Deep Earth DIALOG

Number 4 October 29, 1990

This is the fourth 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 Fluid 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 SEDI workshop at the EGS Assembly in Copenhagen

During the last few years the American Geophysical Union has run at its Fall and Spring meetings a series of highly successful workshops focused on SEDI interests. In Europe, a workshop with a similar focus was held for the first time at the 1990 Assemb ly of the European Geophysical Society in Copenhagen, April 23-27, 1990. The workshop was convened by Durk Doornbos and David Gubbins. Interest in the workshop turned out to be much greater than anticipated, and its duration had to be extended from the originally scheduled half day to one and a half days. The 32 papers presented covered a wide range of topics, including the traditional areas of deep-earth seismology, dynamics, geomagnetism, dynamo theory and core modes. A subset of the papers presente d at the workshop will form the core of an issue of Physics of the Earth and Planetary Interiors, edited by David Gubbins. An informal business meeting was held during the last day of the meeting. The general feeling among those present at this meeting was that the workshop filled a need at the Copenhagen Assembly and that similar workshops should be held at future EGS meetings, provided near-collisions with IUGG meetings can be avoided.

The Second SEDI Symposium at Santa Fe, New Mexico

Introduction

The second SEDI Symposium was convened by Gary Glatzmaier and Paul Roberts at St. Johns College, Santa Fe, New Mexico, 6-10 August, 1990. This was in fact the first stand-alone SEDI Symposium, since the first in Blanes was joint with the Committee for M athematical Geophysics. The theme of the symposium at Santa Fe was "Reversals, secular variation and dynamo theory". As the title suggests, the emphasis was on geomagnetism and paleomagnetism. However, the scope was broadened (after the title had been chosen) to include high-pressure experimentation. The high level of scientific interaction between geomagneticians, paleomagneticians and high-pressure experimentalists was unprecedented, and the meeting was a resounding success. Much of that success wa s due to the hard work of Gary and Paul, who deserve our congratulations and thanks. The organizers and participants wish to thank the Institute of Geophysics and Planetary Physics and the Center for Nonlinear Studies at the Los Alamos National Laborator y, the Center for Earth and Planetary Interiors at UCLA, and the Geophysics Program of the Earth Sciences Division of NSF for their support of the symposium.

The symposium was attended by approximately 90 scientists from 17 countries. One particularly unfortunate absence was that of Paul Roberts, who had to miss the meeting to be with his mother who was in failing health. Paul's presence was missed by all.

The scientific program of the symposium was divided into ten half-day sessions: 1. the main geomagnetic field; 2. the structure of the core and lower mantle; 3. mineral physics; 4. paleomagnetic secular variation; 5. paleointensities; 6. core-mantle coup ling and length of day; 7. core surface fields and motions; 8. reversals and reversal transitions. 9. core dynamics and geodynamo theory 1; 10. core dynamics and geodynamo theory 2. The program consisted of 21 "lead presentations" plus 61 short oral pres entations keyed to posters. Plans are underway to publish proceedings of the symposium as a regular issue of Geophysical and Astrophysical Fluid Dynamics. What follows is a selective summary of some of the highlights of the symposium, compiled by J. Blo xham, M. Brown, J. Cain, D. Fearn, D. Loper and R. Merrill. A modified version of this report is being submitted for publication in EOS.
 
 

The main geomagnetic field

The first session consisted of lead talks by D. Barraclough on modelling the field and recent secular variation and by R. Langel on the accuracy of models at the core-mantle boundary, and posters by J. Bloxham & A. Jackson, V. Golovkov, N.-A. Mörner, and J. Cain. In the general discussion following the presentations, the presence of a 60-year (torsional) oscillation of the geomagnetic field was discussed. Bloxham & Jackson used a steady flow to detrend the data from 1840 to 1990 and concluded that ther e is little evidence for torsional oscillations in the short-period secular variation. S. I. Braginsky noted that, after using a polynomial to remove the long period SV, he had found evidence for such a period. Also, T. Yukutake, using the new "Sompi" sp ectral technique on observatory annual means, found a 60-year variation in all but one spherical harmonic to n = 4, but no evidence of any of the shorter-period harmonics which had been predicted by Braginsky.

 Barraclough concluded that Bloxham's results agreed with those predicted by the DGRF series for 1980, but found less agreement for earlier epochs, especially when the 1945 models were projected to the core-mantle boundary. Cain looked at the energy dens ity computed from the DGRF series and concluded that the concept of energy transfer from the dipole to higher modes does not hold up in detail, except for a weak relation between n = 1 and n = 2.

