| Number 5 | Fall, 1991 |
This is the fifth annual issue of the newsletter of SEDI, an IUGG Union
Committee to Study the Earth's Deep Interior. Requests for additional copies
of this issue, or for copies of the earlier issues, should be addressed
to David Loper, Geophysical Flui d Dynamics Institute, Florida State University,
Tallahassee, Florida 32306-3017, U.S.A. Also news items for the next issue
or notifications of change of address should be sent to the same address.
The first group of presentations on Friday, August 16th, concentrated on the relation between mantle convection, geoid and topography. In the first paper, S. D. King (Univ. California, San Diego) and B. H. Hager (Massachusetts Inst. Tech.), presented th e results of numerical models of subduction with non-linear rheology, with emphasis on the resulting surface geoid and topography. They concluded that the lithosphere and lower mantle are mechanically decoupled, but prefer whole-mantle convection. P. Co lin and L Fleitout (Ecole Normale Supérieure, Paris), noting that mass heterogeneities in the lower mantle contribute to the geoid but not the topography, showed that lateral viscosity variations in the lower mantle do not cause these two surface features to become coupled. E. Okal (Northwestern Univ.) presented evidence that P and S wave diffraction in the D" layer is due principally to variations in temperature and composition. Two-dimensional numerical models of mantle convection with lithospheric pl ates by C. W. Gable and R. J. O'Connell (Harvard Univ.) show that in general for bottom-heated convection the internal convective pattern is different from the plate geometry. V. Dehant (Royal Observatory of Belgium) and J. Wahr (Univ. Colorado) investig ated the core-mantle topography due to internal load within the mantle. They found that a low-viscosity D" layer reduces the topography by roughly a factor of two. J. X. Mitrovica (Harvard Univ.) and W. R. Peltier (Univ. Toronto) combined satellite data on the secular variation of of the geopotential field with sea-level records using dynamic models of the mantle. They found that the data are consistent with a lower mantle having viscosity of either 10E21 or 10E23 Pa s (they preferred the smaller value ).
The next group of presentations dealt principally with dynamical models. First, P. Machtel, P. Weber (U.P.R., Toulouse) and D. A. Yuen (Univ. Minnesota) presented results of an axisymmetric numerical code showing that a large Clapeyron slope for the pha se change at 670 km depth produces convection that appears to be layered. They also concluded that realistic heat fluxes can be achieved only if the temperature of the core-mantle boundary is between 300 and 3500 K. The three-dimensional, spherical-shel l model of mantle convection presented by D. Bercovici (Univ. Hawaii), G. Schubert (Univ. California, Los Angeles) and G. A. Galtzmaier (Los Alamos Natl. Lab.) showed that the modes which interact with the primary (large-scale) modes have triple the waven umber of those modes. I. A.Eltayeb (Sultan Qaboos Univ.) and D. E. Loper (Florida State Univ.) investigated the possibility of a shear-flow instability in the D" layer, but found that a D" layer of uniform thickness is most unstable in the absence of she ar. The interaction of mantle plumes with the core-mantle boundary was studied using an axisymmetric numerical code by L. H. Kellogg (Univ. California, Davis) and S. D. King, who found that if the stabilizing compositional component of the density is at least 7.5 times the buoyant thermal component, the heavy material forms puddles and is not entrained by the plume. In the final presentation before lunch, H. C. Nataf and colleagues (Ecole Normale Supérieure, Paris) described a novel experiment in which a mylar sheet dragged across the top of a vat of syrup simulated the motion of lithospheric plate. This plate induced a descending slab which forced the upwelling motion from the heated bottom boundary to be concentrated in plume-like upwellings.
The last group of presentations on Friday began with D. E. Smylie (York Univ.) and X.-H. Jiang (Inst. Physique du Globe, Strasbourg) describing their numerical calculations of the modes of oscillation of the Earth's core. L. Guoying, P. Longhui and H. H outse (Chinese Acad. Sci., Hubei) discussed the possibility of measuring with a superconducting gravimeter the effect of lateral heterogeneities in the mantle. M. Weber (SZGRD, Erlangen) M. Känig (Univ. Frankfurt) and P. Davis (Teledyne Geotech, Alexandr ia, VA) reported on their attempts to map those regions of the lower mantle which have a P-wave velocity jump of a few percent approximately 300 km above the core-mantle boundary. Existing data allows about 40% of the mantle to be sampled. Using plate-t ectonic reconstructions, M. A. Richards (Univ. California, Berkeley), D. C. Engebretson (Western Washington Univ.), C. Lithgow and H. P. Bunge (Univ. California, Berkeley) and H. P. Bunge showed that current geoid and topography correlate with locations o f subduction in the past 200 myr, and that with a time lag of 20 myr, increases in sea level correlate with increases in spreading rate. In the final presentation of the day, S. K. Runcorn ( Univ. Alaska) presented historical data which supports the idea that there is a long-term interchange of angular momentum between the core and the mantle. There were a total of nine poster presentations associated with this first day of the symposium.
