Deep Earth DIALOG

Number 6 Fall, 1992

The Third SEDI Symposium at Mizusawa, Japan


 

Introduction

The third SEDI Symposium was convened at Mizusawa, Japan, 6-10 July, 1992. The theme of the symposium was "Core-mantle boundary region: structure and dynamics". Prior to the beginning of the scientific portion of the program, a public seminar entitled "Universe and Our Earth" was held on Sunday, July 5, 1992, from 1:30 to 4:30 PM at the Daiko-Ichiban in Mizusawa. The speakers, Prof. F. D. Stacey of the University of Queensland and Prof. T. Yukutake of the University of Tokyo, lectured to an overflow a udience estimated to be 400. Stacey's lecture was translated, while Yukutake spoke in Japanese.

 The symposium was attended by 159 scientists from 14 countries. The scientific program of the symposium ran for 9 half-days and was divided into the following eleven sessions: 1. earth rotation and core-mantle coupling; 2. iron, alloys and silicate met al reactions; 3. plumes: structure composition and origin; 4. heterogeneity of the Earth's deep interior; 5. paleomagnetic inferences about the dynamo process; 6. modern geodetic approach to core studies; 7. progress in geodynamo theories; 8. properties o f core-mantle boundary and D" layer; 9. formation of the core; 10. convection in the mantle and core; 11. geomagnetic variation and core dynamics. Each session was headed by two or three lead-talks, followed by brief presentation of poster papers and c oncluded by a general discussion. 19 lead-talks and 107 posters were presented. Discussions were heated and very fruitful through all the sessions. A limited number of copies of the symposium abstracts are available at Prof. M. Ooe, National Astronomical Observatory, Mizusawa, Iwate Prefecture, 023, Japan. Plans are underway to publish proceedings of the symposium as a special issue of the Journal of Geomagnetism and Geolectricity.

Following the tradition of the first two SEDI symposia, the Mizusawa symposium was marked by a high level of scientific interaction among those in attendance, both during the scientific sessions and at various scheduled (and unscheduled) social events. A distinctive feature of the Mizusawa symposium was the high level of interest and support provided by the citizens and officials of the City of Mizusawa, Iwate Prefecture and the Commemorative Association for the Japan World Exposition (1970). (This inc luded a welcoming banner across the main street of Mizusawa.) Furthermore, the social program for accompanying persons was well planned and enjoyed by all who participated. Much of that success was due to the hard work of the organizing committees, head ed by Takesi Yukutake (Earthquake Research Inst., Univ. Tokyo). All the members of these committees deserve our congratulations and thanks, with special mention of Prof. Masatsugu Ooe (National Astronomical Observatory, Mizusawa) in charge of logistics o f the symposium, Dr. Shuhei Okubo (Earthquake Research Inst., Univ, Tokyo), general secretary of the local organizing committee, Prof. Masaru Kono (Dept. of Earth and Planetary Sciences, Tokyo Inst. Tech.), convener in charge of the scientific program and Prof. Tetsuo Sasao (National Astronomical Observatory, Mizusawa) for his overall support of the symposium as the head of the Mizusawa Branch of the Observatory. Also special thanks go to the Geophysics Program of the Earth Sciences Division of NSF for i ts financial support of US participants in the symposium.

What follows is a selective summary of some of the highlights of the symposium, compiled by O. Anderson, K. Lambeck, J.-L. Le Mou&eumll, D. Loper, P.M. Mathews, H.-C. Nataf and P. McFadden.
 

Earth rotation and core-mantle coupling

What does the variable rotation of the Earth reveal about the properties of, and dynamical processes in, the Earth's interior, and in particular, about the core-mantle interaction? This question was addressed in lead talks by John Wahr (Univ. Colorado, Boulder) and P.M. Mathews (Univ. Madras, India) and in eight poster papers summarized in brief oral presentations. The talk by Wahr and papers by Hans Greiner-Mai (GeoForschungsZentrum, Potsdam), R.S. Gross et al. (JPL, Pasadena and Robert Hooke Institute , Oxford), Y. Yokoyama (Univ. Industrial Technology, Kanagawa), and I. Naito & N. Kikuchi (National Astronomical Observatory, Mizusawa) dealt with Earth rotation variations on decadal time scales; nutations and related phenomena at nearly diurnal periods formed the subject of the talk by Mathews and of the papers by V.V. Bykova (Inst. Physics of the Earth, Moscow), and T. Sato et al (National Astronomical Observatory, Kyoto Univ. and Univ. Tokyo); and other aspects of Earth rotation were considered in pap ers by J. Nastula et al (Polish Academy of Sciences, Warsaw) and V.E. Zharov (Sternberg State Astronomical Institute, Moscow).
 
 

Decade fluctuations of l.o.d. and pole position

In the first lead talk, John Wahr reviewed recent works on the changes in length of day (l.o.d.) and in polar motion, on decadal time scales. He observed that effects of torques on the l.o. d. build up over these time scales and are likely to dominate over effects of moment of inertia changes, while in the case of polar motion, the reverse would be the case. L.o.d. changes on shorter time scales (< 4 years but longer than diurnal) are under stood in terms of angular-momentum exchanges with the atmosphere and oceans, and deformations associated with long-period tides; and the observed secular increase of 1.4 ms/cy, in terms of tidal braking (2 ms/cy), partially offset by an apparent decreasin g trend in the Earth's flattening as inferred from satellite determinations of the coefficient J2 in the Earth's gravity field. Could the remaining decade-scale variations be understood in terms of axial torques exerted on the mantle by the core through t opographic or electromagnetic coupling at the CMB? The torque exerted by the flow of core fluid relative to topographic features on the CMB is estimated from the seismologically determined topography taken together with the flow velocity estimated from c ore-surface-flow models constructed from the observed decade-scale variations of the Earth's magnetic field. The torque thus found predicts too much l.o.d variability, but its magnitude can be reduced drastically with slight changes in either the topograp hy or in the core-surface flow. As regards electromagnetic coupling, the torque is expressible as an integral involving the radial part br of the magnetic field at the CMB and the azimuthal part bf that is induced by relative motion between the conductin g region of the lowermost mantle and the core with its "frozen-in" magnetic flux. The toroidal part bfT of the azimuthal field cannot be uniquely determined from surface observations. However, a partial estimate, based on the secular variation of the po loidal part bfP, yields a torque whose variation, over nearly 100 years from 1880, appears quite similar to that of the torque on the mantle as inferred from the l.o.d. variations. Nevertheless, in order to account for the mean value of the torque, one h as to assume an additional large toroidal field (0.2 Tesla). As regards possible causes of polar motion on decadal time scales, torques would be inefficient, and fluid pressure changes at the CMB (leading to moment-of-inertia changes that affect polar mo tion) would be hard to estimate. Effects due to changes in water storage have been estimated, and appear to show an appreciable correlation, over the period since 1900, with ILS polar-motion records.

 Greiner-Mai concluded, from a comparison of l.o.d. variations with variations in the rotational motion of the core as inferred from the time series, extending back to the 17th century, of the geomagnetic field, that the former may be explained in terms o f core-mantle coupling through the magnetic field at the CMB; variations of the electromagnetic torque on the mantle produced by the core rotation variations were found to be nearly equal to those of the mechanical torques needed for the l.o.d. variations , provided the conductivity at the bottom of the mantle is about 3 x 10E3 1/Sm. But the equatorial components of the electromagnetic torque were found to be too weak, by an order of magnitude, to account for polar motion, and it was suggested that other m echanisms were needed. Yokoyama's paper, on the other hand, suggests the opposite conclusion. Considering the Gauss coefficients gnm and hnm of the geomagnetic potential (rather than the field as a whole), she found a rather close correlation between th e motion, since 1900, of the point having g21 and h21 as coordinates, on the one hand, and the path of the pole on the other. Amplitudes ~ 30 nT were found for oscillations of these coefficients with periods near 30 years, but no quantitative estimates of torques were presented. The results presented by Gross were on decadal variations, as obtained from a polar motion time series spanning 1899--1991 and a length of day series spanning 1656--1991 which were generated by combining the data from all indepen dent observations (made by a wide range of techniques from classical astrometric to modern geodetic ones) available from a variety of sources and agencies. The use of these time series for obtaining information on core-mantle coupling was discussed. Nait o, while confirming the explanation of l.o.d. variations on time scales up to 5 years in terms of angular-momentum exchange with the atmosphere (from a comparison of IRIS l.o.d. data with AAM from JMA data), suggested core-mantle interaction as the cause of l.o.d. variations on 5--10 year scales, citing an absence of any correlation between them and oceanic or atmospheric phenomena. In regard to polar motion, the movement of the residual excitation pole, found after removal of equatorial angular momentum from atmospheric sources, was in the same direction as that based on ILS data but with a 50% higher speed for which melting of the Greenland ice (greenhouse effect?) was offered as a possible explanation.
 
 

Nutations and related phenomena

The second lead talk, by Mathews, highlighted recent advances in observational and theoretical studies on nutations, and discussed the constraints on deep-Earth parameters following therefrom. A near-fourfold in crease, since 1986, of the precision of the estimates of nutation amplitudes, and a correction of 6 parts in 10E5 to the value of the Earth's dynamical ellipticity e ( consequent on a correction found from VLBI and LLR data analyses to the precession rate ) were the key advances on the observational side. The main developments on the theory front were a 10-fold increase in the accuracy of computation of the lunisolar gravitational potential and a consequent increase in the accuracy of the rigid-Earth nuta tion series to 10 microarcseconds, and generalization of the analytic theory of nutations of a nonrigid but idealized Earth to include inner-core effects and to provide a new resonance formula. Nutation amplitudes computed from this theory, when correcte d for the effects, according to available estimates, of mantle anelasticity (for a particular anelasticity model) and of ocean tides, yield almost complete agreement with the real parts of the VLBI-determined amplitudes when the newly corrected value is t aken for the ellipticity of the Earth, the ellipticity of the fluid core is taken to be 4.5% higher than that corresponding to the PREM Earth model, and PREM values are used for all other Earth parameters. The imaginary parts of the amplitudes also come into good agreement when magnetic coupling at the CMB is taken into account, provided a high conductivity ( 5 x 10E5 1/Sm is assumed for the lowermost 200 m or so of the mantle, and the rms magnetic field is taken to have 2.5 times the conventionally esti mated magnitude. An alternative approach to seeking the implications of nutation observations is to determine the region of parameter space within which the values of the Earth parameters should lie in order that the theoretical values of the nutation am plitudes at the various frequencies lie within the respective observed ranges. This approach leads to an upper bound on the ellipticity of the solid inner core, and rather narrow bounds on the effective ellipticity and a compliance parameter of the fluid outer core. The dependence of the actual bounds on the corrections used for mantle anelasticity, etc., leads to the possibility that discrimination among various models of anelasticity may be obtainable, besides more stringent constraints on Earth parame ters, from nutation studies, given the expected improvements in accuracy of determination of nutation amplitudes and in the estimation of ocean tide and other effects in the near future.

 The influence of CMB topography on dynamics of the fluid in the core, and thereby on the nutational motions, was the main topic of Bykova's paper. Possible resonances between a few of the important circular nutations and oscillation modes (of very high spherical harmonic order) of a homogeneous core were exhibited, but were deemed unlikely, for reasonable topographies, to provide an explanation for discrepancies between nutation observations and the theory for an ellipsoidal Earth. Sato presented estim ates of the eigenfrequency and the Q of the free core nutation, obtained from an analysis of superconducting gravimeter records at three Japanese sites. While the frequency was only slightly different from estimates from VLBI and from European SCGs, the Q value was about double that estimated from European tidal data, though still much less than the VLBI estimate.
 