 Bloxham & Jackson concluded that their spline representation gives simple patterns of field change at the core-mantle boundary which are consistent with observatory data showing considerable complexity at the Earth's surface. According to Golovkov, jerk s (discontinuities in secular change) are characteristic of the geomagnetic field's normal behavior. Mörner argued for a precessional motion of the dipole axes as related to the long term irregularities in the Earth's rotation and speculated on involveme nt of the inner core. These latter ideas were not well accepted and alternate explanations were discussed in later sessions of the meeting.
 

Structure of the Core and Lower Mantle

This session began with D. Doornbos reviewing the seismically resolvable properties of the boundaries of the core. His talk highlighted the differences in methodology and uncertainties in results of various attempts to resolve structures near the bounda ries of the core. He concluded that the core-mantle boundary (CMB) is probably quite smooth, but may have "rough spots." The low Q near the top of the inner core is consistent with the idea that the top of the inner core is a "mushy" two-phase zone. Th ese controversial issues were considered from other points of view in short presentations by J. H. Woodhouse & A. M. Dziewonski and by A. Rodgers & J. Wahr.

 F. Stacey presented a model to explain the long-term (~10-100 Myr.) variation in properties of the CMB implied by the reversal record. In his model, gravitationally stable rafts of dense silicate material (i.e., pseudo-continents) drift along the CMB to ward the bases of mantle plumes. As these rafts accumulate at the bases of plumes, they choke the supply of hot material, causing plume to die away. The rafts, by virtue of their thermal insulating properties, also affect flow within the core, possibly controlling (on a short timescale) secular variation and (on a long timescale) the probability of reversal of the magnetic field. This theme was continued when A. K. Goodacre reported on a curious correlation between the directions of plate motions and s patial variations of the Earths magnetic field. He suggested that mantle convection is somehow 'modulating' the main dipole field.

 The session closed with a stimulating talk by D. J. Stevenson of the physical state of the core and the CMB. The theme of his talk concerned processes which are "unavoidable." In particular, he argued that core material could be drawn up into the botto m of the mantle wherever the local stress pattern is tensile. Also, the possibility that mantle material is soluble in the core would lead to ablation of topographic lows on the CMB, with the ablated material redeposited in highs. This process acts to s mooth the boundary, on a timescale which may be as short as several million years.
 

Mineral Physics

In the mineral physics session, R. Böhler, M. Brown, R. Jeanloz and J. P. Poirier discussed mineral-physics contributions to an understanding of composition, temperature and processes in the lower mantle and core. Jeanloz and Poirier agreed that silicat es and iron are highly reactive at high pressure and temperature. Jeanloz attributed this to the fact that iron wets silicates at high pressure and temperature, although it does not do so at low pressure. He claims that this process makes D" the most re active zone in the Earth. As a consequence of this reaction, the core and mantle, which separated at low pressure, are tending to remix, with oxygen diffusing from the mantle into the core and iron moving from the core to the mantle. It is not entirely clear why a sharp core-mantle boundary is observed seismically if these reactions tend to create a diffuse boundary. It may be that the equilibrium compositional distribution, which is a state of uniform energy (chemical plus gravitational), may have a s cale height on the order of a kilometer or less. Verification of this conjecture must wait until the thermodynamics of the reaction are determined.

 Although Jeanloz and Poirier agreed on the reactivity of silicates and iron, they remain apart in their measurements of electrical conductivity. Jeanloz interpreted observed lower mantle conductivities as a result of iron enrichment in a layered mantle model. Poirier, reporting on work with F. Goarant, F. Guyot & J. Peyronneau, attributed the conductive behavior of the lower mantle to the normal effect of pressure and temperature on silicates without the need for iron enrichment. However, Poirier show ed that sample preparation in the form of oxygen-fugacity equilibration can lead to variations in conductivity by three orders of magnitude. He remains cautious in application of the laboratory data to the lower mantle.

 Böhler's determination of silicate densities at high pressure and temperature do not support the chemical layering of the mantle advocated by Jeanloz. Instead, Böhler was able to match closely seismically determined density profiles with no change in co mposition at 670 km depth. Furthermore, using carefully controlled diamond-anvil experiments, Böhler has found that the transition to the perovskite phase is insensitive to iron and silica content. He proposed that 1950±50K be considered a fixed point f or mantle geotherms at 670 km. He also reiterated his finding that the thermal-expansion coefficient decreases strongly with pressure, and may be best parameterized as varying with the fifth or sixth power of the specific volume.