The symposium resumed on Monday, August 19, with presentations focusing on the dynamics of the core and the rotation of the Earth. In an invited presentation J. Hinderer (Inst. Physique du Globe, Strasbourg) reviewed internal dynamical processes in the core and their effect on the surface gravity field. He noted that it will be necessary to stack data from a number of superconducting gravimeters to detect signals from the core. the simultaneous detection of short-period polar motions using VLBI and su perconducting gravimeters was discussed by K. D. Aldridge and W. H. Cannon (York Univ.). P. R. Cummins (Australian National Univ.) and J. Wahr noted that the Q of the Earth's nearly diurnal free wobble detected by IDA tidal data and by VLBI are not in ag reement, and speculated that the ocean corrections may be the cause to the discrepancy. In the second invited presentation, T. A. Herring (Massachusetts Inst. Tech.) reviewed VLBI data and the implications for studies of the dynamics of the Earth's inter ior. B. A. Buffett (Cambridge Univ.) discussed the influence of the toroidal magnetic field or a turbulent viscosity on the nutations of the Earth, through core-mantle coupling, and concluded that significant electrical conductivity within the lower mant le is likely to crucial in the coupling process. A. M. K. Szeto (York Univ.) reviewed the mechanisms which would cause the inner core to rotate differentially with respect to the mantle, and the dynamical and observational implications of this rotation. The relation between thermal core-mantle coupling and variations of the Earth's rotational speed was analyzed by C. Kakuta (National Astronomical Obs., Mizusawa).
In the third invited presentation of the Symposium, J. P. Poirier (Inst. Physique du Globe, Paris) reviewed the physical properties and dynamics of the core. He concluded that compositional buoyancy effects dominate thermal effects, and that Soret effec ts are small. R. Widmer, T. G. Masters and J. F. Gilbert (Univ. California, San Diego) discussed seismic modes that are anomalously split within the outer core. The cause of this splitting is unknown. V. Dehant, J. Hinderer, H. Legros and M. Lefftz (In st. Physique du Globe, Strasbourg) presented analytical computation of the rotational eigenmodes of the inner core. In the last oral presentation on Monday, the structure of the lowermost mantle and the topography of the core-mantle boundary were reviewe d by A. M. Forte, A. M. Dziewonski and R. L. Woodward (Harvard Univ.) in a series of tomographic pictures. They found a consistent correlation between seismic anomalies in the lower mantle (interpreted as thermal anomalies) and the topography. There wer e a total of nine poster presentations associated with this second day of the symposium.
The final session of the Symposium, concentrating on core-mantle boundary topography and core-mantle coupling, was held on Tuesday evening, August 20th, with, not surprisingly, a somewhat reduced audience. The last invited speaker, J. Neuberg (Gadjah Mada Univ.) reported on seismological means of deriving the CMB shape either by performing global inversion of core phases or by probing locally the CMB and D" layer using waveforms of those phases; the amplitude of CMB undulations of wavelengths from 50 to 400 km cannot exceed 2-3 km in the probed area if PcP is observed. A recent analysis of a suite of shear waves allowed T. Lay (Univ. California, Santa Cruz) to infer seismological constraints on the structure of the D" layer and of the E' layer at the top of the core: an anisotropic D" layer is suggested, and the E' region may have lower P velocities than predicted by PREM. A. Jackson (Oxford Univ.) and J. Bloxham (Harvard Univ.) computed the changes of the core angular momentum during the last two centuries using geomagnetic data and a theory of the motions in the core, and compared them to the changes required by length- of-day data to keep the global Earth angular momentum constant; results are encouraging. T. Yukutake (Univ. Tokyo) investigated the electromagnetic core-mantle coupling related to the Earth's tidal deceleration; he concluded that this torque is strong enough to make the core lag about two years, and is probably stronger than the viscous torque. Finally, R. Hide (Robert Hook Institute, Oxford) reviewed the irregular fluctuations of the Earth's rotation. The elucidation of the torque coupling the core and the mantle concomitant with decadal variations needs further progress in the knowledge of the lowermost layers of the mantle; large-scale features of the CMB probably play a part. Seven poster presentations were associated with this last session of the symposium.
Papers based on contributions to Symposium U6 will be published
in an issue of the AGU monographs, edited by J.-L. Le Mouël and
D. Smylie. The deadline for submission of papers is December 1st.
A number of themes that emerged were notable for being common to several of the papers. For example, the variability in global-tectonic style observed on Venus, Earth and Mars is highlighted by the recent spacecraft observations, as described by W. M. K aula (Univ. California, Los Angeles) and was also the subject of thermal-history calculations by D. Breuer and T. Spohn (Westfälische Wilhelma-Universität Münster).
Systematic variations in chemical composition and internal state among the terrestrial bodies is thought to originate during planetary formation, however. H. Wänke (Max-Planck-Institut für Chemie, Mainz) discussed the need tor two components (oxidiz ed and reduced) being mixed in various proportions during accretion to explain such variability among the terrestrial planets. This modification of heterogeneous accretion scenarios was reinterpreted by T. Ahrens (California Inst. Tech.): while accepting the need for reduced and oxidized regimes, he ascribed these to different temporal periods succeeding each other during the accretion-bombardment period. Also, it appears that Jupiter played a significant role in modifying the growth of planets closer t o the Sun, as highlighted by V. N. Zharkov (Inst. Physics of the Earth, Moscow) and alluded to by H. WÄnke.
The concept of a magma ocean on the early Earth is closely linked to the hypothesis that the Moon was formed by a giant impact. L. G. Liu (Australian National Univ.) considered the effects of material addition (from the impactor) and loss (to the Moon) on estimates of the bulk composition of the Earth. thereby revising his previous estimates of lower-mantle and outer-core compositions. Whereas E. Takahashi explored the consequences for lunar formation of a giant impact into a pre-existing terrestrial m agma ocean, Y. Abe (Nagoya Univ.) emphasized the evolution of such a magma ocean as related to the early differentiation of the Earth. In particular, there appears to be only a narrow window of viscosity, hence crystal/liquid ratio, over which crystal-me lt separation is effective.