 

Other aspects

Nastula reported that FFT analysis of a 140-year time series of pole positions from astrometry showed the variations in amplitudes of both the Chandler and the annual components of the Earth's wobble to have common periods of arou nd 40 and 20-25 years, suggesting a common origin for both. Zharov's paper suggested, on the basis of a correlation found between the atmospheric excitation function derived from the time series of Earth rotation parameters (pole positions and UT1--UTC), on the one hand, and the moments of the strongest earthquakes, on the other, that atmospheric processes on the relevant time scales are the cause of both Earth rotation variations and triggering of earthquakes.
 

Iron, alloys and silicate metal reactions

There were two invited papers pertaining to the silicate- metal reactions at core-mantle conditions that bear on the nature of the core-mantle boundary, the first of which was given by J. P. Poirier (Inst. Physique du Globe, Paris) and the second by E.I to (Okayama Univ.), K. Morooka, T. Katsuru and O. Ujikie.

 Poirier's presentation, entitled "Silicate-metal reactions and the nature of the core-mantle boundary",was concerned with how the experimental evidence constrained the predictions of the invasion of the core's metal into the mantle's silicate at the core -mantle boundary. Poirier reported on experiments in his group showing what happened when olivine or enstatite was mixed with iron or iron alloy metals and transformed at high pressure in a laser-heated diamond cell, while the metal was melted. Recovere d samples were examined by transmission electron microscopy and clearly showed that the metal is enriched in oxygen, and the oxides depleted in iron, and further that sulfur enhances the reactions. Molten metal wets the grain-boundaries of the solid phas e, leading Poirier to conclude that the core fluid can rise in the mantle by meters to tens of meters along boundaries. This invasion of metal is not sufficient, however, to significantly perturb the secular variation field observed at the Earth's surfac e.

 The presentation of Ito et al, "Reaction between molten iron and silicate melts at high pressure and temperature", led the authors to conclude that the enrichment of solutes in the outer core is by Si and O as the primary light elements. Ito reported on their experiment, where the starting materials are the same as in Poirier's, but carried out in a large volume press at about 2500°C and 10 to 26 GPa. Quenched samples were examined by x-rays, optical and scanning microscopes, and dispersive micro-analy ses. They found that Si dissolves into iron above 15 GPa; that silicate spherules were immersed in molten iron grains; and that the spherules apparently were immiscible liquid exsolved in the course of cooling. Thus they were led to conclude that coolin g of a seep magma ocean in the primitive earth would lead to extraction of a large amount of SiO2 into the proto-core, changing mantle chemistry from chondritic to perioditic.

 The five contributed talks consisted of two connected with core properties, especially phase diagrams, and three connected with lower mantle properties, especially thermal expansivity.

 O. Anderson's (UCLA) presentation, "The phase diagram of iron and the temperature of the inner core", was a re-evaluation of the phase diagram of iron in view of the recent reports of Tm by Böhler (1992), Ringwood and Hibberson (1990), and Saxena (1992). All of these experiments verify reports of the Tm of iron by Böhler et al (1990) and have lower values of Tm of iron than Williams et al (1986); in other words, the melting curve of iron is much less than previously reported. They also indicate the loc ation of the hcp-fcc-liquid triple point to be at 100 GPa and 2500°K. The melting curve proceeds from this triple point to another triple point connecting liquid-hcp and the high temperature bcc phase at about 195 GPa and 4000°K. The existence of this t riple point is supported by the finding of M. Matsui (1992) from his molecular dynamics study, that the hcp phase probably transforms to a bcc phase as a first-order transition at about 4500°K at 300 GPa. The melting curve then must pass from the hcp-bcc -liquid triple point (at 195 GPa) to the solid-liquid transition (at 224 GPa and 5000°K) observed by Brown and McQueen (1986), which yields Tm ~ 5500°K for pure iron at the liquid-solid core boundary pressure (330 GPa). If the inner core is nearly pure i ron, then the melting temperature at the earth's center would be about 300° larger due to adiabatic compression, yielding Tm ~ 5800°K at the earth's center. Thus the temperature at the earth's center is less than about 6000°K.

 The presentation by T. Yagi (Univ. Tokyo) of the paper, "Phase transformation in the Fe-H System and the process of core formation," by himself, M. Yamakata, W. Utsumi, and Y. F Kukai was concerned with the effect of the addition of hydrogen on the phase boundaries of pure iron. They start with a "sandwich"of iron contained between layers of LiAlH4, subjected to T and P in a large volume press ("Max- 80"), with diffraction profiles measured by x-rays using synchrotron radiation. H2 is released, and Fe- H becomes the impurity in iron. At 6 GPa, this hydrogen-enriched iron passes into the fcc phase via an hcp phase. These phase boundaries of Fe with Fe-H are at considerably lower pressures than for pure iron, suggesting that if water exists in the primi tive materials of the earth, the process of core formation is quite different from those proposed by existing theories. The diffraction profiles in this experiment are remarkably sharp and detailed and leave no doubt about the existence of the multiple p hases at relatively low pressures for hydrogen-enriched iron.

 The presentation of E.Ito (Okayama Univ.) on the paper, "Constraints of lower mantle composition and thermodynamic properties of silicate perovskite,"by M. Akaogi and Ito was concerned with the extent to which accurate knowledge of the thermal expansivi ty at mantle conditions could resolve the controversy over whether the mantle is chemically stratified. They point out that direct measurement of the coefficient of thermal expansion,a, at 1 bar is still immersed in controversy. Their calculation of a p roceeded from their measured heat capacity of a small sample of perovskite using differential scanning calorimetry from which CP was measured from 140-300°K, followed by using Keiffer's model for the density of state. Thus CP was found up to 1000°K. The y used the report of Mao et al (1991) to find a linear equation between a and T at 30 GPa, from which they estimated a = 3 x 10-5 at 1800°K and 30 GPa. This number for a is checked out by calculating g by g = aKSV/CP, resulting in g0 ~ 1.4 ± 0.2. This a reasonable value, and as such is a cross check that confirms their calculated value of a for perovskite at conditions of the upper-lower mantle boundary. Such a value of a is too low to require the presence of excess iron in the lower mantle compared to the upper mantle. This thermal expansivity of perovskite supports the homogeneous mantle model, in which the lower mantle is pyrolite with no thermal boundary layer at 670 km. They also estimate the temperature at the top of the D" layer to be about 25 00°K.

 The presentation of N. Funamori (Univ. Tokyo) on the paper, "Thermal expansion of silicate perovskite at lower mantle conditions - an in-situ x-ray observation," by himself and T. Yagi, was concerned with a direct measurement of a of perovskite at lower mantle conditions. This is to be compared with the calculated a using thermodynamic relationships involving CP and g described in the Akaogi and Ito paper above. Funamori and Yagi started with MgSiO3 enstatite and transformed it into orthorhombic perovs kite using a modified Drickamer-type high pressure cell with sintered diamond anvils allowing a relatively high diffraction angle for the emerging x-ray beam. Using very strong x-rays from synchrotron radiation, high quality x-ray diffraction patterns we re obtained, and the unit cell dimensions were determined with increasing temperature. Thus a direct determination of a = (1/V0) (dV/dT)P was approximately obtained. There is some uncertainty in the result because the pressure increases somewhat in the sample chamber as the temperature increases. Yagi and his group are proceeding with a detailed analysis of a, and will calculate the pressure correction; nevertheless, they are presently able to place limits on a and find that a > 1.7 x 10-5°K at 35 GPa and room temperature. They also found that at approximately 35 GPa, V was proportional to T up to 1900°K, which means that a is independent of T at this pressure. This is in contrast to measurements of a versus T at P = 0, where there is detectable cur vature. They also found that the orthorhombic perovskite structure remained stable up to 1900°K and 35 GPa (that is, into the lower mantle) without any indication of a phase change. This is in contrast to a previous report that a has discontinuity at 90 0°K and 7.2 GPa.

 The presentation of O. Anderson of the paper, "Method of computation of thermal expansivity at extreme conditions: MgO as an example", by himself, Hitoshi Oda, and Donald Isaak, was concerned with establishing an equation for a(T, h) where h is the compr ession V/V0. Their equation was based on measurements of a at high T at P = 0, coupled with thermodynamic identities relating a at P = 0 to (d alpha /d V)T. To quantify this equation, information on a dimensionless parameter k is needed. This can be ob tained by using data derived from the Helmholtz energy, found by the PIB ab initio model of Isaak et al (1990) on MgO. Thus the equation used is strictly true only for MgO, but the constant k is relatively invariant among dense minerals, so that the nume rical values of the results for MgO may be a guide for the behavior of a of perovskite.

 At high T and P the value of a is independent of T at constant P; e.g., a is a constant along isobars. This may explain why Funamori and Yagi (see above) found that V is linear with T at 35 GPa, in contrast to the behavior at P = 0. Also, it follows th at the value of a reported by Akaogi and Ito above (3 x 10-5/K at 1800°K and 30 GPa) is probably close to the ambient value for perovskite. This indicates harmony between the value of a found by Akaogi and Ito and that found by Funamori and Yagi (who rep orted a at 35 GPa and T = 300°K to be greater than 1.7 x 10-5/K).
 

Plumes: structure composition and origin

In the first presentation of this session, Y. Ida (Univ. Tokyo) discussed the style and evolution of upwelling flow in the mantle, as it pertains to the interpretation of hotspots, flood basalts and continental breakup. He noted that the current models of cylindrical plumes do not explain the observed broad linear trends of hotspots, the correlation of hotspot distributions with mid-ocean ridges and the narrow belt of the Dupal anomaly, and consequently proposed a new plume model in which the upwelling of hot, low-viscosity material from the deep mantle is sheet-like, rather than cylindrical. I.S. Sachs (Carnegie Inst., Washington) et al presented the results of 2D and 3D numerical models of plume growth in the presence of ongoing mantle convection. T hey found that the convection strongly modulates the growth and evolution of a plume, with only strong plumes being able to break through to the surface. The surface location of hotspots are the result of the strong modification of plumes from the deep m antle by convective motions confined to the upper mantle, according to the presentation by A. Yamaji (Tohoku Univ.). T. Nakakuki (Univ. Tokyo) et al studied numerically the interaction of an upwelling plume with a phase or chemical boundary at 670 km dept h, with emphasis on the effect of chemical density difference and Clapeyron slope on the ability of plumes to penetrate to the surface. The family of known solitary wave solutions for melt migration in the mantle has been extended by M. Nakayama and D.P. Mason (Univ. Witwatersrand, South Africa), including both refractive and compressive waves.

On a different tack, H.-C. Nataf and J. Vandecar (Utrecht Univ., The Netherlands) described their efforts to image plumes seismically beneath three hotspots in the Pacific Ocean off the coast of Canada (Bowie, Cobb and Anonymous). The two final reports of the session dealt with geochemical signatures of plumes. First, I. Kaneoka (Univ. Tokyo) discussed the constraints on location and form of the plume source region based on the isotopic data. Finally, T. Kogiso and Y. Tatsumi (Kyoto Univ.) presented a comparison of the chemistry of oceanic island basalts found in French Polynesia and in Hawaii.
 