 The iron-melting data from the Berkeley laboratory also experienced new pressure with presentation of Böhler's results from an improved experiment. Incorporating state-of-the-art laser and optical technology, and by replacing subjective criteria for mel ting with quantitative techniques, the new work (extending to 120 Gpa) suggests a modest pressure derivative and a melting point of 3000K at 100 Gpa. Brown critically discussed currently available experimental data for the melting of iron. He suggested that the radiatively determined temperatures of shocked iron interfaces are in accord with his shock temperature determinations. His phase diagram for iron predicts a melting-point temperature for pure iron of 6000K at the inner-core boundary. His prefe rred temperature at the CMB is 3400K.

 Jeanloz suggested that electrical-conductivity heterogeneity at the CMB (caused by reactions between the core and mantle) leads to locally strong, laterally varying and temporally changing coupling between the core and the mantle. This idea echoed throu gh several other presentations.
 

Paleomagnetic secular variation; paleointensities; reversals and reversal transitions

D. Kent summarized marine-sediment data to show that the time-averaged magnetic field (excluding times of reversal events, transitions and excursions) for the past few million years was axially symmetric. Although a geocentric axial-dipole field was a g ood first-order approximation, the time-averaged field also exhibits a small but significant axial-quadrupole component. This quadrupole component is discernibly different for the normal and reverse polarity states. Subsequent discussions produced no co nsensus on the origin of this difference.

 S. Lund provided an excellent tutorial on the problems in obtaining secular variation from sediment data. In addition, he provided evidence that secular variation was nonstationary over the past 50,000 years, and possibly even the past 100,000 years. H is conclusion seemed to be supported by short presentations by R. Dubois and S. Burlatskaya of archeomagnetic and lake-sediment data on secular variation. Lund also argued that the North-American data do not require westward drift to be operating in that region.

 R. Coe gave a first-rate tutorial on paleointensity methods, including discussions of source errors and techniques used to minimize errors. A lead talk by M. Kono, and subsequent short presentations by M. Prevot, P. Pal and J. Shaw dealt with long-term trends in the paleointensity record. The best case for a significant long-term deviation from the present virtual-dipole moment (approximately 8 x 1022 Am2) comes from ocean-intensity data which shows an intensity low over a time span of roughly 50 to 75 Million years in the Mesozoic. This low-intensity interval preceded the Cretaceous superchron (the 30 - 35 million year long interval of no, or few, reversals); the evidence does not support a high intensity during the Cretaceous superchron as had been suggested by a few theoreticians. Kono suggested that even the case for the Mesozoic low intensity might not stand up when more data are acquired. The presentation of E. Tric, A. Mazaud, J.-P. Valet, P. Tucholka & C. Laj illustrated the usefulness of co mbining 10Be and 14C data with paleointensity data to map out variations in paleointensity for the past 80,000 years. In particular, evidence for an average lower paleointensity from 10,000 to 70,000 was presented, along with evidence for variations on a 20,000 year scale.

 K. Hoffman opened his talk with the statement that the "reversal transition interpretations were in transition." This observation was well-illustrated by his talk and those given in subsequent presentations. As pointed out by several speakers, but part icularly by Laj, a disproportionately large number of virtual-geomagnetic-pole (VGP) paths pass through the Americas. This seemed to be independent of site location and also seemed to be reflected in several different reversal transitions stemming from t he Brunhes-Matuyama (B-M) transition back to Miocene transitions. The next most common VGP path appeared antipodal to the Americas. B. Clement provided high-quality data and analyses from three deep-sea sediment records for the B-M transition. The VGP data in this study are most consistent with non-dipolar-field dominance during the middle of the B-M transition. This conclusion is consistent with most previous interpretations of polarity transitions, but appears to conflict with the dipolar dominance seemingly needed to explain the results summarized by Laj. These differences emphasize the need to examine closely the quality of the data used to characterize polarity transitions and to perform careful statistical analyses. At present, a conservative interpretation is that it is not even clear if the middle of most transitions, including the B-M transition, is dominated by an equatorial dipole field or by a non-dipole field! Nevertheless, the numerous transition paths passing through the Americas, or occasionally antipodal to the Americas, was regarded by most paleomagnetists in attendance as one of the most interesting, and puzzling, results presented at the meeting.