The effect of pressure on internal planetary evolution continues to be a rich field of study, especially among experimentalists. N. Kamaya, E. Ohtani and colleagues (Tohoku Univ.) contrasted the mantle structure of Mars (low pressure) and Earth (high pr essure), as deduced from experimental petrology, and C. B. Agee (Harvard Univ.) reviewed the evidence for reversals in crystal/melt buoyancy at depth -- and probably at several depths, as a result of pressure-induced phase changes -- within the Earth's ma ntle or precursory magma ocean. Also, E. Ito and T. Katsura (Okayama Univ.) presented new evidence supporting the idea that core-forming (iron-alloy) liquids react with crystalline silicates at high pressures.
A separate effect of pressure is to stabilize hydrous phases, hence to allow the presence deep in the Earth's mantle of up to several weight percent "water" (hydroxil-based species): an amount corresponding to ~10 - 100 times the mass of the hydrosphere at the surface. This theme was reiterated by T. Ahrens, T. Gasparik (State Univ. of New York, Stony Brook) and R. Jeanloz and colleagues, all three emphasizing the permissive (or suggestive) nature of the existing data. Multi-anvil experiments are provi ding samples for detailed crystallographic characterization, and studies with the diamond-cell offer complementary information on electrical conductivity and strength. The latter indicate that deep-focus earthquakes could at least in part be associated w ith the subduction of hydrous minerals, suggesting that the mantle is currently being recharged with water.
The final paper by Francois (Univ. Liège) et al united many of these themes by showing that there is likely to have been a feedback between the earth's tectonics (mountain building and erosion) and climate over the past 140 million years, as document ed by changes in the marine record of strontium isotopes and carbonate deposition.
Several other talks rounded out the Symposium, which will be summarized
in a proceedings volume that is to be published by AGU within the coming
year.
The session started with 8 mineral physics presentations dealing with partial melting and rheology. First, P. Wyllie (California Inst. Tech.) presented three models for the mantle-volatile system: low oxygen fugacity, high oxygen fugacity and above the solidus-curve critical point. Above the solidus critical point, silicate melt and dense vapor becomes a single "fluid" phase; thus, in partial melts beyond critical conditions, the trace element distribution is obviously different in the other cases sinc e they cannot partition into different phases. Next, S. Mackwell (Pennsylvania State Univ.) showed that the water-derived species that diffuse into dry olivine are protons, which diffuse into and out of olivine easily at high temperatures. Related to th e protonation, high hydrogen activity reduces the strength of olivine up to a factor of three. The loss of strength is due to an increased dislocation density; under lower hydrogen activities, olivine is not weakened. D. C. Rubie (Bayerische GeoinstitÜt , Bayreuth) reported an extension (by a factor of 20) of the pressure range available for examining dislocation densities of olivine under hydrostatic conditions and controlled chemical environment, thus avoiding the need for long extrapolations of low-pr essure data. H. Spetzler (Univ. Colorado) described a new and very sophisticated technique for measuring seismic attenuation, spanning 13 orders of magnitude in frequency, in single crystals as thin as 100 mm. From deformation experiments under constant torque on a KCl-H2O system, T. Watanabe (Univ. Tokyo) showed that this system changes from a non-Newtonian to Newtonian rheology as the melt fraction increases above the critical level of 10%.
T. J. Ahrens concluded from shock wave results on molten silicates (komatiites and MORB) that the density of the melt exceeds that of the solid at some critical pressure, in general agreement with the results of Agee and Walkers' sink-float experiments. He suggested that olivine and clinopyroxene are neutrally buoyant at 250-300 km depth and garnet-majorite is neutrally buoyant at 500-1700 km depth. This result could remove one of the major arguments against the giant-impact hypothesis of the formation of the moon. Using the above solid and melt densities, S. Franck (FHD, Potsdam) presented results of his thermal-evolution calculation of a global magma ocean using simple stability estimates and cooling time calculations. He surmised that the crystall ization of a magma ocean cannot wholly explain a separate chemical evolution of the two mantle regions. H. W. Green (Univ. California, Davis) illustrated that the phase transformation of olivine (Mg2GeO4) to spinel can produce shear fracture under condit ions where nucleation and growth are just possible. He concluded that deep-focus earthquakes are produced in downgoing slabs when the rocks under stress warm to the threshold of nucleation of the denser phase of olivine.
R. Böhler (Max Planck Institut für Chemie, Mainz) introduced a new CO2-laser-heating technique in the diamond-anvil cell that makes it possible to measure silicate phase transitions accurately beyond the P-T conditions possible in large presses. First results on perovskite constrain the temperature at 670 km depth to 1900 ± 100 K. F. Guyot (Inst. Physique Globe, Paris) demonstrated phase changes in germanate olivines by XANES on samples in the diamond-anvil cell. It appears that the divalent c ation shell is modified at 7 GPa in Ca2GeO4 and at 13 GPa in Mg2GeO4 and that CaMgGeO4 amorphized at 17 GPa. A. Chopelas found phase changes in the high-pressure polymorphs MgSiO3 clinopyroxene at 5 GPa, majorite at 28 GPa and perovskite at 37 GPa by Ram an spectroscopy; the spectra revealed marked changes in the number of peaks and their frequencies as well as in the pressure derivatives of the modes, suggesting drastic changes in compressional properties. This implies that long extrapolations of physic al properties measured near ambient conditions on these materials may not be valid. G. D. Price (Univ. College London) showed that molecular dynamical simulations can predict the physical properties of mantle materials at high pressure or temperature con ditions quite accurately, particularly volumes. First attempts on prediction of melting based on the mean-square displacements of atoms in conjunction with the Lindemann's melting law yielded results on perovskite very near to experimental values.