Heterogeneity of the Earth's deep interior

In the first lead talk of this session, Y. Fukao (Nagoya Univ.) et al described the seismic tomogram of the Earth's mantle that they have recently obtained using a variable-resolution mesh, with emphasis on the image of upwelling and downwelling convecti on currents. They find a low-velocity anomaly in the central Pacific which is identified as the image of a plume rising from the D" layer. This image is traced up through the lower mantle well into the transition zone toward the Polynesian Superswell. The high velocity image of the subducted slab beneath the western Pacific shows extensive spreading in the transition zone. The authors suggest that the downwelling slab tends to stagnate in the transition zone under a subtle control of the 660km discont inuity, allowing a partial mass exchange between upper and lower mantle. A broader summary of mantle tomographic results was given by F. Gilbert et al (IGPP, Scripps, La Jolla) in the next lead talk. They noted that short-period data sees a discontinuit y at 200km depth that is not present in the long-period data, while the long-period data sees one at 520km depth that has not been seen in the short-period data. Both data sets reveal discontinuities at 410 and 660km depth. The depth variation of the 66 0km discontinuity is typically 20-30 km, and indicates a negative Clapeyron slope. Topographic imaging of the core-mantle boundary is difficult; a topography of order 2km seems to fit the seismic and other data best. Core-sensitive modes reveal an anoma lous splitting which is difficult to interpret in terms of any reasonable model of a fluid outer core. In the third lead talk, R.L. Woodward et al (Harvard Univ.) reviewed seismic and geodynamic constraints on the large-scale structure of the mantle, emp hasizing the accuracy and reliability of the model results. In one test, the results of two inversions, one with 670km discontinuity explicitly included and one continuous model, are found to yield virtually identical velocity variations, and the continu ous model shows abrupt changes in heterogeneity at 670km depth. They find that mantle heterogeneity is most difficult to constrain in regions where its amplitude is low. Also, radial resolution is relatively low in the core-mantle boundary region, but t he conclusion that heterogeneity increases near the base of the mantle appears to be robust. Seismic tomographic inversions are found to be in agreement with the observed large-scale nonhydrostatic geoid.

D. K. Chowdhury (Indiana Univ.-Purdue Univ., Fort Wayne) studied the depth of the subducted plate in the Tonga-Karmadec region of the pacific Ocean using core-reflected phases, and found a distinctive velocity change under the Karmadec region to a minimu m depth of 1000 km, suggesting a possible penetration of the plate. In a pair of presentations, R.J. Geller et al (Tokyo Univ.) described the use of the direct solution method to determine the laterally heterogeneous upper mantle structure. It is import ant to determine these structures accurately as a preliminary to inverting for deeper structures. S. Tanaka & H. Hamaguchi (Tohoku Univ.) inverted travel times of long-period SKS and S2KS core phases and found a pattern suggestive of stable stratificatio n in the outer core. Since a seismically observable density has an enormously large dynamical effect, this possibility deserves further investigation. In a related presentation, H. Hamaguchi et al. reported on a search for hidden symmetry in the mantle and core, principally using P and S travel times. They found that the symmetry axes for the heterogeneities in the lower mantle and the upper-most core are spatially identical to each other and are located in the center of the residual geoid high, close to the equator. These axes are located close to the principal axis of the perturbation in moment of inertia for the crust, and the Earth's most active volcanoes, Hawaii and Nyiragongo/Nyamuragira are located near these axes.

The J-Array, which is a large seismic network covering the Japan Islands, has been used by T. Shibutani et al (Kyoto Univ.) to search for core-reflected waves. These have been used to calculate the ellipticity of the inner core and the density jump at t he inner-core boundary. Current efforts are underway to improve station corrections and thus improve the stacking method which can better reveal deep structure. A revised model of the P-velocity structure at the base of the outer core was proposed by S. Kaneshima et al (Tokyo Univ.) using broad-band and short-period array seismic data, in which the velocity is less than that proposed by PREM. I. Nakanishi (Hokkaido Univ., Sapporo) using PKP data also finds the P velocity to be less than PREM at the bas e of the outer core.
 

Paleomagnetic inferences about the dynamo process

Papers and posters covered a wide range of topics, including secular variation, paleointensity, the effects of changes in angular momentum, measurement methods for long cores, and of course the question of preferred paths for VGPs (virtual geomagnetic po les) from reversal transitions.

 H.Tanaka (Tokyo Inst. of Tech.) and co-workers presented a new method for estimating paleointensities. They showed that this method may give reliable results with rocks that would have been unsuitable for previous methods. Tanaka also presented informa tion on a paleointensity database from volcanic rocks - a database from which it is now possible to start drawing some useful conclusions.

 Several papers suggested links between variations in the geomagnetic field and changes in the Earth's moment of inertia from, for example, changes in the volume of ice sheets. During discussion, D. Loper noted that the core can easily accommodate such c hanges and that it is hard to identify a mechanism whereby they would affect the geomagnetic field.

 H. Oda, M. Torii and co-workers (Kyoto Univ.) presented a method for obtaining and assessing paleomagnetic records from sediments, particularly from long cores.

 From the perspective of structure and dynamics of the core-mantle boundary region, the question of preferred paths for VGPs from reversal transitions probably attracted the most attention. N. Niitsuma (Shizuoka Univ.) and co-workers presented a high-res olution record from the Boso Peninsula, Central Japan. M. Hyodo (Kobe Univ.) presented an Upper-Olduvai transition record from Java, in which the VGPs are longitudinally confined, but well away from the hypothesized preferred bands. X. Quidelleur and co -workers (Inst. Phys. Globe, Paris) attempted to obtain transition records from continental sediments in Bolivia, but the observed polarity change in the record is abrupt.

 C. Laj and co-workers (CNRS, Gif-sur-Yvette) used the Rao test and the Rayleigh test to determine whether the apparent preference for particular longitudinal bands is statistically significant. They concluded that it is, They also examined whether the observations could be produced by smoothing of the signal in sedimentary rocks, and concluded that this is most unlikely.

 P. McFadden (Bureau of Mineral Resources, Canberra) developed a statistical method specifically designed to test the hypothesis of two antipodal bands of preferred location for transitional VGPs. In concert with Laj and co-workers, he concluded that it is most unlikely the observations could have been obtained by chance from a uniform random distribution, and that some explanation is required. He noted two strong concentrations of site longitudes centered 90° from the centers of the preferred bands for VGPs. He also noted that the currently available database favors the hypothesis that some mechanism biases the longitude of transitional VGPs a small amount towards 90° away from the site longitude, and that the concentration of site longitudes produces the concentration of transitional VGP longitudes.
 

Modern geodetic approach to core studies

Broadly defined, geodesy is concerned with the study of the shape and deformations of the Earth and involves the precise measurement of the motions of the planet, of the surface deformations and of the gravity field and its time dependent behavior. Thes e observations provide information on the Earth's behavior in the frequency range spanning the seismic and geological observations of the dynamic and kinematic behavior of the Earth, although the lower limit is usually limited by factors such as the diffi culty of funding and maintaining long-term observation programs, the difficulty of maintaining homogeneous data sets at a time when technological advances are rapid, and the difficulty of maintaining long-term observation programs in the face of changing scientific trends.

 Evolution of the geodetic measurements over recent decades has been most significant and sub-centimeter accuracies are being achieved in position determinations, sub-multi-arc-second accuracies are achieved in direction measurements and nanogal accuracie s have been achieved in gravity observations. Thus, a new range of physical phenomena can be expected to rise above the noise spectra of the measurement process and subjects that were largely of theoretical interest are now becoming matters of some urgen cy.

 Geodetic measurements may not seem to be an ideal way for studying the core but when the core is inaccessible anything that contributes takes on significance. Gravity meters on the surface of the Earth lie far from the source of any perturbations in the core arising from core motions and some of the other measurements reflect globally integrated effects of mass redistributions. The level of complexity is raised by the fact that many other parts of the earth contribute to the measured spectrum, be it on e of the time dependence of gravity, deformation or planetary rotation. Oceans and atmosphere are particular nuisances at the shorter periods but tectonic influences also become important as adding to the background noise. In many instances these effect s are larger than the sought core phenomena and they will require painstakingly careful corrections, as was illustrated by the paper by Goodacre et al (Geological Survey, Canada) related to high-precision gravity measurements. These are the tedious yet e ssential parts of geodetic science if any core effects are to be observed.

 What core effects can be measured from geodetic measurements, other than density and constraints on the density gradient from planetary mass and moment of inertia determinations? One class of problems is the detection and identification of core modes th rough the changes in vertical gravity associated with fluid oscillations in the core or with oscillations of the solid inner core within its fluid envelope. Another class of problems is concerned with the core fluid rotating bodily about an axis that is not aligned with the rotation axis of the mantle. The session on Modern Geodetic Approaches to Core Studies was almost wholly devoted to these two classes of problems reflecting two major developments in geodetic instrumentation. The superconducting gra vimeter (SCG) has now been developed to the stage where the detection of core modes becomes feasible. As noted by Y. Imanishi (Univ. Tokyo) et al, Superconducting gravimeters are now routinely achieving a sensitivity and accuracy that should allow the de tection and identification of core oscillations. The session began with a lead presentation by D.E. Smylie & X. Jiang (York Univ.) on core oscillations and their detection in superconducting gravimeter records using the rotational splitting of modes. Th e possible detection of three resonances in the record was discussed, but this identification is still somewhat controversial. Generally, longer records, freer from discontinuities in the data and free from other perturbations, will be required before co nfidence can be had in any observations of these modes. The second presentation by D. Kakuta et al (National Astronomical Observatory, Mizusawa) dealt with the deformations of the Earth and thermal core-mantle coupling, with emphasis on their effect on t he length of day. The deformations induce changes in the l.o.d. by changing the angular momentum of the mantle, while core-mantle couplings change principally its angular momentum.

 The claim of possible detection of core oscillations made by Smylie was challenged by D. Crossley (McGill Univ.) who noted that the eigenfrequencies of the core modes must be calculated using dynamic, rather than static, load Love numbers in parameterizi ng the effect of the mantle on the core oscillations. When this is done, the agreement between calculated and observed periods for the Slichter modes of the core as reported by Smylie becomes questionable. There was some discussion of this point followi ng the presentation, but the issue does not yet appear to be resolved.

There were a number of presentations concerning the operation of superconducting gravimeters in Japan including two presentations by Y. Imanishi (Univ. Tokyo) et al and S. Kaneshima et al (Univ. Tokyo) describing the operation of the superconducting grav imeter in operation at the Esashi Laboratory, with emphasis on the attempt to detect core modes, a report by I. Nakagawa et al (Kyoto Univ.) on the possible detection of core modes using two superconducting gravimeters installed at Kyoto University and la ser strainmeters at the Amagase Observatory, and a description by N. Seama et al (Univ. Tokyo) of the operation of a superconducting gravimeter at Kakioka, Japan. All of these operations are yielding preliminary results, but it is too soon to state with any certainty whether anyone has detected core oscillations, as argued by in the presentation by C. Ooshima et al (Toyama Univ.).