 W. Lowrie presented an excellent review of how various reversal chronologies have been constructed. Although both missed and spurious reversal events are likely to be present in any given reversal chronology, there is clear evidence of long-term non-sta tionarity in the rate at which reversals occur. It is also possible that smaller-scale structures, such as small periodicities, are superimposed on the predominantly stochastic reversal record. Lowrie concluded that at present there was no robust eviden ce for any short events during the Cretaceous superchron. R. Merrill also discussed the non-stationarity of the reversal-chronology record based on his work with P. McFadden and suggested the characteristic time for this variation was roughly 180 million years and reflected processes acting in the mantle. These processes would affect the boundary conditions at the core-mantle interface. In addition, Merrill showed how the paleosecular-variation data from lava flows could be used to obtain estimates of the relative strengths of the primary (dipole) and secondary (quadrupole) dynamo families,using the fact that the VGP scatter is roughly proportional to latitude for dipole fields and almost independent of latitude for quadrupole fields. This allowed a t est of prediction made in 1988 by Merrill and McFadden that there was a greater probability for a reversal when the ratio of primary to secondary dynamo family was low. Analyses of paleosecular-variation data from rocks with ages ranging from the present back to mid-Mesozoic have been made and strongly support their prediction; in particular, the primary to secondary family ratio was maximum during the Cretaceous superchron.

A. Ruzmaikin advocated the use of fractals to characterize the variations in the reversal chronology record. In contrast, McFadden pointed out that one needed several orders of magnitude more data than likely ever would be available to prove that proces ses such as (deterministic) chaos were responsible for the variability in the reversal-chronology record. He pointed out that although it would be a mistake to assume that chaotic processes did not occur in the core, there appeared to be superior ways of analyzing the data than fractal-type analysis.
 

Core-mantle coupling and length of day

The session began with lead presentations by L. V. Morrison & F. R. Stephenson on changes in the length of day (LOD) since 500 BC and by J.-L. Le Mouël on core-mantle coupling mechanisms. Morrison noted that since the advent of atomic clocks in 1955, an nual changes in the LOD on the order of 1 millisecond are detectable, but prior to that, only grosser trends are observable. The total change in LOD since 500 BC is 50 milliseconds. There is evidence of non-tidal torques of magnitude 2 x 10E16 Nm acting over the past 1000 years and maximum torques of order 10E18 Nm acting for shorter periods of time. Le Mouël's review concentrated on two possible coupling mechanisms: electromagnetic and topographic. He concluded that electromagnetic coupling is only m arginally capable of producing torques of the necessary magnitude. On the other hand, topographic coupling seems to be too large by as much as two orders of magnitude, unless the flow pattern is "nearly orthogonal" to the topography.

 In spite of Le Mouël's conclusions, topographic coupling was ignored in a number of following short presentations by A. Jackson & J. Bloxham, F. H. Busse, J. Love & J. Bloxham, M. A. Celaya & E. R. Benton and B. A. Buffet, T. A. Herring, P. M. Mathews & I. I. Shapiro, which concentrated on electromagnetic couping alone. Busse's presentation stressed the possible importance of lateral variations in the conductivity of the lowermost mantle, which may affect the standing/drifting field pattern, and even th e dynamo process itself.

 Buffet & others, noting that Jeanloz has argued for the existence of a thin metallicized layer at the base of the mantle, reported on their study of the effect on the forced nutations of the Earth of the electromagnetic torques acting in a highly conduct ing layer some 500 m thick. Their study is concerned with the dissipative part of the coupling; topographic coupling, being locally conservative, has no dissipative part. They could explain the out-of-phase components in the nutation (apart from those a ttributable to tidal dissipation and mantle anelasticity) if the conductivity of this metallicized layer is comparable to that of the core.
 

Core surface fields and motions

This session consisted of a number of presentations concerned with mapping the magnetic field and the fluid flow at the core surface. The session began with a lead paper by J. Bloxham & A. Jackson on fluid flow near the surface of Earth's outer core. B loxham & Jackson reviewed the underlying theory of determining fluid flow near the core surface from geomagnetic-field models, highlighting the problems caused by the inherent nonuniqueness in inversions for core surface motions, and focusing on the well- determined aspects of the flow. In the second part of their talk they discussed some recent attempts to reconcile the toroidal and tangentially geostrophic flow constraints, and work combining the radial and the horizontal components of the magnetic fiel d in the inverse problem.

C. Constable, R. Parker & P. Stark presented magnetic-field models for 1945 and 1980, produced using a new parameterization of the field based on a tessellation of the core surface with spherical triangles. By inverting the two datasets simultaneously a nd adding appropriate constraints, they calculate the most similar models for the two epochs satisfying both the data and Backus's flux constraints. In the following paper Parker discussed some convergence problems associated with magnetic-field modellin g. As the number of data are increased, the errors on the model do not decrease with the reciprocal of the square root of the number of data, as is the case in classical finite dimensional least squares. Indeed, Parker argued that relatively small datas ets (say several thousand data) are about as good as extremely large datasets.