T. J. Shankland (Los Alamos National Lab.) investigated the nature of the bonding of iron in fayalite by resonant photoemission spectroscopy and found similarities to previous results for wÜstite, hematite and magnetite, suggesting that iron-bearing sili cates can behave electrically as transition metal oxides. T. Yagi showed that erroneously indexing the x-ray diffraction pattern of MgSiO3 tetragonal garnet in the diamond-anvil cell as a quasi-cubic structure yields an anomalously high bulk modulus. By using the correct indexing and by carefully analyzing the many overlapping peaks in the diffraction pattern, a bulk modulus of about 160 GPa is obtained. This bulk modulus agrees with that extrapolated across the pyrope-enstatite join for cubic garnets and is much lower that most other previous estimates.
Moving away from mineral physics into mineral dynamics, X. Sun (Beijing Univ.) calculated the effect that three different models of retrograde migration of trenches has on mantle convection. She showed there was generally a larger influence of velocity and kinetic pressure and a smaller effect of temperature. Then, D. Giardini (Inst. Nazionale di Geofisica, Rome), substituting for his co-author P. R. Lundgren (Jet Propulsion Lab., Pasadena), suggested from evidence of two differing mechanisms of earthq uakes occurring near 670 km depth, i.e., (1) bending and (2) selective shearing, that subducting slabs do not penetrate into the lower mantle. Further evidence supporting this concept includes horizontal distribution of seismic events away from the traje ctory of subduction (Banda Sea and Fiji plateau) and thickening of the seismogenic portion of subducting flow (Tonga). D. M. Tralli (Jet Propulsion Lab., Pasadena) examined P and S seismic anomalies in order to constrain composition (Fe/Mg content and mi neral phase) and temperature of the mantle. He suggested some variation in iron content and/or temperature to account for the up to 0.2 km/sec P-wave and up to 0.4 km/sec S-wave anomalies in the upper mantle. E. M. Chesnikov (Inst. Physics of the Earth, Moscow) proposed that non-spherical inclusions produced during partial melting events under non-hydrostatic stresses may contribute to seismic anisotropy. The degree of seismic anisotropy depends on the shear stress in the partially melted zones. B.Bir ger (Inst. Physics of the Earth, Moscow) showed a series of equations delineating his new non-linear rheological model of solid flow. Using this model results in a viscosity for materials associated with post-glacial rebound which is three orders of magn itude lower than those associated with convective flow.
On the second day of the symposium, D. Giardini showed that two different stress states exist in subducted slabs in the Western Pacific (Kurile and Tonga arcs), especially near 670 km depth. Families of large events differing by 90° co-exist in the same locations, which is not expected in the faulting of homogeneous material. W. Su (Harvard Univ.) examined mantle heterogeneity from a different point of view by looking at recordings of over 400 earthquakes spanning 12 years, in particular, at the long w avelength features. He found that the anomalies in the mantle can be related to surface tectonic features. M. S. Patterson (Australian National Univ.) presented measurements of shear modulus and internal friction on dunite performed on a new instrument that observes forced torsional oscillations at seismic frequencies at high P and T. Results suggest that the internal friction is mainly concentrated within the olivine grains and that 1/Q is only mildly frequency dependent but increases exponentially wi th T. Thus, solid state anelastic relaxation is consistent with losses observed seismically in the upper mantle. T. J. Shankland, substituting for his co-author P. A. Johnson (Los Alamos National Lab.), described a new method for measurement of P-wave t ravel times at two different frequencies simultaneously in natural rock vis an interference method. This method appears to alleviate some of the attenuation problems due to the non-linear elastic-wave interactions. Along similar lines, B. P. Bonner (Law rence Livermore National Lab.) studied the non-linear mechanical response under strain arising from both grain-scale microcrack and macroscopic features using the same method introduced by Shankland. Particularly large changes in compressional velocity w ith load were found for partially saturated tuff.
R. Jeanloz (Univ. California, Berkeley) presented new results
on electrical conductivity of lower-mantle assemblages (perovskite plus
oxide phases) at lower-mantle conditions, including some on samples obtained
from J.-P. Poirier. The new results are in agreement with those of Poirier's
lab, except for a Mg0.9,Fe0.1 sample. Jeanloz maintains that the composition
and valence state are the determining factors for conductivity whereas
temperature and pressure play minor roles. K. Kurita (Univ. Tsukuba) p
roposed a liquid immiscibility model for Fe-S-O and Fe-S-C systems as a
mechanism to stratify the outer core. He included some discussion on how
the outer core should crystallize. However, these systems have not been
examined at the relevant P-T conditi ons. C. McCammon (Bayerische Geoinstitüt,
Bayreuth) resolved the wüstite paradox. Previous results showed that
the equilibrium fraction of iron in wüstite (FexO, x=0.92) first increased
then decreased with increasing synthesis pressure when the iron fra ction
is calculated form the lattice parameter, a variation at odds with the
trend of bulk modulus with iron content. Finally, M. Niazzi (Lawrence Berkeley
Lab.) re-examined the attenuation characteristics of the shallow inner
core by analyzing over 30 s hort period-vertical seismograms in the distance
range of 140-160 degrees. Fourier spectral ratios of two different seismic
wave phase provide estimates for attenuation versus depth below the ICB.
