Seki and Ito (1993) showed that the geomagnetic polarity reversals had a power-law distribution and presented a simple model in which the geodynamo was assumed to be a system of magnetic spins in a critical phase-transition state. We present an improved, more realistic model, and obtain a power exponent in agreement with the observed value, which is about -1.5. The revised model is a Coupled Map Lattice (CML). A CML is a dynamical system with discrete time and space, but continuous state. In the present model, elementary dynamo evolves autonomously according to the Lorenz map obtained from Rikitake dynamo dynamics. We examine the behavior of the system and the distribution of polarity reversal intervals for various values of parameters. We find some sets of the parameters which yield a power exponent close to -1.5.

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The total within-site dispersion of paleomagnetic data results from natural misalignment processes and experimentally produced dispersion. Although some of the sources of dispersion may be determined by experiments, this is not possible for others. The total accuracy of the paleomagnetic method so far has been determined only on historic lava flows. Thirty-seven Quaternary basaltic lava flows from the Eifel, Germany, have been re-sampled using the same outcrops as before. The new site-mean directions do not differ significantly from those of the original study. The angular distance between pairs of site-mean directions may be approximated by a Fisher distribution. Assuming that both studies are affected by the same natural dispersion processes, we obtain as the best value for the experimental dispersion s_exp = 4.33°. The total within-site dispersion shows a log-normal distribution with a mean of s_tot = 5.39°. From these values we calculate that the dispersion due to natural processes in the Eifel volcanic field is s_nat = 3.21°.

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We conducted onshore and offshore magnetic surveys on and around the southern Boso Peninsula, Honshu, Japan, and observed prominent large amplitude anomalies along the Mineoka Ophiolite Belt, and long wavelength low anomalies to the south of the belt containing short wavelength isolated anomalies. The magnetic structure was modeled by using three-dimensional magnetic prisms and basement with about 1 A/m of magnetization. At the Mineoka Belt, the top of the magnetic prisms is located at the ground surface, and these bodies are elongate in the vertical direction, with high angle magnetic inclinations. Magnetic basement exists at shallow depth beneath the belt. The magnetic basement traces the bottom surfaces of the magnetic prisms and forms a graben structure. In the south of the Mineoka Belt, thin sheet-like magnetic prisms with low magnetic inclinations are assumed at 1-3 km depth. The magnetic structure implies the tectonic process of the formation of the Mineoka Ophiolite Belt. The belt could be fragmented pieces of an oceanic plate emplaced at a paleo-plate boundary, which originated in low latitude and was transported by obduction to the present place via northward drift.

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A new index AES-80 was derived in order to provide a measure of the geomagnetic activity at high southern latitudes (Ballatore et al., 1998). The comparison between the southern hemisphere AES-80 with the northern hemisphere PC index is reported here for the period May 1994-April 1995. It is found that during southward IMF the two parameters record the geomagnetic disturbances rather similarly. In contrast, during northward IMF, only AES-80 is capable of recording geomagnetic activity occurring in the nominal polar cap region. In agreement with this finding, a slightly higher correlation coefficient is found between AES-80 and PC during southward IMF than during the northward IMF condition. Although at any time the two indices reflect opposite seasons, the overall correlations found between the two indices are significant, independent of the specific UT interval considered. The UT modulation (which should possibly be taken into account in the use of the indices) has been calculated for the AES-80 and the PC index. The results show that, for the specific period considered, the PC index dependence on UT is more significant than that of the AES-80 parameter.

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A new technique for continuation of the ground magnetic field caused by ionospheric currents to the ionosphere in spherical geometry is presented that makes use of elementary ionospheric current systems, which were introduced by Amm (1997) in extension of an earlier work by Fukushima (1976). The measured ground magnetic disturbance is expanded in terms of the ground magnetic effect of a spatial distribution of such elementary current systems. Using a matrix inversion technique, the scaling factors for each elementary current system, and therefrom the ionospheric equivalent currents are calculated. The technique can be applied to both global and local scales. Its advantages compared to the common field continuation techniques with Fourier (local scale), spherical cap (local to medium scale), or spherical (global scale) harmonic expansions are: 1) No fixed limitation of the spectral content has to be given for the whole analysis area, as it has to be done for the other techniques by truncation of a series expansion. 2) The locations of the elementary current systems can be chosen freely, such that they are most suitable with respect to the available measurement sites or the type of current system to be analysed. Results of the new technique are discussed in comparison to results of the spherical cap harmonic expansion method for a model of a Cowling channel.

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We present a possible theoretical model for the generation of travelling Alfven waves by external compressional disturbances in a two-dimensional inhomogeneous plasma. The transformation of a compressional wave mode into Alfven waves guided by the magnetic field is considered in a region with decreased Alfven velocity. This region represents, on the one hand, a waveguide for compressional oscillations, and on the other hand, a resonator with opaque boundaries for Alfven waves. The wave transformation is most effective in a resonant case, when the frequency of a compressional mode approaches the local Alfven frequency of a resonator. This model is applied to the interpretation of Pc3 observations in the cusp region. The mechanism can ensure the selective amplification of magnetosheath turbulence in the Pc3 band in the high-latitude cusp.

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Detailed magnetic surveys carried out in the Suruga trough area, where the Philippine Sea plate subducts beneath Japan, revealed some local magnetic anomalies on the overriding plate side. We first show that these anomalies reflect seamounts sitting on and subducting with the Philippine Sea plate, since no magnetic sources corresponding to the anomalies are identified in the seafloor topography. An important issue to be addressed is how these seamounts affect the generation of a great interplate earthquake at the Suruga trough (often referred to as the Tokai earthquake). So we made a numerical simulation of earthquake generation, applying the rate- and state-variable friction law to the plate interface. Here we incorporated the seamounts in the simulation as an additional normal stress exerted locally to the interface. We then found how the seamounts affect the generation of an interplate earthquake depends on whether or not they are located in the seismogenic zone. The most notable effect is that the amount of slip immediately before the main shock, often called a pre-slip, tends to be enhanced if they are located inside the zone.

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