 The nature of the Earth's rotation, both the angular velocity and the orientation of the direction of the rotation axis, is a function of the properties of the liquid core, and to a lesser degree of the properties of the inner core. To a high degree of precision the core effects can be explained by relatively simple models in which the core fluid motion is modelled as a nearly rigidly rotating body about an axis that is not quite coincident with the axis of the mantle. But to obtain higher levels of co nstraint on core physics from earth rotation observations will require a much improved instrumentation as well as the ability to filter out other irregularities in the rotation. The session did not yield new results but new instrumentation techniques wer e discussed in a set of four presentations by O. Kameya et al, H. Hanada et al, T. Sasao et al. and N. Kawano et al. (all of the National Astronomical Observatory, Mizusawa) concerning the construction of a 10m antenna for VLBI observations at Mizusawa an d its incorporation into an antenna network. This instrument will be used to study irregularities in the rotation of the Earth, possible core-mantle coupling, as well as the structure of the lunar core.

 On both fronts it is still early days in providing constraints on the physics of the core from geodetic observations. The results, however, are promising and the putative observations are providing a new impetus for renewed theoretical modelling of thes e modes.
 

Progress in geodynamo theories

This session consisted of three lead talks, by P. H. Roberts (UCLA), S. I. Braginsky (UCLA) and G. Glatzmaier (Los Alamos National Lab.), plus 13 poster presentations. The session was opened with a talk by P. H. Roberts on "Ideal and resistive instabili ties in the Earth's fluid core", who reviewed mean-field theory and a-w dynamos, then discussed various types of instabilities hat can occur in the core. Most ideal and resistive instabilities are found to be of small scale. Next S. I. Braginsky, in a p resentation entitled "events on the core-mantle boundary, and the geodynamo", gave arguments in favor of a strongly stratified compositional layer at the top of the outer core, which he called the "hidden ocean". According to Braginsky, this layer is abo ut 80 km thick and has a Brunt–Väisälä frequency close to one day. This level of stratification will produce a hydromagnetic oscillation having a 60-year period. Such a level of stratification can be achieved if the compositional variation is of order 1 0-4 in the layer. This is to be compared with a typical value of 10-8 in the bulk of the core. The mechanisms of "rain" and "evaporation" which maintain the layer in equilibrium are not yet clear, although the rain is presumably a consequence incomplete mixing of the compositionally buoyant fluid parcels which are generated at the inner core boundary by the solidification process. If such a layer exists at the top of the core, this would preclude the possibility of significant lateral variation in the heat flux across the CMB; the combination of stable stratification and lateral heat-flux variation would induce very large thermal winds, but these are not observed in the secular variation record. G. Glatzmaier, speaking for himself and P. H. Roberts, r eviewed the current status of "numerical solutions of the geodynamo". The kinematic dynamo problem is now well understood, and attention is focused principally on the "intermediate" problem in which the alpha coefficient and the thermal wind are prescrib ed. Work is also underway on development of a numerical code for the full dynamo problem in which the forcing is prescribed in terms of sources and sinks of buoyancy of either thermal or compositional origin.

Two of the poster presentations appeared to have particular relevance to the dynamo problem. Y. Honkura, T. Iijima and M. Matsushima (Tokyo Inst. Tech.) presented results of an analysis that showed that reversals of a model magnetic field driven by cool ing from above can be suppressed by the presence of lateral temperature variations on the top boundary. This conclusion appears to contradict the hypothesis of Gubbins that reversals are triggered by variations in these boundary conditions. The resoluti on of this difference will be an important step toward a better understanding of the geodynamo. G. Hulot and J.-L. Le Mou'l (IPG, Paris) presented the results of their study of the statistical properties of the geomagnetic field. They find that the hist orical non-dipole field has a correlation time of 200 years, and that the flow appears to be tangentially geostrophic. However, they note that flows compatible with arbitrary synthetic data can always be found.
 

Properties of core-mantle boundary and D" layer

The session was initiated by lead presentations given by D. Doornbos (Univ. Oslo) and T. Lay (UC Sata Cruz). In his talk, Doornbos reviewed the difficulties in imaging the large and small scale structures on th core-mantle boundary. He noted that the P -wave speed in the core is too large in the PREM model and that the ISC data set has systematic errors. As a result, the large-scale topography is as yet undetermined, and remains hidden in the noise. The topographic high in the Pacific region reported by some tomographic inversions appears to be an artifact of the poor data coverage; the likely magnitude of the topography is not more than a few km. In fact, the magnitude of the regional topography in various models appears to be inversely correlated w ith the data coverage, suggesting that the core-mantle boundary may be quite smooth,with topography less than 1 km. In his talk, Lay concentrated on the possible detection of regions of rapid seismic variation 250 to 300 km above the core-mantle boundary . The current status of the data is perplexing, with the region beneath Alaska showing a reflector in the S-wave data, but none in the P-wave data. As usual, the spotty coverage of the CMB by the seismic data presents a problem in development of a consi stent model of the lowermost portions of the mantle.

M. Kato et al (Kyoto Univ.) reported that no reflector has been seen beneath the western Pacific using P-wave data recorded by the J-Array, a seismic network covering Japan. On the other hand, H. Sylvain and H.-C. Nataf (Ecole Normal Superieure, Paris) report the positive detection of such a layer beneath northern Siberia using P-wave data recorded in France. This observation was apparently confirmed by M. Weber (Seismologisches Zentralobservatorium, Erlangen)

The role of temperature variations at the base of the mantle in controlling core processes was investigated by F. D. Stacey (Univ. Queensland). Specifically, he analyzed the usual assumption that the seismic variations detected in the lower mantle are d ue to temperature variations. Thermodynamic estimates of the sensitivity of seismic parameters to temperature variations show that a 1% variation in P-wave speed requires a 400K variation in temperature, with a corresponding density variation of 25 kg m- 3. For velocity anomalies thousands of kilometers in extent, Stacey finds this to be a dynamically untenable conclusion, and suggests that the velocity anomalies must be interpreted as due to compositional variations. consequently, we cannot appeal to t omography for evidence of temperature variations in the lower mantle. He also argued that variations in the electrical conductivity of the D"layer must be due to compositional,rather than thermal, effects.
 

Formation of the core

The session began with a lead talk by D. Stevenson (California Inst. Tech.) which focused on five questions pertaining to the formation and evolution of the core: 1. Under what circumstances can a core form at all? 2. How and when does a planetary core form? 3. What is the "initial state" (composition, temperature, density and phase as a function of radius) of the resulting core? 4. What is the subsequent thermal and compositional evolution of the core? 5. What are the connections of all these consid erations to observables? David gave his own speculations on the answers to these questions and, as usual, came up with ample food for thought. Perhaps the most striking new idea is that the core may have been strongly stably stratified immediately follo wing its formation, and that this led to layering which persists to this day.

The formation of the core has been simulated numerically by R. Honda (Nagoya Univ.) et al, who found that the viscosity of the silicate proto-core must be less than 1026 Pa s (which is lower than current estimates) if the process is to occur quickly (les s than 109 yr). M. Kumuzawa et al (Univ. Tokyo) considered the effect of outer-core dynamics on the growth of the inner core, noting that the symmetry of outer core motions imposed by the rotational constraint may affect the local rate of freezing onto t he inner core. This disparity in growth rate would require an axisymmetric pattern of creep in the inner core in order to maintain its spherical shape. This creep could align the crystals of the inner core, causing the observed anisotropy of seismic wav e speed in the inner core. Ohtani (Tohoku Univ.) et al show that there is no evidence for a density cross-over between peridotite and magma, which was thought to exist at a depth of about 250 km, when the actual high-pressure experiments are performed. Thus the accumulation of olivine by flotation is not likely to occur in the magma-ocean stage of the Earth.
 

Convection in the mantle and core

The possible exsolution of the light element in some depth range in the outer core leads to growing interest in the understanding of "bubble-driven" convection. Two presentations on this topic, presented jointly by Y. Abe, S. Kobayashi, and Y. Fukao (Na goya Univ.), clarifyed the dynamics of such systems. They indeed display rather non-intuitive behavior, such as flows with a very large horizontal scale, or flows in which the light bubbles sink locally, due to the effect of apparent compressibility.

 M. Kuri and K. Kurita (Univ. Tsukuba) demonstrated the use of microcapsuled liquid-crystal thermometers as a powerful tool for investigating thermal convection in the laboratory.

 Those attending the SEDI conference were the first to hear of the "Braginsky number", defined by P. Cardin (Ecole Normale Supérieure) and P. Olson (Johns Hopkins Univ.) as the ratio of compositional to thermal buoyancy fluxes. They estimated this number to be of the order of 3 in the core. By performing laboratory experiments with similar Braginsky numbers, they conclude that the structure and dynamics of the outer core is probably dominated by thermal processes.

 Using a linear theory that includes the magnetic field, S. Yoshida and Y. Hamano (Univ. Tokyo) have investigated the effect of temperature heterogeneities at the core-mantle boundary on flow in the outer core, as well as its role in dynamo action.

 M. Ogawa (Ehime Univ.) presented a numerical study of coupled magmatism-mantle convection, with applications to Venus and the Earth. The higher surface temperature of Venus tends to produce a hotter but much more chemically stratified mantle (depleted ov er enriched components) than on Earth.
 

Geomagnetic variation and core dynamics

This last session was less focussed than the others. The first speaker, T. Yukutake (Univ. Tokyo) addressed a long-debated question: what is the behavior of the geomagnetic non-dipole field in the Pacific? Basically, using available declination and inc lination data along a 20°N circle of latitude, he showed that an intense focus of the non-dipole field existed in the Pacific around 1620; the non-dipole field in this area was weak only from 1700 to now. This result is confirmed by secular variation dat a from Japan and archeomagnetic data from Hawaii. The conclusion is that the Pacific area is not a special one where the dipole field dominates and the non-dipole field is suppressed, contrary to what has been often claimed. This conclusion is welcome s ince all the reasons invoked to explain this special behavior of the geomagnetic field were rather artificial.

 N. H. Rotanova (Izmiran, Moscow) addressed the question of the 60, 30 and 20 year variations of the geomagnetic field. She determined these components of the secular variation in a large enough number of observatories to be able to outline their geograp hic distribution, which appears to be similar for the three periods. Some attendees pointed out possible pitfalls due to the use of the maximum entropy analysis.

 A somewhat provocative talk was given by A. K. Goodacre (Geol. Surv. Canada) about a correlation between the directions of motion of most of the major lithospheric plates and a postulated electric potential defined by cancelling the radial component of t he motional induced electric field (u x B) in the core. Some difficulties with relevant time constants of plate tectonics and secular variation of the geomagnetic field were raised during the discussion.

 I. Fujii et al (U. Tokyo) described their intention to measure the voltage between the ends of the the submarine cable between Ninomayia and Guam ( 2700 km), which will provide information on the electrical conductivity of the deep mantle and, possibly, will allow the detection of core-generated signals. Powering the cable will allow ocean-bottom observatories to be run. During the discussion other countries were encouraged to begin to use other abandoned submarine cables.

 Newly published geomagnetic charts of Japan for the epoch 1990.0, based on measurements in 105 first-order repeat stations and 872 second-order geomagnetic stations, were presented by I.Kaido (Geographical Survey Inst, Tsukuba). At the same time tempora l changes of the field at all the repeat stations have been examined; it appears that the secular variation displays different patterns in North-Eastern and South-Western Japan. The question was raised whether some kind of abnormal secular variation (not core-generated) could possibly be present in Japan, as has been found in some smaller islands.

 S.-Z. Ma et al (Inst. of Geophysics, Chinese Academy of Sciences, Beijing) propose to use the Hamilton formalism in inverting secular variation data to get the fluid flow at the top of the core. They argue that this formalism is extremely succinct, allo wing easy incorporation of the symmetry properties of the solution in the conservation laws of the dynamic system, and is well fitted to the problem of the flow computation. Practical applications of Hamilton algorithms will be welcome.