P. Roberts (Univ. California, Los Angeles) reported work with M. Kono (Tokyo Inst. Tech.) on the adjoint kinematic dynamo problem. This study helps to explain why dipole and quadrapole modes are excited with almost equal ease. From their viewpoint the more important application is to the weakly nonlinear theory of an alpha squared-dynamo model with quenching which they discussed in another session, "Numerical modelling of planetary dynamos." Results of alpha squared dynamos without quenching were pres ented by R. Hollerbach (Univ. Newcastle upon Tyne). The excited mean magnetic field drives mean motion so the system is fully nonlinear. Particular emphasis was placed on the interaction between magnetic modes of dipole and quadrapole type. C. Jones (U niv. Newcastle upon Tyne) reported related studies of aw-dynamo models, which again emphasized nonlinear features, particularly Taylors' constraint as modified by viscous effects through Ekman layers on the core-mantle boundary.
Thermal convection in the Earth's core was considered by K. K. Zhang and D. Gubbins (Leeds Univ.) They were particularly concerned with consequences of lateral variations of the heat flux in the lower mantle. In their models, this forces convective cur rents in the upper regions of an otherwise stably stratified fluid shell which they believe may be responsible for secular variation of the magnetic field. Zhang reported the results of his numerical investigations. Another model of thermal convection i n a non-uniformly stratified layer was discussed by J. Boda (Univ. Komensky). The combined influence of large Coriolis and Lorentz forces was considered and it was found that strong shear in the zonal flow localized the convection. Resistive instabiliti es of sheared magnetic fields which took energy from the magnetic field, rather than being driven by buoyancy forces, were investigated by W. Kuang (Univ. California, Los Angeles) and P. Roberts. Roberts discussed the nonlinear development of the initial ly linear modes of instability paying particular attention to the role of the excited geostrophic flow.
A recurring theme concerned the nature of the geostrophic motion. That is the axisymmetric azimuthal flow which depends only on the radial distance from the rotation axis. This motion is rapidly accelerated by the Lorentz force. In equilibrium situati ons, Taylor's condition states that the mean value of the azimuthal component of the Lorentz force on the circular cylinders of fluid flow must vanish. This may be modified by core-mantle coupling. Usually viscous effects are invoked. D. Jault (IGN, Sa int-Mandé), on the other hand, described joint work with J.-L. Le Mouël (Inst. Physique Globe, Paris), in which bumps on the core-mantle boundary might lead to a net azimuthal component of the surface pressure force. This, in turn, can balance a non-vanishing mean Lorentz force. These pressure forces lead to a torque between the core and the mantle. M. R. E. Proctor (Cambridge Univ.), in a joint paper with D. R. Fearn (Glasgow Univ.), was particularly concerned with non-axisymmetric magnetic fi elds, which would result from a fully convective dynamo. He expressed doubts that Taylor's condition could be met simply by invoking electromagnetic coupling with a weakly electrically conducting mantle. He suggested that some viscous coupling may be es sential. He also reported a modified version of Taylor's condition which is appropriate when the core-mantle boundary is not quite spherical as modified by surface modulations. M. St. Pierre (Univ. California, Los Angeles), in a joint paper with P. Robe rts, showed that a class of solutions satisfying Taylor's constraint remain stable, when inertia was reinstated.
Work on fully consistent convective dynamos was reported by a number of authors. G. Glatzmaier (Los Alamos National Lab.) and P. Roberts have formulated a set of equations governing such a model and Glatzmaier reported progress on his numerical code to solve the problem. W. Hirsching (Univ. Bayreuth) described joint numerical studies with F. H. Busse (Univ. Bayreuth) on convective dynamos in a fluid shell. They were particularly concerned with electromagnetic coupling both with a central rigid core an d an outer, poorly conducting mantle. The coupling causes the inner sphere to rotate differentially with respect to the outer boundary and influences the convective columns which occur. Busse, in a joint paper with J. Wicht (Univ. Bayreuth), described a n elementary dynamo which can occur as the result of a simple relative flow adjacent to a boundary. The essential ingredient for dynamo operation is lateral variations of the conductivity in the solid boundary.
On a different theme, W. Marzocchi and F. Mulargia (Univ. Bologna) considered geomagnetic reversals. Marzocchi described statistical evidence for no significant period in the inter-reversal times, in agreement with a stochastic mechanism for their origi n. On the other hand, they report a 15 m.y. periodicity in the rate of reversal occurrence present in many geomagnetic scales.
D. R. Fearn (Univ. Glasgow), who gave the first paper (by Fearn, Lamb & McIntyre), was an invited speaker and spent part of his allocated time giving a review of the topic of the session from an applied mathematical point of view. It is known that the field strength required for instability can be comparable with the inferred toroidal field strength in the core, thus justifying the study of instabilities as part of our understanding of the dynamics of the core. He then went on to present results from a study of the weakly non- linear regime in a cylindrical geometry. I. A. Eltayeb then discussed the propagation of waves in a thin annular shell in a variety of ambient magnetic fields with several different temperature gradients. In the weak field regime the propagation and stability of waves within a highly rotating fluid contained in a sphere in the presence of a co-rotating magnetic field is known to be strongly influenced by the boundary of the sphere. The introduction of a solid inner core influences the propagation and stability of the waves in a profound way. The motions take the form of annular cylindrical cells. When the inner core is present the nature of the motions that can occur within the annular cylindrical surface touching the inner core at its equator are different from those motions occurring outside that surface. W. Weiglhofer (Univ. Glasgow) & D. R. Fearn (presented by Weiglhofer) examined the linear stabilities present in a rigidly rotating sphere in the magnetospheric approximation. They found that the field gradient and resistive instabilities were most important for the parameter regime most likely to represent that of the EarthUs core.