The fourth talk returned to core motions, with C. Voorhies discussing an extension of his earlier steady motions work to flows at finite magnetic-Reynolds number. Unique solutions can be found even at finite magnetic-Reynolds number, provided both the f low and diffusion are steady. Further results on steady flow were presented by K. Whaler in the following talk, who argued that although the poloidal part of the flow is much weaker than the toroidal part it is nonetheless resolved. Unsteady motions wer e discussed by E. Benton & M. Celaya: they showed that the flow can be determined uniquely locally even if the magnetic Reynolds number is finite and the flow is unsteady, provided the time-dependence can be represented by a truncated Taylor expansion. J .-L. Le Mouël re-examined the underlying assumptions on which core-flow inversions are based, finding that they are well justified, and argued that the tangentially geostrophic flow hypothesis is valid even if the top of the core is strongly stratified (a lthough then the shear at the core surface would be very strong). He also raised some questions about the validity of using the horizontal components of the field to map the flow.

The following three papers were largely concerned with dynamics. J. A. Thomsen & S. London presented further results on trapping of MHD waves in the core, the waves being confined outside of a cylinder coaxial with the rotation axis. A. Anufriev showed how reflections of MHD waves at the core-mantle boundary can give rise to an alpha effect, though the magnitude decreases as the toroidal field increases, thus limiting growth of the toroidal field. Y. Honkura & H. Takayanagi presented a cellular automa ta calculation of the rise through the outer core of buoyant material liberated at the inner core boundary. T. Yukutake presented a model of non-dipole secular variation due to changes in the dipole field induced by fluid flow at the core surface.

The formal session concluded with a lead presentation by D. Gubbins, K. Zhang & K. Hutcheson on dynamics of the secular variation. In the first part, they discussed some recent results in kinematic-dynamo theory, showing that the addition of a stagnant layer of fluid at the top of the core increases the efficacy of simple laminar dynamos. In the second part, they discussed core motions, addressing again the issue of reconciling toroidal flows and tangentially geostrophic flows, raised earlier by Bloxha m & Jackson, and Le Mouël, Gubbins proposing that they are reconciled through small-scale motions.

A lively discussion period then followed. The discussion focused on the problem of deciding what constitutes an acceptable model; most core flows are fit to magnetic-field models rather than the original magnetic observations, and it is difficult to dis criminate between different flow models on the basis of their fit to field models when the uncertainties in the field models are unquantified. Although the solution might seem to be to fit the original data, a similar problem is then encountered as to wh at constitutes an acceptable fit to the data. It seems that geomagnetic-field modelling and core motions modelling are likely to continue to be somewhat subjective for the foreseeable future.
 

Core dynamics and geodynamo theory

The final day with three lead talks and no fewer than 19 short presentations was a marathon session, strictly chaired by "race officials" A. M. Soward and D. E. Loper. The fundamental problem of geodynamo theory, Taylor's constraint, was reviewed by C. A. Jones. In the Earth's core, the dominance of the Coriolis force permits neglect of inertial forces for timescales relevant to the geodynamo. This neglect means that the azimuthal torque due to Lorentz forces integrated over any cylinder coaxial with the rotation axis is unbalanced in the interior of the core. This torque must therefore either vanish (the Taylor regime) or be small and be balanced by coupling forces at the CMB (the Ekman regime). Jones discussed the role of Taylor's constraint in th e context of both mean-field dynamos (with prescribed alpha and omega effects) and magneto-convection (convection in the presence of a prescribed magnetic field). For the former, immediately above the (linear) critical dynamo number, the system is in the Ekman regime, and the amplitude of the field is small (controlled by the weak coupling at the CMB). At some higher dynamo number there is typically a transition to the Taylor regime where the field amplitude is much larger, controlled by some nonlinear effect other than the Taylor constraint. The picture for magneto-convection is similar, but in at least one model it can be shown that no Taylor regime exists.

The Taylor-constraint theme was continued in several other presentations. The transition from Ekman to Taylor regimes in spherical mean-field dynamos was discussed by C. F. Barenghi and R. Hollerbach. Barenghi described various modes (steady, oscillato ry, vacillating, and chaotic) found in the Ekman regime. Hollerbach used a linear friction and found results similar to those obtained using a normal viscosity. A. M. Soward highlighted the problems caused by the Taylor constraint in magneto-convection problems when heat diffuses much slower than magnetic field. There is then a conflict between the high shear associated with Taylor's constraint and the inhibiting effect of shear on thermal convection.