 The westward drift of the geomagnetic field was viewed from a novel point of view by S. Yoshida and Y. Hamano (Univ. Tokyo). they proposed that this phenomenon is caused by the variations of the length of the day through topographic coupling between the core and the mantle; a poloidal field which appears to drift westward is generated by twisting a toroidal field on the bumpy core-mantle boundary. It is then possible to infer information on the core-mantle boundary topography form the observed westward drift. During the discussion, questions were asked about the cause of the variations of the length of day in the proposed mechanism.
 

The SEDI business meeting at Mizusawa

The meeting was convened by the SEDI Chairman, D. Doornbos, at 6 PM on Tuesday, July 7, 1992. There were 53 people from 9 countries in attendance. This report constitutes the official minutes of that meeting.

 The first item of business were a series of brief reports of national SEDI activities; the Canadian GGP by D. Crossley (McGill Univ.); the Chinese SEDI organization by S.Z. Ma (Chinese Acad. Sci., Beijing); the Japanese Earth's Central Core Project by T. Yukutake (Univ. Tokyo) [Workshops in Mizusawa in December 1991 and Kyoto in April 1992, Second progress report published in March 1992 - 56 papers, 40% in English]; the US Geomagnetic Workshop by P. H. Roberts (UCLA); the US CSEDI initiative by T. Lay UC Santa Cruz; the Russian Geomagnetic Initiative by V. Golovkov (Izmiran, Russia). Reports of these activities appear under 'Summaries of meetings of interest to SEDI' or 'Projects and National Activities' below.

 There was no old business. The first item of new business was the possible publication of proceedings of Mizusawa Symposium. M.Kono (Tokyo Inst. Tech.) proposed that the proceedings of the Symposium be published as a special issue of the Journal of Geo magnetism and Geolectricity. He noted (1) that he currently serves as Chief Editor of this journal and would be willing to serve as one of the editors of the special issue, (2) the journal has published special issues devoted to proceedings in the past ( 3) the special issue will appear 3 to 4 months after the last paper is received and (4) the Journal is planning to broaden the scope of papers that it publishes to include all geophysical and geochemical disciplines.

 Next, K. Yokoyama (Natl. Astronomical Obs., Mizusawa) presented a resolution in support of NASA's VLBI program which is being threatened with severe budget cuts. SEDI acknowledges the value of VLBI and encouraged Yokoyama to solicit letters of support f rom distinguished scientists familiar with the scientific value of VLBI.

 Two invitations were presented for the location of SEDI Symposium in 1994; D. Crossley presented an invitation to hold the 1994 symposium in Banff, Canada, and S. Z. Ma presented an invitation to hold it in Beijing, China. The Canadians have reserved t he Centre for Fine Arts in Banff for the week of June 20-24, 1992. This is the only week available for the SEDI meeting at this locality. Since this week of the year has traditionally been used by the Committee on Mathematical Geophysics for its biennia l meeting, D. Crossley wrote to A. Tarantola proposing that a joint CMG/SEDI meeting in Banff in 1994. That proposal was declined. A perceived disadvantage with the Chinese invitation is that the present meeting is being held in the Far East, and a geog raphical diversity of locations of the SEDI meetings is deemed desirable. Consequently, the Canadian's invitation was accepted, in spite of the conflict with the CMG meeting. (Note that the conflict with CMG has now been resolved; see the following story .)

 Announcements of SEDI sessions at other meetings (see "Future meetings of interest to SEDI"), including a SEDI session being organized for the next meeting of the European Union of Geosciences to be held in Strasbourg, April 4-8, 1993, and a symposium on the major discontinuities of the inner Earth (i.e, at the ICB, CMB and 670 km depth) being organized U. Christensen, T. Lay and F. D. Stacey for the IASPEI Assembly to be held in Wellington New Zealand in January, 1994. There was some discussion of the possibility of having a SEDI-related session organized for the IAGA assembly to be held in Buenos Aires in August 1993, but no firm decision was made.

 The meeting was adjourned at 7: 25 PM.
 

1994 SEDI symposium

The next SEDI meeting will be held at Whistler Mountain, British Columbia, the week of August 8-12, 1994. The change of venue from that originally proposed (and accepted by the participants) at the SEDI symposium in Mizusawa was deemed necessary by some members of the Canadian geophysical community to avoid conflicts with the Annual CGU Meeting in Banff in May and the Mathematical Geophysics Meeting the same week. Planning for the new site has already begun and accommodation reserved; the participation of the CGU has been welcomed. Canadian organizers hope to get out a first circular within a few months.

 Whistler Mountain is a perennial favorite amongst the skiing community of Western North America. As far as the local facilities are concerned, August represents the off season so a good selection of different types of accommodation is available. Whistl er is a relatively remote site, some 80 miles North of Vancouver, and transportation from the Vancouver airport will of course be arranged by bus. Accompanying visitors will find the range of shopping opportunities less than in a major city, but there a re abundant opportunities to walk and hike in the area. The mountains around Whistler are not as spectacular as the Rockies, but the area is nonetheless beautiful.
 

Summaries of meetings of interest to SEDI


 

Report of the fifth symposium of the project "Central Core of the Earth"

The fifth symposium of the project, "Central Core of the Earth" was held in Mizusawa, 24-26 December, 1991. The symposium was convened by M. Ooe, T. Yukutake and Y. Honkura. More than 100 people attended the symposium and 48 papers were presented. The symposium included 6 sessions: (1) high-pressure experiments, (2) analyses of paleo and secular magnetic data, (3) calculation techniques for geomagnetic problems, (4) dynamics of the core and the mantle, (5) seismological observations and VLBI, and (6) core formation. Many fruitful results were presented and active discussions ensued during the symposium.

 The symposium was opened with a lead talk by T. Yukutake. In the first session, T. Yagi introduced high-pressure measurement using the sintered-diamond anvil and three papers followed his talk. T. Kondo reported that a state of 30 GPa and 1600°C was g enerated with the MA8 anvils. N. Funamori described successful experiments with X-ray diffraction in situ under these conditions. T. Kato reported on phase transitions in MgSiO3 with the instruments. His preliminary results suggested that the pressure of the phase transition boundary between ilmenite and perovskite is a few GPa higher than that of previous experiments. If seismic-wave velocity has a discontinuity at 670 km caused by phase change between ilmenite and perovskite, the results suggest tha t the phase transition boundary possesses a larger negative slope than -6MPa/°C. Although there still remains some problems, their papers showed promise of high-pressure and high-temperature experiment with using the new material, sintered diamond.

 Paleomagnetic data analyses and secular variation were discussed in the second session. One of the topics was the possibility of recovering the correct magnetic field from paleomagnetic records. M. Torii indicated a difficulty in comparing various pale omagnetic data obtained from different sediments. Some problems remain in normalizing the data. H. Oda suggested a method to deconvolute data of pass-through measured core sample in which magnetic field data are convoluted. H. Shibuya indicated differe nce between record of sediment cores and that of volcanic rocks. the Milankovitch cycle was the second topic of the session. T. Yamazaki and M. Seki each showed paleomagnetic records which have Milankovitch cycle and asked why changes in the geomagnetic field exhibit the same time scale as changes of ice sheets. Geomagnetic secular variation in the Pacific region was discussed at the last of the session both by T. Yukutake and by K. Hinata. While the Pacific region has said to be a dipole window, both of them conclude that this is not a persistent feature of the field.

 The third session consisted mostly of technical reports of analyses of the magnetic field and of calculations of the dynamo equations. E. Mochizuki and Y. Yokoyama suggested the necessity of hourly measurements of Gauss coefficients during several decad es to derive several-year variations, including for example, magnetic jerk and 5-year fluctuation. They first suggested that problems associated with deriving the hourly coefficients include removal of the external magnetic field and the effect of inhomo geneous distribution of observatories. They introduced new techniques to overcome these problems. M. Kono and Y. Honkura respectively developed computer programs to calculate eigenvalue problem of kinematic dynamos and time variation of MHD dynamo. The y showed some results of the calculation.

 The core-mantle interaction was the main topic of the fourth session. Various mechanisms of core-mantle coupling were discussed, based on recent observational data from seismicity, geomagnetism and the Earth rotation. S. Tanaka showed the structure of the topmost core by analyzing seismic body waves. Y. Fukao identified a super plume originating at the CMB under the Pacific region by analyzing P-wave and free-oscillation data. K. Tamaki also supported an existence of super plume in the Cretaceous per iod by comparing volumes of sea mounts and spreading rates of ocean basements. M. Matsushima derived thermal distribution at the CMB from geomagnetic data. C. Kakuta showed that irregular change of the length of day could be explained by thermal couplin g. S. Yoshida considered topographic Rossby waves beneath the CMB and suggested that westward drift may depend on the topography of the CMB. Y. Yokoyama showed that 30-year variations were commonly observed both in the Earth rotation and magnetic field and indicated that paths of the polar secular motion and tesseral Gauss coefficients in the g-h diagram have remarkable similarity. M. Kumazawa gave new sight into the core dynamics based on the evolution of growing Earth. He suggested that the whole cor e was stratified stably at the first stage of the Earth's history and was stirred by the tidal resonance of inertial gravity waves at 3.5 Gyr or at 2.7 Gyr ago.

 Projects of seismological measurements and VLBI system were mainly reported in the fifth session. Y. Hamano, M. Kumazawa, and K. Hirahara respectively reported on ocean basement measurement system, narrow-band seismometer, large-scale seismic array. Th e VLBI measurement system is mostly due to the National Astronomical Observatory, Mizusawa division. T. Sasao, T. Hara, H. Hanada, O. Kameya, and Y. Tamura reported on their VLBI system that is planned to have high resolution and long duration observatio n.

 The last session was mostly on the core formation. H. Yurimoto, H. Uegomorin, and K. Kurita respectively discussed core formation considering Ni, Co budget, shape and size of metallic phases in meteorites, and sweeping of metallic phase. T. Hirata repo rted a result of experiment of breaking viscoelastic materials by liquid injection to yield viscoelastic fingering. R. Honda reported a result of a numerical simulation of the core formation.

 The most important results of the symposium are: (1) working hypotheses are presented that are subject to be test by computations and observations and (2) new instrumentations have been developed, especially the instruments of super-conducting gravimeter (no paper was presented), narrow-band seismometer, VLBI and large-scale seismic-array system. There are two approaches to the Earth's core: to the present core and to the evolving core. The models by the first approach can be checked by the above devel oping measurements. But,the models for evolving core appear difficult to test their validity in this state. We hereafter need to acquire paleo-geophysical evidences to confirm the theories.

 This report was contributed by Y. Yokoyama (Univ. Industrial Technology, Kanagawa, Japan).
 

Summary of NATO ASI in Cambridge in September, 1992.

A NATO Advanced Study Institute on the subject "Theory of Solar and Planetary Dynamos" was held at the Isaac Newton Institute for Mathematical Sciences, Cambridge, U.K. from 20th September to 2nd October 1992. This was within the framework of the extend ed programme of the Isaac Newton Institute on Dynamo Theory (July to December 1992). The two-week ASI consisted of a series of invited lectures which will be published in a first volume of Proceedings by Cambridge University Press, and a large number of contributed papers, which will be collected in a second volume.