After coffee, the emphasis was more on geomagnetic data and what might be learned from them. C. Voorhies (NASA, Goddard), giving a paper by Voorhies & M. Nishimaha, discussed how Ohmic diffusion could be introduced into the problem of solving for fluid flow at the core-mantle boundary (CMB) from geomagnetic field models. E. R. Benton & M. Celaya (both at Univ. Colorado; presented by Benton) presented a solution for an unsteady CMB flow with specific (polynomial) time dependence, again with flux diffusion. The importance of wave propagation to the study of hydromagnetic phenomena in planetary and stellar interiors was stressed by R. Hide (Robert Hooke Inst., Oxford) who predicted that the study of the linear waves propagating on non-axisymmetric basic states is the next step following the almost full understanding of linear waves on axisymmetric basic states. G. Hulot & J.-L. Le Mouël (both at Institut de Physique du Globe, Paris; presented by Hulot) examined the symmetry properties of CMB flows inferred from geomagnetic field models, suggesting that equatorial symmetry and symmetry about the EarthUs center might be useful approximations which reduce the size of the numerical inverse problem. R. G. Davis (Univ. Leeds) & K. Whaler (presented by Whaler) had obtained a series of CMB flows for 1915-1975 by assuming the flow was steady over time intervals of a decade. They deduced that changes in amplitude of the flow were more significant than changes in the flow pattern, in general, although there was a reversal of the flow direction beneath the northern Atlantic around 1940. J. Bloxham (Harvard Univ.), on the other hand, had obtained a good fit to the data for the last century and a half by assuming a steady flow over that period. Bloxham also questioned some of the symmetry approximations proposed by Hulot & Le Mouël, which he did not believe were justified by the data.
The session was lively and reasonably well attended. Particularly
pleasing was the emergence of points of contact between the dynamo theorists/applied
mathematicians and those working with geomagnetic data. Hopefully by the
time of the next IAGA meeting those links will have become closer still.
"Hors d'oeuvre": a session on the influence and modeling of realistic material properties, and the link with plates and surface observables. N. Ribe (Yale Univ.) presented a thin-shell theory for modeling large lateral viscosity variations in the lithosphere, and pointed out an unexpected scaling relation for stresses. The role of phase transitions, and its recent revival for explaining partly layered convection, was discussed by P. Machetel (CNES, Toulouse). We then had a very healthy discussion on the influence plates have on several observables: poloidal-toroidal partitioning (R. O'Connell, Harvard Univ.), mean lithospheric rotation (O. Cadek, Ecole Normale Superieure, Paris), geoid signature of hotspots (P. Colin, Ecole Normale Superieure, Paris), etc.
"Entree": How can we model compositional heterogeneities in a convecting mantle ? Take a blob of heavy stuff, and stick it into a convecting system. It gets sheared, and soon distorted, and filamented. How far can we, and should we, go in following this filamentation ? What is the dynamical behavior of the system ? We got some answers from U. Christensen (Max-Planck-Institut für Chemie, Mainz), C. Kincaid, and Poliakov (Univ. Minnesota). However, the results from different methods to the 'exercise' proposed by H. Schmelling (Univ. Bayreuth) showed that key questions such as 'how fast does the mixing take place ?' get widely different answers, at present...
"Main course": 3-D convection! Most people working in this rapidly growing field were present. After an introduction to the physics of convection, by F. Busse (Univ. Bayreuth), we had a rather thorough survey of the state of the art, with contributions from G. Houseman (Monash Univ.), L. Cserepes, U. Christensen , G. Glatzmaier (Los Alamos National Lab.), J. Baumgardner (Los Alamos National Lab.), B. Travis (Los Alamos National Lab.), and C. Gable (Harvard Univ.). 3-D codes for temperature-dependent viscosity were described (U. Christensen ), and first results for spherical convection in the inner core (a SEDI special !) were proposed (H. Harder, Max-Planck-Institut für Chemie, Mainz, P. Machtel & P. Weber, CNES, Toulouse).
"Entremets": 3-D benchmark. Benchmarks are one of the specialties of the European Club. Their goal is to compare and cross- check the different numerical methods used in Geophysics, and to provide numbers for validating new codes. The first attempt was the 2-D benchmark proposed at the first Neustadt meeting. It enabled the comparison of 10 different codes, and lead to a publication (Blankenbach et al., 'A benchmark comparison for mantle convection codes', Geophys. J. Int., 98, 23-38, 1989). This time, test cases were proposed for 3-D convection, in Cartesian and spherical geometries, with constant and variable material properties. More than 6 different groups brought preliminary data. In addition, measurements from laboratory experiments have been obtained for the Cartesian cases. Those who would like to contribute can get the material from Uli Christensen (e-mail: URC@DGAIPP1S.bitnet) for the Cartesian benchmark, and from Gary Glatzmaier (gag@lanl.gov) for the spherical one.