The dominant role of the Coriolis force and the consequent Taylor constraint has impeded progress towards numerical simulation of the full hydromagnetic geodynamo driven by convection. Two broadly similar numerical approaches that should produce results in the not too distant future were presented by D. R. Fearn, M. R. E. Proctor, C. C. Seller, C. F. Barenghi, C. A. Jones & A. M. Soward and by G. A. Glatzmaier & P. H. Roberts. A rather different approach (using computer algebra) was described by M. Kon o & T. Nakajima. An intermediate step between such models and the mean-field dynamos discussed earlier was presented by S. I. Braginsky. He has taken his model-Z dynamo, prescribed an alpha-effect as before, but determined the toroidal flow from the mean momentum equation together with an equation describing the diffusion and advection of the density excess, C. So far no finite-amplitude solutions have been found; they either decay if the buoyancy force is too small or blow up. The diffusion of C is ba sed on a turbulent theory (Braginsky & V. P. Meytlis); turbulence is due to a local buoyancy instability, giving an effective diffusivity of C that is highly anisotropic, dependent on the magnetic field, and comparable in magnitude with the molecular magn etic diffusivity.

Studies of magnetic-field stability will be important in interpreting the results of numerical geodynamos and provide constraints on what field configurations may be found in planetary interiors. Two studies (by W. S. Weiglhofer & D. R. Fearn and W. Kua ng & P. H. Roberts) have shown that resistive instabilities can be unstable at moderate field strengths [O(.01T)], and so may play an important role in core dynamics.

The nature of compositionally driven convection was the subject of two presentations. J. W. M. Bush, J. Bloxham & H. A. Stone considered the rise of an immiscible fluid blob in a non-magnetic but rapidly rotating system. D. E. Loper & H. K. Moffatt tac kled a similar problem in which Lorentz and Coriolis forces are comparable in magnitude and presented results for the rise rate of a rigid horizontal cylinder through a horizontal magnetic field through a fluid rotating about the vertical axis.

In earlier sessions we heard much about the CMB and the lateral variation of its properties. E. W. Bolton & B. S. Sayler described a theoretical and experimental investigation of convection with lateral variations in heating. With uniform heating, drif ting waves are found. Periodic heating favours a "locked" convection structure. For heating with both uniform and periodic components, there is competition between the two influences and an intermediate "stick and slip" regime exists.

M. Matsushima & Y. Honkura described an interesting calculation based on field data and the assumption that poloidal field is generated by the interaction of poloidal flow with a strong toroidal field. An iterative approach is used, starting with a gues s at the poloidal flow and ultimately calculating a new poloidal flow, the process being repeated until it converges.

Two talks dealt with how paleomagnetic data can help in our understanding of the geodynamo and constrain theoretical models. R. T. Merrill reviewed the paleomagnetist's art. He discussed the timescale on which the reversal frequency changes [O(180Ma)], and noted that it is comparable with timescales characteristic of the mantle. Merrill also speculated on the role the interaction of dipole and quadrupole fields plays in reversals. Is the Cretaceous quiet zone a period of dipole dominance? With the ve ry patchy paleomagnetic record, P. L. McFadden emphasized the importance of sound statistical technique in drawing conclusions from paleomagnetic data. Occurrence of reversals is described by a Poisson process. This may constrain theoretical models.
 

The SEDI Business Meeting at Santa Fe

In the afternoon of 7 August, 1990, a business meeting of SEDI was chaired by E. R. Benton, Chairman of SEDI, and attended by 34 scientists from 11 countries. This report constitutes the official minutes of that meeting.

 The first item of business was the announcement of the formation of a nominating committee to chose candidates for the SEDI offices for 1991-1995. The composition and charge of this committee are described in the next section of this DIALOG. This was f ollowed by announcements of upcoming meetings of interest to SEDI members; these are described below.

 SEDI has received two invitations for the site of the next SEDI symposium top be held in 1992. One is from the Canadian SEDI committee to hold the symposium at Chateau Montebello, 18-22 May, 1992. The second invitation from the Japanese SEDI Committee is to hold the Symposium at Mizusawa, Japan, 17-21 August, 1992. This would be just prior to the International Geological Congress to be held in Kyoto from 24 August to 3 September, 1992. The pros and cons of these two invitations were discussed at leng th, but no decision was reached, in part because a scientific program and set of convenors had not yet been identified for either. The choice between the two invitations was left up to the Officers and Executive Committee of SEDI, once a principal conven or had been identified and a tentative scientific program had been outlined.