 The meeting opened with lectures from P.H. Roberts (U. California, Los Angeles) on the fundamentals of dynamo theory, the aim of the lectures being to prepare the ground for later speakers. The basic principles of dynamo action were set out, and in part icular the theory of mean field dynamos, together with numerical results for fully three-dimensional kinematic dynamos, were presented. The theme of a-w type dynamos was taken up by A. Brandenburg (Nordita, Copenhagen), who described a variety of numeric al models including nonlinear dynamical feedback mechanisms. The results of mean-field models were then compared with the results of direct simulation of fully three-dimensional hydromagnetic convective turbulence. These simulations reveal the developme nt of highly intermittent magnetic fields, as observed on the solar photosphere.

 An extensive review of observational data relating to solar and stellar magnetic fields, coupled with a discussion of appropriate dynamo models, was presented by N.O. Weiss (DAMTP, Cambridge), who argued the case for locating the solar dynamo at the base of the convection zone where the effects of magnetic buoyancy can be balanced by a downward flux expulsion mechanism. Weiss then described work on model dynamical systems based on nonlinear dynamo waves, which are capable of reproducing the solar activi ty cycle, and of predicting a wide range of behaviour in other stars.

 Two lectures on the theory of magnetoconvection were presented by M.R.E. Proctor (U. Cambridge), the first on compressible magnetoconvection, with emphasis on the solar and stellar situations, and the second on magnetoconvection in a rotating fluid, with emphasis on the magnetostrophic regime where Coriolis and Lorentz forces are of comparable importance. In this regime, instability can proceed most easily, an important consideration in the dynamo context.

 The question of energy sources for planetary dynamos was addressed by W.V.R. Malkus (MIT), who set out to eliminate the possibility that rotational energy (tapped through precessional or tidal effects) might provide the dominant source of energy, but who concluded that, in the case of the Earth, the question is still wide open! Malkus demonstrated that tidal straining can induce violent inviscid instability in a rotating fluid. This instability, analysed further by R.R. Kerswell (U. Newcastle upon Tyne ) in one of the contributed papers, results from a vorticity-stretching mechanism, and does not saturate at small amplitude. Malkus concluded his contribution by appealing for restraint in the promotion of new theories of the geodynamo: as much effort s hould be devoted to the elimination (as to the promotion) of theories that are in conflict with observation.

 The topic of fast dynamos, currently attracting much attention, was introduced by A.M. Soward (U. Newcastle upon Tyne), who first discussed basic mechanisms associated with linear flows (i.e. flows with uniform velocity gradient). Soward developed a hie rarchy of dynamo models stemming from the stretch-twist-fold and stretch-fold-shear mechanisms, and indicated the nature of current research still directed at the elusive goal of providing a convincing analytic proof of fast dynamo action (i.e. kinematic dynamo action with growth rate on the convective time-scale).

 There were about thirty contributed papers during the first week of which about a dozen were primarily concerned with the geodynamo. K. Zhang (U. Exeter) considered the Poincaré equation governing inertial waves in a sphere, and demonstrated a focusing effect whereby the waves may be trapped in a neighbourhood of the equator. D. Sokoloff (Moscow State U.) considered excitation of a random magnetic field by a random motion, and interpreted geomagnetic secular variation in terms of intense flux ropes gen erated by such dynamo action. A. Ruzmaikin (Inst. Terrestrial Magnetism, Troitsk) considered the role of the solid inner core in planetary dynamo action, and emphasised the possible importance of the singular shear layer located on the cylinder circumscr ibing the inner core and parallel to the rotation axis. C.F. Barenghi (U. Newcastle upon Tyne) studied nonlinear dynamos in a spherical shell, adopting the 'intermediate' approach in which differential rotation and a effect are prescribed; account is ta ken of Taylor's constraint, and the amplitude of the nonlinear solution is determined either by viscous core-mantle coupling, or by the back-reaction of Lorentz forces, depending on the intensity of the forcing. A.Y.K. Chui (U. Cambridge) described an ex tension of the Bullard disc dynamo incorporating thermal forcing, as in the Welander loop; he identified configurations for which reversals of the field could occur. P. Cardin (Ecole Normale Superieure) described laboratory experiments involving thermal and chemical convection in a rapidly rotating sphere, and argued on the basis of these experiments that thermal convection probably dominates over compositional convection in the outer core. I. Cupal (Czechoslovak Acad. Sci.) considered the time evoluti on of solutions of the 'model Z' equations, and identified two time-scales, a short time-scale related to diffusion of the azimuthal field in the core-mantle boundary layer, and a long time-scale associated with diffusion of the meridional field throughou t the core. M.G. St. Pierre (U. California Los Angeles) investigated the strong field branch of the Childress-Soward dynamo in a rapidly rotating Bénard convection layer; he retained the inertial term in the calculations, in order to follow the time evo lution of the system, and demonstrated converged solutions for small values of the Ekman number. D.E. Loper (Florida State U.) focused attention on the problem of eruptions from the mushy zone at the inner core boundary. The problem is to determine the structure of the buoyant parcels that originate in this region, and the manner of their rise towards the core-mantle boundary. Loper attacks this problem by considering the drag experienced by a body of known shape moving at a prescribed velocity, taking full account of Coriolis and Lorentz forces. V. Zheligovsky (Academy of Sciences, Russia) demonstrated dynamo action associated with a non-axisymmetric Beltrami flow (vorticity everywhere parallel to velocity) in a sphere. A remarkable feature of the s olutions was that the growth rates obtained were higher (by a factor of about 2) than the maximal Lyapunov exponents associated with the chaotic trajectories of the flow considered.

 The main geophysically oriented lectures in the second week were given by D.R. Fearn (Glasgow U.) and S.I. Braginsky (U. California Los Angeles), who both addressed the nonlinear dynamics of the core. Fearn gave a full account of the problems arising fr om Taylor's constraint in the context of a rapidly rotating core permeated by a strong magnetic field, and showed that the resolution of these problems (does the small viscosity control the flow or does a 'Taylor state' develop?) is not yet upon us. He al so described recent developments in the construction of a fully three-dimensional convective dynamo model. Braginsky, on the other hand, gave a definitive description of 'Model-Z', his own prescription for overcoming Taylor's constraint; while the asymp totic state that he suggests has been found in some computations of `intermediate models' (for which the a effect is prescribed), there are still questions over the forms of the a effect that enable the Model-Z balance to occur.

 An entirely different approach was adopted by E. Knobloch (U. California Berkeley), who applied modern ideas of bifurcation theory in the presence of symmetry to the problem of instability in a rotating sphere. He explained, among other things, that the presence of rotation will in general lead to travelling-wave solutions, and showed how in the context of nonlinear dynamo waves (due to the a-w mean field mechanism) the interaction between different hemispheres can be simply explained by interaction te rms applied to the wave evolution equations, reducing the role of the actual physics to the selection among a small number of different bifurcation sequences. B.J. Bayly (U. Arizona) left the realms of fluid flow entirely and gave an exposition of the th eory of mappings with diffusion as models of the fast dynamo process. The simplification provided by the concept of a mapping means that the fundamental features of the stretching, folding and twisting of magnetic field lines, that lie at the heart of th e fast dynamo process, may be more easily described. U. Frisch (Observatoire de Nice) described recent work on the properties of Burgers' and related equations at high Reynolds number; these simplified model equations can tell us a great deal about turb ulence in real fluids. Finally, E.A. Spiegel (Columbia U.) lectured on the phenomenon of chaos in the solar dynamo, and various simple models that can be constructed to model the intermittent behaviour of the sunspot cycle.

 The contributed talks covered a wide range of subjects from fast dynamo theory to aspects of galactic magnetic fields. On the geophysical side, F.H. Busse (U. Bayreuth) discussed the role of lateral variations in mantle conductivity in maintaining coup ling between core and mantle, such as appears to be required by the existence of stationary features of the non-axisymmetric magnetic field. Clearly inhomogeneities fixed in the mantle can help to account for these standing components, and the associated torques can be time-dependent; these may be observable under suitable circumstances. R. Hollerbach (U. Exeter) and M.R.E. Proctor (U. Cambridge) discussed the solution of the magnetostrophic equations with non-axisymmetric forcing in the presence of an inner core. Although Taylor's condition is automatically satisfied, the inner core produces severe discontinuities in the velocity on a cylinder aligned with the rotation axis and tangent to the inner core. Such a singularity must be removed either by a complex viscous boundary layer or else by suitable modification of the forcing term through the induction equation.

 D.R. Fearn gave an account of the possible magnetic instabilities of a rotating fluid with a toroidal magnetic field imposed; such instabilities, which occur (when the field distribution is suitable) for values of the Elsasser number of order unity (and so appropriate to the Earth), may provide the trigger for reversals and act as an important limiting mechanism for the field strength. Finally J. Wicht (U. Bayreuth) described his dynamo model, developed in collaboration with Busse, in which the flow is very simple but the dynamo is engendered by lateral variations of conductivity at the CMB. It seems that the dynamo only works efficiently for persistent flows rather than oscillatory ones.

 There were several talks on solar and galactic dynamos. D. Moss (Manchester U.) showed how an azimuthal variation in a galactic a effect could lead to significant non-axisymmetric fields in the resulting dynamo. Even when the a effect is axisymmetric, nonlinear calculations suggest that the non-axisymmetric field can persist for long times. M. Foth (U. Sternwarte) described preliminary results for a galactic dynamo model using toroidal coordinates. R.M. Kulsrud (Princeton U.) and S. Anderson (Somervi lle MA) made another contribution to the debate on turbulent diffusion by performing an analysis of the initial value problem for a kinematic dynamo in Fourier space. They showed that local equipartition was reached on short timescales and that this was likely to retard growth of the mean field when Lorentz forces were included, thus adding weight to the thesis of Vainshtein and Tao. E. Zweibel (U. Colorado) discussed the effects of ambipolar diffusion on galactic dynamo models, and presented some resu lts on calculations of dynamo waves performed with M.R.E. Proctor. P.C. Matthews (U. Cambridge) and P.A. Fox (NCAR) reported calculations of magnetoconvection in a compressible atmosphere; Matthews concentrated on three-dimensional flows with emphasis on pattern selection while Fox focused more on eruptions of flux and other potentially observable effects, using sub-grid scale methods. T. Prautzsch (U. Sternwarte) described a dynamo model for the overshoot region of the solar convection zone, while M. G hizaru (Astronomical Inst., Romanian Academy) described approximate models of wave interactions in this region.

 In the fast dynamo context, there were several papers on the magnetic fields excited by the so-called ABC Beltrami flows. A. Pouquet (OCA, Nice) and O. Zheligovsky discussed the stability of these flows when subject to small viscosity and a weak drivi ng force; it is found that there is a gradual increase of the disorder of the flow as the Reynolds number is increased, taking the form for larger Reynolds number of successions of quasi-steady ABC flows with differing coefficients. D.J. Galloway (U. Sy dney) described stability calculations on flows related to the ABC form; a flow with non-net helicity is shown to have probable fast behaviour. Comparisons are also made between fully diffusive ABC calculations and the diffusionless results of Gilbert; some inconsistencies seem to remain here. B. Galanti (Inst. Computational Fluid Dynamics, Tokyo) and A.D. Gilbert (U. Cambridge) both described nonlinear investigations of ABC and time-dependent flows, respectively. Galanti found that large length scal es were preferred for the resulting dynamos, while Gilbert used a simple averaged model of the Lorentz back-reaction, which may not give accurate information on the dominant feedback mechanisms. K. BAJER gave an interesting example of a Stokes flow with chaotic streamlines between two spheres rotating at different rates. A. Brandenburg described numerical simulations of hydrodynamic and hydromagnetic turbulence at small values of the diffusivities, showing that 'near-singularities' produce a multifracta l scaling near the Kolmogorov cut-off. Finally, A.M. Soward gave an analytical description of fast dynamo action in pulsed Beltrami flows in the limit of slow pulsation frequency. This tour-de-force will hopefully set the scene for more analytical demon strations.