"Dessert" was served after U. Hansen (Univ. Koln) showed us his latest video-production: TDie HuegelU, on the behavior of a chemically distinct DS layer in a convecting mantle. Sweet syrups and other delicacies were the heart of this last session, with results from laboratory experiments on variable viscosity (C. Lithgow, Univ. California, Berkeley, E. Giannandrea, Max-Planck-Institut für Chemie, Mainz, A. Davaille, Institut de Physique du Globe, Paris), plates and plumes (H.-C. Nataf), and insulating continents (L. Guillou, Institut de Physique du Globe, Paris).
The Workshop was followed by a newly born 'Workshop on the Modeling of Lithosphere Dynamics', but this is another story...
Next there were a number of brief reports of scientific activities endorsed by, or of interest to, SEDI. A. W. Green summarized the status of Intermagnet (see DIALOG #2 & 3) and noted that they are planning an array of ocean-bottom geomagnetic observato ries which they hope will be able to image events and features on the core-mantle boundary. The status of ISOP (see DIALOGS #1 & 3 and the report of IASPEI Workshop SW12, above) was described by D. Doornbos, noting that a full-time coordinator has been h ired and a newsletter has been started. For more information about ISOP, please contact the coordinator: Eric A. Bergman, National Earthquake Information Center, USGS, MS 967, P.O. Box 25046, Golden Co 80225, USA. The activities within Japan related to the Earth's Central Core Project were described by T. Yukutake (see DIALOG #3 & 4 and the summary below). The French have recently organized a SEDI national group, concentrating on seismology and high-pressure physics, and have had one meeting so far; se e the separate report in the following section. Finally J. Hermance gave a brief description of a planned US National Geomagnetic Initiative; this new initiative is summarized below.
Perhaps the most important item on the agenda was the consideration of an invitation from T. Yukutake, on behalf of the Japanese National Astronomical Observatory, to hold the next SEDI Symposium in Mizusawa, Japan. 6-10 July, 1992. The topic of the Sym posium will be "The Core-mantle Boundary Region: Structure and Dynamics". After some discussion of the timing of the meeting and the detailed wording of the title, the invitation was accepted by acclimation. The Symposium is described in a following sec tion.
The final action taken at the business meeting was consideration of a request for endorsement of a Global Geodynamics Project (GGP), made by D. Crossley (McGill Univ.), on behalf of a consortium of Canadian scientists. After a presentation and brief dis cussion of the project, it was formally endorsed by a voice vote of those in attendance at the meeting. A summary of the GGP is given below.
Members of the geomagnetic community are proposing to define and implement a long-term, multi-agency effort using satellite, aircraft, ship and surface measurements (observatories, surveys, regional arrays, etc.) to systematically characterize the spatia l and temporal behavior of the earth's magnetic field on local, regional and global scales. The acquisition, analysis and interpretation of data from such studies would involve the major scientific institutions in the country: universities (through NSF), the USGS, DOE, DOD (through Navy and Air Force), NASA, NOAA, state agencies and private industry. Whereas much of the research (both the basic and applied components) would proceed in the traditional mode of individual investigator or small investigator team projects, this program would require - because of the long lead-time and high cost in providing some of the essential facilities (e.g., satellites, survey aircraft and ships, surface observatories, data repositories) - that specific attention be giv en to developing appropriate mechanisms for nurturing on-going communication and coordination among the principal participants.
For more information regarding this activity contact: Prof. J.F.
Hermance, Geophysical Electromagnetics Laboratory, Department of Geological
Sciences, Brown University, Providence, RI 02912-1846 USA.
Precise global measurements of the earth's gravity field are essential to answer a number of important questions in geophysics: (a) Do internal gravity waves (inertial waves if the core is neutrally stratified) exist in the Earth's liquid core and are th eir gravitational effects at the Earth's surface detectable? (b) What is the gravity effect of the global atmospheric loading and mass re-distribution on the solid Earth? (c) Through global tidal analysis, can we refine estimates of the nearly diurnal f ree wobble of the Earth and models of oceanic loading on the solid Earth? (d) What changes in gravity are associated with slow and silent earthquakes, tectonic motions, sea-level changes and post-glacial rebound? (e) Can we monitor the location of the r otation pole of the Earth on a time scale of minutes?
The GGP contains two linked aspects, one international and one Canadian. The international aspect is an agreement by the existing SG groups to (a) upgrade existing SG facilities to a common standard of data acquisition, (b) participate in the six-year o bservational period by maintaining the SGs in good operating condition at fixed locations and (c) exchange raw gravity (and other important supplemental) data through high-speed computer links. Funding for this aspect of the GGP will be the financial res ponsibility of the supporting governmental organizations of each of the national SG groups. The second aspect of the GGP is a project funded by the Natural Sciences and Engineering Research Council of Canada. This project will (a) enhance the Canadian g ravity program by the purchase of two new SGs to form a mini-network of three instruments used both for Canadian gravity studies and for the GGP project and (b) establish an International SG Data Center in Canada. This center will support the internation al GGP effort, facilitating both the data processing and the development of analysis protocols. The Data Center will assist in establishing the high-speed data links between the SG groups and, over the lifetime of the project, will transfer the data proc essing expertise and analysis programs to the user community.
For more information concerning the GGP, contact D. Crossley, Geophysical Laboratory McGill University, 3450 University Street, Montreal, PQ, Canada H3A 2A7.
