Showing papers by "IPG Photonics published in 1986"

The region on the core—mantle boundary where a geostrophic velocity field can be determined from frozen-flux magnetic data

Abstract: Summary On the spherical surface S just below the core-mantle boundary layer, let u be the core-fluid velocity, Br the radial magnetic field, θ the colatitude, and φ=Br sec θ. In the approximation where the flux is frozen and u is tangentially geostrophic, u can be determined from Br and ∂tBr at all points of S which are connected to the geographical equator by level lines of φ. At the other points of S, u is determined by Br and ∂tBr only up to an unknown arbitrary tangentially geostrophic circulation around the level lines of φ. The frozen flux*** approximation will always fail in a ‘leaky belt’ of approximate width | ν▿2 (rBr) |/| ▿1▿1· (Bru) | radians centred on the ‘leaky curve’ where ▿1· (Bru) = 0. Here ν is the magnetic diffusivity of the core and r−1▿1 is the surface gradient on S. The leaky belt includes those points at which null-flux curves appear or disappear. The geostrophic approximation will always fail in a narrow belt centred on the geographical equator. Neither failure interferes with the determination of u from Br and φtBr unless the belt widths are wider than the horizontal scale of u. Numerical calculation of u from Br and φtBr can be carried out using an explicit complete basis for the space of geostrophic motions on S. The basis fields are linear combinations of one, two or three surface vector spherical harmonics, and as predicted by Benton, they force the geographical equator to consist always of the same fluid particles. Each basis field, and every tangentially geostrophic flow, produces constant core fluid pressure on the geographical equator.

Motions at the core surface derived from SV data

Abstract: Summary. Examples of core motions which generate the observed secular variation field – as given by various models for 1970 and 1980 – from the main field have been computed in the frozen flux approximation, assuming that the spectrum of the motion is of low degree and decreases with wave-number. No mode of degree > 4 in the expansion of the motion can be derived with any degree of confidence. Among the low degree modes, some appear to be stable (they come out with the same magnitude whatever the inversion scheme used). The flow made of these stable modes is then examined. An outstanding feature of the flow is the body westward drift. But it seems necessary, if one looks for such a regular flow, to consider both toroidal and poloidal components, which would imply upwelling and down-welling in the upper layers of the core. The toroidal part of the flow appears to be enhanced by the 1969 impulse, although its geometry is nearly unchanged. On the contrary the geometry of the computed poloidal part is different in 1980 and in 1970;

3‐dimensional structure of the Indian Ocean inferred from long period surface waves

Abstract: To improve the lateral resolution of the first global 3 - dimensional models of seismic wave velocities, regional studies have to be undertaken. The dispersion of Rayleigh waves along 86 paths across the Indian Ocean and surrounding regions is investigated in the period range 40 - 300 s. The regionalization of group velocity according to the age of the sea floor shows an increase of velocity with age up to 150 s only, similar to the results in the Pacific Ocean. But here, this relationship vanishes more quickly at long period. Therefore the correlation of the deep structure with surface tectonics seems to be shallower in the Indian Ocean than in the Pacific Ocean. A tomographic method is applied to compute the geographical distributions of group velocity and azimuthal anisotropy and then the 3-D structure of S-wave velocity. Horizontal wavelengths of 2000 km for velocity and 3000 km for azimuthal anisotropy distribution can be resolved. Except for the central part of the South East Indian ridge which displays high velocities at all depths, the inversion corroborates a good correlation between lithospheric structure down to 120 km and surface tectonics: low velocities along the central and southeast Indian ridges, velocity increasing with the age of the sea floor, high velocities under African, Indian and Australian shields. At greater depths, the low velocity zones under the Gulf of Aden and the western part of the Southeast Indian ridges hold but the low velocity anomaly of the Central Indian ridge is offset eastward. The low velocity anomalies suggest uprising material and complex plate boundary.