 The meeting was ended with a summary and round-table discussion led by H.K. Moffatt (U. Cambridge), who outlined his theory of dynamo action in the core due to blobs of buoyant fluid rising from the inner core, and concluded that the subject, as evidence d by the contributions at the meeting, was in an exceptionally healthy state!

 This report was prepared by H. K. Moffatt and M. R. E. Proctor.
 

Projects and National Activities


 

AGU SEI (Studies of the Earth's Interior) Committee

The chairman of this committee is appointed for a two-year term by the President of the American Geophysical Union, and is charged with coordinating SEDI-related activities within the AGU. The chairman for the period 1990-92 has been Thorne Lay (UC Sant a Cruz) and the remaining members of this committee have been C. Bina, J. Bloxham, R. Carlson, B. Hager, R. Hemley, L. Kellogg, E. Knittle, P. Olson, C. Prewitt, M. Richards, P. Shearer, and D. Stevenson. This is a brief summary of the activities of this committee during the past two years.

 A principal function of the committee has been to organize special sessions at the regular meetings of th AGU. Those arranged in the past two years include: (a) "Structure and Dynamics of the Core-Mantle Boundary Zone" organized by J. Bloxham at the Fal l 1990 meeting, (b) "Mantle Plumes from Bottom to Top" organized by Louise Kellogg and Rick Carlson at the Spring 1991 meeting, and (c) "The Upper Mantle: Discontinuities and Composition" organized by Elise Knittle at the Fall 1991 meeting. There were many SEDI related special sessions organized for the Spring 1992 meeting, but none were formally under the SEI committee. These special sessions have been very interdisciplinary and have been well attended. The AGU continues to highlight SEDI theme sect ions in the meeting abstract volume.

 In October 1990 and article entitled "Studies of the Earth's Deep Interior: Goals and Trends" by T. Lay, T. Ahrens, P. Olson, J. Smyth and D. Loper was published in Physics Today. This article was reprinted in Japanese in May 1991 issue of Parity Magazi ne. The goal of the article was to provide visibility for SEDI science efforts and to attract researchers from the physics community.

 At the Fall 1991 AGU meeting the AGU-SEI committee met and discussed the emerging initiative for CSEDI. After discussion of this initiative a letter was sent to Mike Mayhew expressing the enthusiastic support of the AGU-SEI community for the CSEDI initi ative.

 Jeremy Bloxham (Harvard Univ.) has been named chairman of the AGU-SEI committee for the period 1992-94.
 

US CSEDI Initiative

A US SEDI activity was initiated in the summer of 1991 with an informal group discussion at the IUGG General Assembly in Vienna. Subsequently the plans for the activity were debated in a series of three workshops: October 31 - November 1, 1991 and Janua ry 11, 1992, at Cal Tech and May 15-16, 1992, at Harvard, involving US members of SEDI and members of the SEI (Studies of the Earth's Interior) committee of the AGU. The activity is now called CSEDI, for Cooperative Studies of the Earth's Deep Interior.< P> CSEDI is a community initiative aimed at making major advances in understanding how the Earth works. This will be accomplished by the facilitation and encouragement of collaborative projects that would bring together investigators from different institu tions and disciplines to attack fundamental problems related to the state and dynamics of the Earth's deep mantle and core, their influence on the evolution of the Earth as a whole and on processes that affect the Earth's surface.

 During the spring and summer of 1992, Rick O'Connell led an effort to prepare a science plan that spells out the scientific motivation for the CSEDI initiative, presents a number of examples of collaborative projects that could be carried out under this initiative, and proposes a plan of operation and management of the initiative. Two distinctive features of the proposed initiative are its minimal administrative structure, consisting of a Chairman, Secretary and a small coordinating committee, and that the initiative and its officials will have no direct voice in the process of funding CSEDI projects.

 The CSEDI initiative was approved at a general meeting held at MIT September 11-12, 1992, which was attended by more than 100 interested scientists from 45 universities government laboratories and private institutions in 21 states and the District of Col umbia. The preliminary version of the science plan was approved and an interim coordinating committee was formed, consisting of T. Ahrens (Cal Tech), J. Bloxham (Harvard), D. DePaolo (UC Berkeley), R. Jeanloz (UC Berkeley), T. Jordan (MIT), L. Kellogg (U C Davis), T. Lay (UC Santa Cruz), D. Loper (Florida State), R. O'Connell (Harvard), and A. Zindler (Columbia). This committee has a tenure of one year and has been charged with (1) finalizing of the science plan, (2) publicizing the formation of CSEDI, for example with an article in EOS and/or an article in Review of Geophysics, (3) promoting the CSEDI initiative to NSF, NASA, DOE, etc., (4) preparing a proposal to be submitted to NSF about December 1, 1992, for the financial support of CSEDI for the n ext year or two, (5) organizing and hold a general meeting in mid 1993, (6) encouraging proposals for CSEDI workshops to be submitted to NSF, (7) providing liaison with the IUGG SEDI organization, (8) proposing a long-term administrative structure for CSE DI.

 Copies of the science plan are available from R. O'Connell, Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Rd., Cambridge, MA 02138.
 

Canadian Global Geodynamics Project

The GGP is currently in a pre-planning phase. In Mizusawa, a meeting between the Canadian and Japanese superconducting gravimeter (SG) teams was held and the state of the Japanese SG operations (with 4 instruments, more than any other country) was revie wed. Subsequently, at a Workshop on High Precision Tidal Processing just concluded in Bonn, it was determined that several SG groups do not anticipate being ready to begin a long observation period until 1994. This delay is due to the necessity of upgra ding some of the data acquisition systems and to allow new instruments to be sited, as anticipated in the original working document. Nevertheless, cooperative data exchange between the Canadian and French SG teams has already been realized and the Canadi an SG group is the first to make the raw SG data generally available through INTERNET.

 It was agreed in Bonn that specifications for the data acquisition systems be discussed at the International Symposium on Earth Tides, to be held in Beijing in August 1993. The Canadian component of the GGP, dealing with the funding of a Data Centre and two new Canadian SGs, after some delay, is now ready to be presented to NSERC for evaluation.
 

Chinese SEDI organization

In the Spring of 1992, the China SEDI Committee was established as a Committee of the China IUGG, with Ouyang Ziyuan (Institute of Geochemistry, Chinese Academy of Sciences) as Chairman, Xu Wenyao as Vice Chairman, and Ma Shi-Zhuang as Secretary (both in Institute of Geophysics, Chinese Academy of Sciences). This committee organized the first China SEDI workshop in the Summer of 1992 in Weihai, Shandong Province. The meeting reviewed current Chinese SEDI activities on the subjects of mantle dynamics, c ore-mantle interactions and geodynamo theory, high-temperature and pressure experiments, dynamical geodesy and others, with 45 papers being presented in 5 half-day sessions. The proceedings will be published in the first issue of volume 7 of Geophysical Progress (in Chinese), to appear in 1993.
 

Japanese Earth's Central Core Project

Efforts of the Japanese SEDI community of this year were concentrated on organizing the Third SEDI Symposium, "Core-mantle boundary region: Structure and dynamics." As reported in the lead story, the symposium was held at Mizusawa for July 6 to 10. Sin ce mid-1991, three scientific meetings have been held in Japan. One was held at the time of spring general meetings of the Earth Sciences societies in Kyoto in April, 1992. A special session, "Earth's central core," was convened on April 7-8, and 43 pap ers were presented. The other two symposia were organized under the program, "The Earth's central core." The first, on"Early history of the Earth and processes taking place in the deep Earth," was held at Tsukuba University October 7-8, 1991. The 13 rev iew talks were followed by heated discussion and comments. The other was the fifth symposium on "The Earth's central core," held at National Astronomical Observatory, Mizusawa, December 24-26, 1991, where 48 papers were read.

 The next symposium on "The Earth's central core" is scheduled for January 20-22, 1993, in Tokyo. A special session will be convened again for the studies of the Earth's core during the general meetings of the Earth Sciences societies, which is to be hel d in Tokyo in March, 1993.

 The second volume of "The Central Core of the Earth", the progress report of the Japanese research program on the Earth's central core, was published in March of this year, which included 56 papers, some in English and other in Japanese. Copies are avail able from T. Yukutake, Earthquake Research Institute, University of Tokyo. The National Astronomical Observatory plans to install a super-conducting gravimeter in the Antarctic with the cooperation of the National Polar Research Institute. The gravimeter will embark for the Antarctic on November 14, 1992.

 Progress has been made in many areas such as seismic tomography of the deep mantle, the study of geomagnetic variations related to long period climate changes, high pressure experiments, theoretical and experimental studies of compositional convections. They may be referred to the above mentioned progress report or the symposium abstract book.
 

Scientific Council on Geomagnetism (Russian Geomagnetic Institute)

This Council was created in 1957 as an interdisciplinary paleomagnetic commission to coordinate research carried out in Institutes of Academy of Sciences, Ministry of Geology and Universities. In 1962 the commission was accepted by the Academy as its in formal consulting body. Its first chairman was Prof. B.M.Yanovsky. After his death in 1967 the commission was re-organized as the Council on Geomagnetism headed by two co-chairmen: Prof.V.A.Troitskaya (magnetospheric phenomena) and G.N.Petrova (magnetis m of the solid Earth). In recent years its co-chairmen have been Prof. M.S.Zhdanov and Prof. G.N.Petrova. Prof. A.N.Khramov, Prof. V. I. Golovkov and Dr. T.S. Gendler (secretary) are members of its executive committee. The Council consists of twelve wo rking groups: 1. Main Geomagnetic Field. 2. Magnetospheric Phenomena. 3. Magnetic anomaly of continents. 4. Magnetic anomaly of oceans. 5. Electromagnetic induction. 6. Tectonomagnetism. 7. Paleomagnetism and Tectonics. 8. Fine structure of paleomagnetic field. 9. Magnetic Stratigraphy. 10. Rock-magnetism and paleofield intensity. 11. Magnetic mineralogy. 12. Instruments and Observatories.

 The main task of the Council is the creation of a scientific and friendly atmosphere to facilitate easier and pleasant communication between magnetologists of the former USSR. Each year we organize four or five workshops or All-Union Seminars which attr act up to 100 participants. Interdisciplinary congresses of magnetologists are organized every four years with 400 or more participants from the USSR and 20 or 30 guests from abroad. The last congress took place at Suzdal in March 1991. By having its m eetings in different circles, the Council fostered the formation of new teams of magnetologists, and also helped and supported experienced colleagues.

 New political and economic conditions have forced the Council to look for new forms of its work, perhaps as an International association. But our immediate task is to help magnetic observatories and small teams of magnetologists to survive with financia l support from the Russian Government and, maybe, some grants of foreign scientific foundations.

 This report was contributed by V. Golovkov (IZMIRAN).
 