 Other business included an affirmation of SEDI's continued support for ISOP (see DIALOGs #1 and #3 for descriptions of this project) and a brief report by T. Yukutake on the plans of the Japanese SEDI gro up to use recently abandoned submarine cables running from Japan through Guam to Hawaii in an attempt to measure of the electric field produced by the toroidal magnetic field leaking from the core.

 There was an informal discussion concerning the scientific format of the Santa Fe meeting, which consisted of a fixed number of invited 40-minute lead talks and larger number of 10-minute oral presentations linked to posters. The major complaint of the present format seemed to be that there was insufficient time to view the posters. A number of suggestions were made for improvement of the format, but there was no clear consensus how best to modify the format.

 The final item of business was the following proposed resolution, put forth by D. E. Loper: "SEDI, recognizing that the changes that have swept the countries of Eastern Europe in the past year have made travel by scientists from those countries to the up coming IUGG General Assembly in Vienna much easier politically, but more difficult economically, urges the IUGG to seek funds, from the EEC and elsewhere, which would be used to reduce or eliminate the registration fees of those scientists." After some d iscussion, it was agreed that the best way to convey this sentiment was in the form of a letter from the Chairman of SEDI, E. R. Benton, to the Chairman of IUGG, V. I. Keilis-Borok. (Subsequently it was decided that this petition would be more effective if it were directed to the local organizers. Consequently, on August 28th, E. R. Benton wrote a letter to Prof. Peter Steinhauser, chairman of the Local Organizing Committee for the IUGG General Assembly in Vienna, urging him to seek funds to be used to reduce or waive registration fees of scientists from Eastern European countries.)
 

Nominating Committee formed

The proposal to the IUGG which was approved at the Vancouver General Assembly in 1987 stipulated that SEDI be guided by an Executive Committee, headed by a Chairman, a Vice Chairman and a Secretary General. The terms of these three offices would be for four years, with the first term running from 1 September, 1987, through 31 August, 1991. The officers for the present term are E. R. Benton, Chairman, D. Doornbos, Vice-Chairman and D. E. Loper, Secretary General. The proposal further stipulated that "E xecutive Committee members and Officers for subsequent four-year terms shall be chosen at a meeting of interested scientists at the appropriate IUGG General Assembly."

 To facilitate this process, E. R. Benton has appointed a nominating committee consisting of:

 Thomas Ahrens, Seismological Laboratory, 252-21, California Institute of Technology, Pasadena, CA 91125; Tel - (818) 356-6906; Fax - (818) 568-0935.

 Jean-Louis Le Mouël, Institute de Physique du Globe, 4 Place Jussieu, 75252 Paris Cedex 05, France; Tel - 33-1-43294454; Fax - 33-1-43264029.

 H. Keith Moffatt, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 9EW, U.K.; Tel - 44-223-337856; Fax - 44-223-337918.

 Paul Roberts, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024; Tel - (213) 206-2707; Fax - (213) 206-3051.

 Douglas Smylie, Department of Earth Sciences, York University, Downsview, Ontario M3J 1P3 Canada, Tel - (416) 736-5245; Fax - xxx.

 Takesi Yukutake, Earthquake Research Institute, University of Tokyo, Bunkyo-Ku, Tokyo 113, Japan.

 This committee is charged with compiling a slate of names of people who are willing and qualified to serve on the Executive Committee and as Officers of SEDI. This slate is to be presented for approval at the SEDI business meeting at the Vienna General A ssembly. Those in attendance at this meeting will then have the option to approve the nominees or to make any changes they deem appropriate.

 Anyone wishing to provide input to the nominating committee, such as recommending names of nominees for executive committee or one of the offices, is urged to contact one of the members of the committee.
 

Upcoming meetings

A workshop on "Intraplate volcanism: The Reunion hot spot" will be held on Reunion Island 12-17 November, 1990. The scientific sessions will include (a) active hot spots, (b) modelling the plumbing system and eruption forecasting of intraplate active vo lcanoes, (c) intraplate volcanism and seamount genesis, (d) hot spot genesis. For further information concerning this meeting, contact Mme. Catherine Netter, Observatoires Volcanologiques, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Pa ris Cedex 05, France.

 At the AGU 1990 Fall Meeting to be held in San Francisco, December 3-7, 1990, a special session on "Structure and Dynamics of the Core-Mantle Boundary Zone" will be convened by Jeremy Bloxham. This session, which is jointly sponsored by the Geodesy, Geo magnetism & Paleomagnetism, Seismology, Tectonophysics Sections of AGU and by SEDI.

