in the quiet ionosphere are studied using the universal variation of the F₂-layer critical frequency, i.e. the dependence of the instantaneous mean-longitudinal value of foF₂ at a fixed latitude on universal time (UT). Algorithms for the universal variation calculation have been developed and are used to process the ionospheric vertical sounding data for 1957-1990. It is shown that the foF₂-universal variation forms are essentially different for solar minima and maxima years. In the solar minima years, the foF₂-universal variation is similar to the known universal variation of the atmospheric electric field near the Earth's surface which has a maximum at 19 UT. There is no foF₂-universal variation seasonal change in solar minimum years. The foF₂-universal variation form changes for solar maximum years have a minimum of foF₂, at 16-17 UT, and there are small seasonal changes in the foF₂-universal variation. Such behaviour of the foF₂-universal variation suggests that under the quiet conditions in solar minimum years the electric field penetrates from the lower atmospheric layers up to the F₂ layer of the ionosphere.

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The magnetic field data from the Engineering Test Satellite -VI (ETS-VI) have been analyzed to investigate the occurrence distributions of pulsations in Pc 3-5 frequency ranges in the magnetosphere. The observation of ETS-VI covered the invariant latitude (ILAT) range of 64.5°-69°ILAT near the geomagnetic equator (-10°-20° magnetic latitude) at all magnetic local time (MLT). Magnetic pulsations were selected by the Fast Fourier Transform method and checked by visual scanning if they have continuous waveforms. From the occurrence distributions of pulsations, we have found distinctive features in the following pulsations: (1) azimuthal Pc 5 pulsation; (2) azimuthal Pc 3 pulsation; (3) radial Pc 4 pulsation on the dayside; (4) azimuthal Pc 4 pulsations on the nightside. In respect of the first three types of pulsations (i.e., the azimuthal Pc 5 pulsation, the azimuthal Pc 3 pulsation, and the radial Pc 4 pulsation on the dayside), the results presented in this study confirm the previous results obtained by other satellite observations. The azimuthal Pc 4 pulsations on the nightside were observed in continuous waveforms lasting for about 10 minutes. Although the azimuthal Pc 4 pulsations on the nightside start at almost the same time as substorm onsets, they are different from Pi 2 pulsations in the magnetosphere. They are observed frequently in the MLT range of 23-04MLT with an occurrence peak at 01-02MLT.We suggest that the azimuthal Pc 4 pulsations on the nightside are excited through coupling to the fast mode Alfvén waves which were launched at substorm onset.

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Auroral tomography observations have been carried out in March, 1995, as a joint international campaign between Sweden and Japan. Three unmanned Swedish ALIS stations (Kiruna, Merasjärvi, Tjautjas) and two Japanese JICCD sites (Abisko, Nikkaluokta), geographically separated by about 50 km at higher latitudes, were operated to capture multi-station monochromatic tomography images at 557.7 nm wavelength using CCD cameras. All cameras were pointing to one of the predetermined directions to secure a common field of view. Several images of auroral arcs, mostly for the core region right above Kiruna, have synchronously been taken by the multi-station imaging system. Tomographic inversion analysis for four-point images was carried out using the algebraic reconstruction technique. Reconstructions of a curved arc and of a double arc system suggest promising application of this technique to the retrieval of three-dimensional auroral luminosity.

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The time series of the interplanetary and Earth's magnetotail magnetic fields are analyzed. The data subsets taken by the IMP 8 spacecraft outside and inside of the Earth's magnetosphere show clearly the fractal properties: their “box dimensions” are found to be close to 1.7 and 1.5 respectively. It is shown that data gaps have a little effect on the results. The magnetic field increment distributions are found to be non-Gaussian, but self-affine.

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