Report of the workshop of the US National Geomagnetic Initiative

In 1989 the US Geodynamics Committee of the National Academy of Sciences appointed an ad hoc Task Group, headed by J. Hermance (Brown Univ., Providence) to mobilize a geomagnetic initiative and to define specifically its task and focus. These efforts cu lminated in a workshop on the US National Geomagnetic Initiative held at the National Academy of Science in Washington D.C. on March 16 - 20, 1992, and attended by about 75 scientists representing all disciplines of geomagnetism. The Initiative is an act ivity to define and encourage the implementation of an on-going effort - coordinated among universities, government agencies, and industry - to characterize systematically the spatial and temporal behavior of the Earth's magnetic field on local, regional, and global scales, in order to understand physical processes in the Earth and its environment, and to apply this understanding to a variety of technical and societal needs. The workshop provided a forum in which a cross-section of the geomagnetic commun ity discussed all aspects of the initiative and during which a report to the USGC was finalized.

 The workshop comprised both plenary sessions, where all participated, and working groups for specialized topics. The working groups were of two varieties: one dealing with topics in geomagnetic studies, i.e. the main field and studies of the core and d ynamo, electromagnetic studies of the crust and mantle, studies of the Earth's lithosphere, and studies of ionospheric and magnetospheric fields, and the other dealing with operational issues, i.e. data platforms, data archiving and retrieval, programmati c issues. Each working group wrote a report dealing with its topic. The main report itself consists of the main issues and recommendations raised in both the working groups and the plenary sessions.

This was the first time such a workshop has been held for the discipline of geomagnetism. The interactions between the various groups was strong and healthy. Common concerns, methodologies and interests were identified. In many cases, it was discovere d that nearly identical instrumentation would be appropriate for studies by different disciplines. This will presumably lead to specific interactions in the near future.

 Among the major needs identified, five were of particular interest to SEDI, namely: (1) the expansion of the present network of geomagnetic observatories into one with uniform geographic location, (2) approval and launch of the ARISTOTELES mission, or eq uivalent, (3) a major effort in archeomagnetism and paleomagnetism, (4) continuous monitoring of the near-Earth geomagnetic field from spacecraft, and (5) greater resources to be allocated to core magnetohydrodynamics and geodynamo theory.

 The report is being prepared as an official report of the US national Academy of Sciences.
 

Future meetings of interest to SEDI

As usual, the annual Fall meeting of the American Geophysical Union (held this year in San Francisco, 7-11 December, 1992) will have a number of sessions and presentations of interest to SEDI. Most noteworthy among these is Union session 1 "Studies of t he Earth's Deep Interior: Addressing Fundamental Problems", being convened by T. Jordan of MIT.

 Two sessions of SEDI interest will be convened at the EUG meeting to be held in Strasbourg, France, 4-8 April, 1993. A session on "The mantle transition zone: composition, discontinuities and dynamics" will be convened by G. Bussod (Bayreuth), H. Paulss en (Utrecht), Ph. Machtel (Toulouse) and J. Neuberg (Leeds). Also, a session on "Structure and dynamics of the Earth's inner and outer cores" will be convened by A. Jackson (Oxford), F. Busse (Bayreuth) and J. Hinderer (Strasbourg). For information abou t the meeting, contact Organizing Committee of EUG VII, Geological Survey of the Netherlands, P.O. Box 157, 2000 AD Haarlem, the Netherlands.

The 7th Assembly of IAGA will be held in Buenos Aires, August 8 - 20, 1993. Sessions of interest to SEDI include (1) Models of the geodynamo and core-mantle coupling, (2) Geomagnetic secular variation, (3) Paleosecular variation - direction and intensit y, (4) Nature of geomagnetic reversals, (5) External/internal relations and spatial variations of geomagnetic disturbances at the surface of the earth (6) Geomagnetic secular variation: analysis, interpretation and origin. For further information, write to Local Organizing Committee, 7th IAGA Assembly, Castilla de Correo 106 Sucursal 28 - (1428) Buenos Aires, Argentina.

The 27th General Assembly of IASPEI will be held in Wellington NZ, January 10 - 21, 1994. Sessions of interest to SEDI include (1) Deep-earth discontinuities: configuration and dynamics, (2) structure and composition of the Earth's interior and their re lation to planetary evolution, (3) seismic tomography and mantle dynamics. for further information contact The Secretary, IASPEI 94, Institute of Geological and Nuclear Sciences, P.O. Box 1320, Wellington, New Zealand.
 

Remembrance of Ned Benton.

The last item is the sad news of the death of Edward R. "Ned" Benton on January 25, 1992, who served as the first Chairman of SEDI from 1987 to 1991. The following is a summary of Ned's scientific career, assembled with the help of a number of his frien ds.

Ned's scientific career began with the completion in 1960 of his doctoral thesis in Applied Mathematics on "Aerodynamic origin of the magnus effect on a finned missile" under the supervision of Professor Arthur E. Bryson, Jr. at Harvard University. For the first few years thereafter, he continued work in applied mathematics on the supersonic magnus effect, principally as a Assistant Senior Engineer at Arthur D. Little, Inc., in Cambridge, Mass.

Ned began the transition to his main career interests in geophysics by serving as a lecturer in mathematics at the University of Manchester for the 1962-63 academic year. Following this 'early sabbatical', he moved to Boulder, Colorado, to take a positi on as a Staff Scientist at the National Center for Atmospheric Research (NCAR) in the fall of 1963. The following year he also became a lecturer in the Department of Astro-Geophysics (now the Department of Astrophysical, Planetary and Atmospheric Science s) at the University of Colorado (UC), thus beginning an employment oscillation between NCAR and UC that lasted through 1977. The milestones along the way included academic appointments at UC to Assistant Professor in 1965, to Associate Professor in 1967 and Full Professor in 1971, and serving as Assistant Director of the NCAR Advanced-Study Program from 1967 to 1969, UC Departmental Chairman from 1969 to 1974, Special Assistant to the President for University Relations of the University Corporation for Atmospheric Research (the parent organization for NCAR) from 1975 to 1977 and Associate Director of the Office of Space Science and Technology of UC from 1986 to 1987.

Ned's transition from engineer to geophysicist began with the publication in 1964 of a study of zonal flow inside an impulsively started rotating sphere. This work blossomed into a series of fundamental studies of the combined effects of rotation and ma gnetic fields on confined fluids, culminating in 1974 with a review of spin up. During this period, Ned also had a secondary interest in solutions of Burghers' equation. The next geophysically related area in which Ned took an interest was kinematic dyn amo theory, with a series of three papers published in 1975-1979 on Lortz-type dynamos. These papers developed a systematic categorization of these simple kinematic dynamos and provided some useful insights into their possible structures. This work was not continued as Ned's interest was soon drawn in other directions.

The work up until 1979 was prelude to his major scientific contribution in the area of inversion of the geomagnetic field. In that year he published a burst of papers on this topic, highlighted by papers on "Magnetic probing of planetary interiors" and "Magnetic contour maps at the core-mantle boundary". Earlier in the mid-60s workers had peered through this geomagnetic window on the Earth's core, but when it was shown that the view was both incomplete and flawed, interest in this subject had languishe d. Following Ned's revitalization of the subject, it has flourished, with contour maps of magnetic field and velocity at the top of the core being published by a number of groups. Now accurate determination of these features is deemed to be a vital part of solving the dynamo problem.

Many of Ned's colleagues know him best for his service as Chairman of SEDI (Study of the Earth's Deep Interior) from 1987 to 1991. SEDI began as an idea at the IAGA Scientific Assembly in Prague, Czechoslovakia, in August, 1985, when IAGA Working Group I-2 (on Theory of Planetary Magnetic Fields and Geomagnetic Secular Variation) called for the creation of a project entitled International Study of the Earth's Core and Lower Mantle, with acronym ISECALM. Ned co-chaired an effort to obtain approval for ISECALM as an official program of IAGA and IUGG, and served as its first Chairman from 1987 to 1991. During his term as Chairman, SEDI held symposia at Blanes, Spain, in June of 1988, and Santa Fe, New Mexico in August of 1990, as well as a large number of special sessions at various meetings of AGU, EGS, IAGA and IASPEI. Also, a number of national SEDI groups were formed, including those in Britain, Canada, France, Japan and the United States. Several of these groups have been successful in institutin g national scientific projects with the help of SEDI. Also, SEDI has endorsed several new scientific projects, including ISOP, INTERMAGNET and the Canadian GGP. All this activity has been a great boon for the deep-earth geoscientific community and has h elped these studies maintain their visibility and viability. This success is due in large part to Ned's steady hand at the helm during SEDI's crucial formative years.

Ned received a number of honors for his scientific research and service, including NASA's Group Achievement Award in 1983, a Scholarship from the Cecil H. and Ida M. Green Foundation for Earth Sciences, and election as Fellow of the American Geophysical Union shortly before his death.

The following are some personal comments made by Ned's colleagues and friends.

"He was an outstanding lecturer, extremely clear, well organized. His work was rigorous, careful, but still creative and insightful. Ned was a very kind man, who genuinely cared about the interests of others. And he was a very friendly man, always chee rful."

"His lectures were gems, beautifully prepared and presented. Just as impressive was his obvious concern that his students really understand the subject. He cared about students. To me, Ned epitomized the role of the true teacher, which is not to cover material, but to uncover it. All of us who labor in university education hope to achieve some measure of immortality in this world through publications that will stand on library shelves and, especially, through the students who will carry on the work w hen we are gone. Ned Benton certainly achieved this."

"Ned helped to build a department that has astrophysicists, atmospheric and planetary scientists working together --- he had a gentle diplomatic touch, yet was forceful and energetic, and was always able to express his real enthusiasm for building such a n unusual interdisciplinary department."

"Ned has inspired many students about how mathematical physics can be used to make detailed predictions, and how seemingly small features in the flows, called boundary layers, can have dominant control over the major flows. We call some of these by nam es like Ekman layers and Hartmann layers, possibly to make it all seem more mysterious, but Ned was a creative master at working with these, and teaching others about them. You can be remembered by who were your students, and Ned does very well by that m axim."

"Ned was an outstanding teacher, an elegant scientist, and an exceptionally fine man. The better I got to know him, the more I liked him. He was brilliant, yet rock solid, patient, warm, caring, and kind."

"I am grateful for having known and worked with Ned Benton, and having enjoyed his friendship. Ned combined the talents of a brilliant scientist with the temperament of a splendid, caring human being."

"Ned was a gentleman and a scholar: knowledgeable, understanding and eloquent. He loved the outdoors -- particularly running and backpacking. He was charitable and soft spoken. He just wouldn't voluntarily say unkind things about people -- either in general or in particular. Of course, his tolerance of human error did not extend to technical errors; yet when he found technical flaws, he would point them out in a gracious way. Along with his uncanny insight and organizational ability, this quality m ay help explain why he was a natural leader of so many activities: from special projects for the University to SEDI and other initiatives espoused by the scientific community."

"One felt that here was not only a completely honest individual, but also one who you could rely on for help in time of need. One cannot think of him without recalling the stalwart efforts he made to advance his field, for example by his commitment of t ime and effort to ensure that SEDI was launched successfully, even at times when he was sick and must have been in pain, though being Ned he never complained of his affliction to my knowledge. I feel proud to have known him."

"Ned always had an enormous amount of time for others, perhaps to the detriment of his own career at times."

"The loss of his scientific talents, leadership ability and companionship will be felt strongly by his many colleagues, but he leaves a legacy in his scientific works, the strength of SEDI and many fond memories of him as a good friend."

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