Takesi Yukutake

A stratified core motion inferred from geomagnetic secular variations

Abstract An examination of the westward drift of the geomagnetic field indicates that the drift velocity is almost independent of latitude, suggesting a uniform rigid rotation of spherical shape. When the geomagnetic field is separated into standing and drifting components and expressed in a spherical harmonic series, a lack of sectorial terms is noted in the standing field. It is shown that these features are well explained by a stratified core model. The core is supposed to be stratified near the surface where toroidal fluid motions are predominant. In the deeper part, the fluid motion is two-dimensional, forming Taylor columns. A simplified core model is assumed to represent these features, in which the core is divided into two parts, an outer spherical shell that rotates westwards at a uniform rate of 0.3° y −1 and a central sphere in which the two-dimensional columnar motions reside. The toroidal motions in the outer spherical interact with the dipole field to induce the drifting field, whereas the columnar motions generate the standing field through interaction with a toroidal field. It follows that a small velocity as 5 × 10 −3 cm s −1 for the stratified motion is sufficient to create the observed drifting field.

Review of the geomagnetic secular variations on the historical time scale

Abstract In view of the classification of the geomagnetic field into its axisymmetric and non-axisymmetric parts, studies of geomagnetic secular variations on the historical time-scale are reviewed. The westward drift of the geomagnetic field, which is one of the most conspicuous features of its secular variation, is examined first. The non-axisymmetric field during the past several hundred years can be well approximated by the superposition of two constant-magnitude fields, a standing and a drifting field, whose lifetimes are supposed to be longer than 1000 years. It is pointed out that the sectorial term of the non-dipole standing field is small compared with the drifting one. The lack of the n = m = 2 term of the standing field is particularly remarkable. On the other hand, the equatorial dipole field is likely to consist of two components which are both drifting. One drifts westwards with a normal velocity and the other eastwards with a small velocity. Besides the pronounced westward drift in an east-west direction, the poleward movements of particular foci of the secular variation are noted. This may, however, be related to the rapid growth of the axisymmetric quadrupole field. The time variation of the dipole field is briefly examined. As far as the data on the historical time scale are concerned, an antiparallel relationship seems to exist between the variations in the dipole and the quadrupole field. As the dipole moment decreases, the magnitude of the quadrupole moment increases. Finally, characteristic oscillation periods of the dipole field are examined. Although the data are few, a 60–70-year period, a 400–600-year and a 8000-year period emerge as the dominant periods.

A preliminary study on variations in the Gauss coefficients of the geomagnetic potential over several hundred years

Abstract Geomagnetic secular variations are examined in terms of time variation in the Gauss coefficients. Major parts of the variations over several hundred years can be represented by a two mode model which consists of a standing and a drifting field. When the Gauss coefficients are plotted on a diagram with g n m in the abscissa and h n m in the ordinate, the drifting component describes a circle. However, some of the observed coefficients depict an elliptical trajectory rather than a circular one. Improvement of the model is attempted in two different ways. One is to assume time variability of the amplitude of the drifting component. The other is to introduce another drifting mode. Selecting a few spherical harmonic terms, variations in the Gauss coefficients since A.D. 1600 are analysed. When the amplitude of the drifting field is assumed to vary, the observed nature of the elliptical trajectory is well represented. In this case, phase velocity also changes with time. It is large while the amplitude is small, and it is small while the amplitude is large. Three mode models, in which an eastward drifting mode is incorporated, approximate the observed variations as well, not only for the period over several hundred years but also for the last several decades. In this model the westward drifting mode dominates the eastward mode.

EMRIDGE: The electromagnetic investigation of the Juan de Fuca Ridge

From July to November 1988, a major electromagnetic (EM) experiment, known as EMRIDGE, took place over the southern end of the Juan de Fuca Ridge in the northeast Pacific. It was designed to complement the previous EMSLAB experiment which covered the entire Juan de Fuca Plate, from the spreading ridge to subduction zone. The principal objective of EMRIDGE was to use natural sources of EM induction to investigate the processes of ridge accretion. Magnetotelluric (MT) sounding and Geomagnetic Depth Sounding (GDS) are well suited to the study of the migration and accumulation of melt, hydrothermal circulation, and the thermal evolution of dry lithosphere. Eleven magnetometers and two electrometers were deployed on the seafloor for a period of three months. Simultaneous land-based data were made available from the Victoria Magnetic Observatory, B.C., Canada and from a magnetometer sited in Oregon, U.S.A.Changes in seafloor bathymetry have a major influence on seafloor EM observations as shown by the orientation of the real GDS induction arrows away from the ridge axis and towards the deep ocean. Three-dimensional (3D) modelling, using a thin-sheet algorithm, shows that the observed EM signature of the Juan de Fuca Ridge and Blanco Fracture Zone is primarily due to nonuniform EM induction within the ocean, associated with changes in ocean depth. Furthermore, if the influence of the bathymetry is removed from the observations, then no significant conductivity anomaly is required at the ridge axis. The lack of a major anomaly is significant in the light of evidence for almost continuous hydrothermal venting along the neo-volcanic zone of the southern Juan de Fuca Ridge: such magmatic activity may be expected to have a distinct electrical conductivity signature, from high temperatures, hydrothermal fluids and possible melt accumulation in the crust.Estimates of seafloor electrical conductivity are made by the MT method, using electric field records at a site 35 km east of the ridge axis, on lithosphere of age 1.2 Ma, and magnetic field records at other seafloor sites. On rotating the MT impedance tensor to the principal axis orientation, significant anisotropy between the major (TE) and minor (TM) apparent resistivities is evident. Phase angles also differ between the principal axis polarisations, and TM phase are greater than 90° at short periods. Thin-sheet modelling suggests that bathymetric changes accounts for some of the observed 3D induction, but two-dimensional (2D) electrical conductivity structure in the crust and upper mantle, aligned with the ridge axis, may also be present. A one-dimensional (1D) inversion of the MT data suggests that the top 50 km of Earth is electrically resistive, and that there is a rise in conductivity at approximately 300 km. A high conductivity layer at 100 km depth is also a feature of the 1D inversion, but its presence is less well constrained.