As the tenure of this grant is approaching its end it is appropriate to give an update on its progress. The axisymmetric problem in isolation is an alpha squared-omega dynamo. Significant progress has been made in looking at the nonlinear aspects of th is problem incorporating Taylor's constraint. The non-axisymmetric problem is at its final testing stage and much of the work required to link the axisymmetric and non-axisymmetric problems together has also been completed, so it is hoped that preliminar y hydrodynamic dynamo results will be forthcoming in the next year. To permit this and further developments, we have recently heard that SERC has provided funding for a further 3 years.
The scale of the project has expanded, both geographically and
in terms of manpower. Chris Jones moves to Exeter at the beginning of 1992
to take up a chair and Carlo Barenghi joins the list of investigators after
taking up a lectureship at Newcastle.
The first circular about the Symposium has been distributed. If you have not received one, please contact Masaru Kono [via mail at Department of Applied Physics, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152 Japan; via fax at (81-3) 3727-4662; via email at mkono@cc.titech.ac.jp], and inform him of your interest.
There has been some talk of holding down costs by arranging group flights to Tokyo from the West coast of the United States and from Europe. A group fare can be arranged with a minimum of ten people. Typically this amounts to roughly a 20% savings. Anyone interested a group flight from the United States should contact David Loper [via mail at Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, Florida 32306-3017, U.S.A.; via voice telephone at (904) 644-6467; via fax at (904) 644-8972; via email at loper@carmen.gfdi.fsu.edu] before February 1, 1992. Similarly, anyone interested in a group flight from Europe should contact Durk Doornbos [via mail at Institute of Geophysics, University of Oslo, 0315 Oslo 3, Norway; via voice telephone at (47-2) 455756; via fax at (47-2) 455269; via email at durk.doornbos@use.uio.no] before February 1st. If enough people express an interest we will look into arranging group flights.
The XVII General Assembly of the European Geophysical Society will be held in Edinburgh, Scotland, 6 - 10 April, 1992. Symposia of particular interest to SEDI include "Dynamics and Structure of the Earth's core and lower mantle", "3D structure and dynam ics of the Earth's mantle" and "Kinematics and dynamics of plate boundaries". For further information, contact the EGS Office, Postfach 49, Max-Planck-Str. 1, W-3411 Katlenburg-Lindau, Germany. Fax: (49) 5556-4709.
The 19th International Conference on Mathematical Geophysics will be held in Taxco, Mexico, 21-26 June, 1992. For more information about this conference, write to Jorge Lomnitz-Adler, Instituto de Física, UNAM, Apartado Postal 20-364, Mex. 01000, D. F. México. Fax: (525) 548-3111.
The University of Cambridge has recently established the Isaac Newton Institute for Mathematical Sciences under the direction of Sir Michael Atiyah. The Institute will run extended programs (generally of 6 months duration) on topics of current interest in the mathematical sciences. One of the two programs chosen for the inaugural period of July to December 1992 is Dynamo Theory, which is being organized by H. K. Moffatt, M. R. E. Proctor, U. Frisch and A. M. Soward. For more information concerning thi s program, write to Prof. H. K. Moffatt, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Silver Street, Cambridge, England CB3 9EW. Fax: (44-223) 337918 or 312984.
A symposium on "The Cosmic Dynamo" is to be held in Potsdam, 6-13 September, 1992. For more information, contact Prof. F. Krause, Akademie der Wissenschaften, Zentralinstitut für Astrophysik, 15 Potsdam, Germany.
The 29th International Geological Congress will be held in Kyoto, Japan from 24 August to 3 September 1992. Symposia of particular interest to SEDI include "Physical properties, chemical compositions and dynamics of the mantle and core", "Geological rhy thms and the Earth's planetary dynamics", "Generation, segregation, ascent and storage of magma" and "Comparative planetary geology and tectonics". For information concerning the Congress, write to Organizing Committee, 29th IGC, P.O. Box 65, Tsukuba, Ib araki 305 Japan.
The 7th Scientific Assembly of IAGA is to be held 8 - 20 August, 1993 in Cordoba, Argentina. To receive the first circular for this Assembly, write to Asociaci—n Argentina de Geof'sicos y Geodestas, 7th Scientific Assembly IAGA, CC 106 Suc 28, (1428) Bu enos Aires, Argentina. The Fax number is (54-1) 791-2658.
The existence of SEDI during the past 4 years indeed appears to have alleviated these problems. The issuing of this newsletter and the biennial SEDI Symposia (1988 in Blanes, 1990 in Santa Fe and 1992 in Mizusawa) certainly have contributed to this succ ess. However, perhaps an equally important factor is that individuals have been able to use the SEDI structure to initiate new programs and organize meetings. Thus national SEDI groups have been formed in several countries, and the visibility of SEDI-re lated research has been much increased by special SEDI symposia at the regular meetings of national and international Unions and Societies.
Then, coming back to the beginning, should SEDI assume an active role also in the process of identifying and coordinating major research tasks? The issue was discussed at the last business meeting in Vienna, without leading to a clear conclusion. It is clear however that, while the present SEDI Committee is not designed to execute or administer cooperative research projects, the SEDI structure itself is well-suited to stimulate and support initiatives leading to such projects. But the initiative should come from individuals, realizing that SEDI can be used to establish the necessary contacts and coordination. Implicitly it has been stated in earlier Dialogs (among others in the form of an appeal by our former chairman, Ned Benton) that the usefulness of SEDI is determined by its users. Obviously this is true also for the next four years. We look forward to hearing from you. D. Doornbos, Institute of Geophysics, University of Oslo, Blindern, 0315 Oslo 3, Norway; phone (47-2) 455756, fax (47-2) 45526 9; email durk.doornbos@use.uio.no.