Free decay of non-dipole components of the geomagnetic field

Abstract Free decay of the non-dipole magnetic field is studied when the core is rotating relative to the conducting mantle. In the first place, the decay time for a stationary core is calculated, giving the time constants of 7300 years for the field n = 2 and 4400 years for n = 3. Secondly the effects of the weakly conducting mantle both on the decay constants and on the rotation of the fields are discussed. The results show, however, that these effects are negligibly small (less than 1%) when the conductivity of the lower mantle is assumed to be 10 −9 emu.

Seismic resistivity changes observed at Aburatsubo, central Japan, revisited

Abstract Earths resistivity variation has been continuously recorded in a vault at Aburatsubo, central Japan, since 1967. In the previous works, tidal, coseismic, and precursory resistivity variations were reported, which were explained by strain-induced changes. Based on the recent data obtained by a digital recorder and laboratory experiments newly conducted on the rocks electrical properties, we have reexamined the nature of the seismic resistivity changes. New findings so far obtained from in-situ observation are as follows. (1) A coseismic step takes place on arrival of the S-wave rather than that of the P-wave. (2) The polarity of the coseismic step shows no correlation to the source mechanism of the earthquake, but shows a clear seasonal variation; i.e. positive steps appear in summer but are negative in winter. (3) There is a seasonal variation in the Earths resistivity with minimum and maximum in October and in April, respectively. The polarity of coseismic resistivity steps has a good correlation with the rate of seasonal variation of the ground resistivity itself. Combining these observations with results of recently made laboratory experiments, we present a tentative model for coseismic resistivity step generation which proposes mixing of temperature stratification in the shallow ground due to ground oscillation by seismic S-waves.

OFFSHORE EMSLAB: objectives, experimental phase and early results

Abstract A large electromagnetic (EM) experiment dedicated to the exploration of lithosphere and asthenosphere associated with a spreading oceanic plate over its various tectonic regimes, from ridge accretion to subduction, was carried out during the second half of 1985 over an area extending from the Juan de Fuca Ridge, eastward across the coastal zone, the Cascade area and beyond. Referred to by the acronym ‘EMSLAB’, for ‘EM Sound of Lithosphere and Asthenosphere Beneath’ the Juan de Fuca Plate, the experiment involved two principal arrays, one on land, the other on the adjacent seafloor. We report here on the outcome of the oceanic portion of EMSLAB, named OFFSHORE EMSLAB for convenience. The OFFSHORE EMSLAB array of seafloor instrumentation included 40 self-contained and free recording units including magnetometers, electrometers and other oceanographic devices. The latter were intended to provide information on the EM fields generated by the interaction of oceanic motions with the main Earths field, so as (1) to decontaminate ionospheric signals prior to magnetotelluric interpretation, and (2) to illustrate the beneficial contributions that EM observations may provide to ocean studies. The collected database is described and assessed, and an early illustration of its information content is given.

A model of the geomagnetic 60-year variation

A model of the geomagnetic 60-year variation is proposed. The 60-year variation is regarded as a magnetohydrodynamic oscillation originating in a thin layer at the top of the core. Two types of magnetic field and two types of fluid motion are mainly considered in the layer, which are poloidal magnetic mode (1, 0), toroidal magnetic mode (2, 0), toroidal velocity mode (1, 0), and poloidal velocity mode (2, 0). When the magnetic force and the Coriolis force are dynamically dominant, these four fields make a closed system and cause magnetohydrodynamic oscillations in the layer. Steady magnetic fields of higher modes generated in the deeper part of the core induce fluctuating fields when they pass through the layer. The induced magnetic fields have a common period and two different types of phase. Observed amplitudes of these fields are mostly explained when the maximum velocity in the layer is 10−4 m/s.

A review of studies on the electrical resistivity structure of the crust in Japan

Electromagnetic investigations of active faults revealed existence of low resistivity zones that had developed along the fault. Water bearing rocks in the fault fractured zone are considered to be responsible for the low resistivity. In some cases, surface water is likely to be connected to the focal depth of 15 to 20 km through the low resistivity fault.

A generating process of geomagnetic drifting field

The geomagnetic field is comprised of drifting and standing fields. The drifting field has two remarkable features. One is predominance of sectorial harmonics when the field is expressed in a spherical harmonic series, and the other is uniform drift rate irrespective of harmonics. We consider that the drifting field is a product of interaction of the core flow with the axial dipole field near the surface of the core. The key to the predominance of sectorial harmonics is in the boundary condition on the electric current at the core–mantle boundary. If we take the mantle to be an electrical insulator, the electric current normal to the boundary must vanish. This strongly constrains the surface flow. The toroidal flow becomes the flow with the sectorial harmonics predominant. Then, the sectorial toroidal flow, interacting with the axial dipole field, induces the poloidal field in which the sectorial harmonics are predominant. This is the observed type of drifting field. The uniform drift rate, the second nature of the drifting field, seems to suggest that the surface part of the core is rotating westwards as a whole. Subsequently, the sectorial type toroidal flow embedded in the westward-rotating surface layer is considered as the cause of the drifting field.