 A NATO Advanced Study Institute on "Fluid motions in the solar system: a comparative approach" will be held in Fairbanks, Alaska, 18-29 June, 1991. For information concerning this meeting, contact one of the convenors: S. Keith Runcorn or Davis B. Stone, both at Geophysical Institute, University of Alaska, Fairbanks, AK 99775-0800; Tel - (907) 474-7558; Fax - (907) 474-7290.

At the next General Assembly of IUGG to be held in Vienna 11-24 August, 1991, Union Symposium 6, on "Dynamics of the Earth's deep interior and Earth Rotation", should be of particular interest to SEDI members. For information concerning this symposium, contact one of the four convenors: Kurt Lambeck, Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia; Jean-Louis Le Mouël, Institute de Physique du Globe, 4 Place Jussieu, 75252 Paris Cedex 05, France; Davi d E. Loper, Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, FL 32306, U.S.A.; Douglas E. Smylie, Institut de Physique du Globe de Strasbourg, 5 Rue René Descartes, 67084 Strasbourg Cedex, France. Of course there will be a num ber of Association symposia of interest to SEDI members, as well.

 Prior to the General Assembly in Vienna, a workshop on "Numerical modelling of lithospheric and mantle dynamics" will be held in Germany 6-11 August, 1991. For further information, contact Harro Schmelling, Department of Mineralogy and Petrology, Uppsal a University, Box 555, S-75122 Uppsala, Sweden, Tel - 46-18-182563 or Uli Christensen, Max-Planck-Institut für Chemie, Saarstrasse 23, D-6500 Mainz, Germany, Tel - 49-6131-305389.

 Susan Friedlander and Misha Vishik are planning a conference on "Magnetohydrodynamic Stability in Geophysics" to be held in Suzdal, U.S.S.R., 25-31 August, 1991, i.e., immediately following the General Assembly in Vienna. For further information concern ing this conference, contact Susan Friedlander at the Department of Mathematics, Statistics and Computer Science, University of Illinois at Chicago, Box 4348, Chicago IL 60680, Tel - (312) 996-3041.
 

SEDI Symposium 1992

Following the SEDI business meeting on 7 August, 1990, further informal discussions were held concerning the next SEDI Symposium. As a result of these discussions, E. R. Benton, wrote to M. Kono on August 27th, 1990, officially inviting him to act as co nvenor of the third SEDI Symposium, to be held in Japan during the summer of 1992. The suggested title of the symposium is "Lateral heterogeneity of the transition between core and mantle". The symposium is expected to include the following topics: (1) seismic structure; (2) geochemistry; (3) high-pressure experiments; (4) dynamics; (5) effects on secular variation, core-mantle coupling and the dynamo process.
 

Global Geodynamics Project

The Canadian SEDI group, led by K. Aldridge, D. J. Crossley, L. Mansinha and D. E. Smylie, is spearheading an effort to launch a 5-year observational period with a global network of superconducting gravimeters. The goals of this Global Geodynamics Proje ct include acquisition of a data set consisting of simultaneous, continuous and uninterrupted high-precision vertical-gravity observations at a number of international stations, and establishment of a centralized resource center to assist in collection an d dissemination of data. The main purpose of the project is to provide data relevant to a number of geophysical phenomena including terrestrial free vibrations, Earth tides, internal waves in the core, nearly diurnal free wobble of the Earth and the effe ct of polar motion on gravity. For more information concerning this project, contact David Crossley, Department of Geological Sciences, McGill University, 3450 University St., Montreal, Quebec, Canada H3A 2A7.
 

ISOP Progress Report

SEDI continues to endorse the establishment of an International Seismological Observing Period. It is increasingly obvious that the currently available data base at the International Seismological Centre, while indispensable for establishing the large-s cale aspherical structure of the Earth, cannot provide the resolution needed to answer crucial questions concerning the fine structure such as the nature of the core-mantle boundary region. Preparations for ISOP have progressed steadily since the last pr ogress report (see Dialog #3). Preliminary science and management plans have been reviewed by the newly formed ISOP Executive Committee at its first meeting at the ISC in Newburg in June, 1990. Although the ISOP will be a truly international effort, for logistic reasons the initial coordination of the project will be done from NEIC in Golden, Colorado. A copy of the ISOP science plan may be obtained from E. R. Engdahl, U.S. Geological Survey, Denver Federal Center, P.O. Box 25 96, MS 967, Denver, CO 80225, U.S.A.
 

Japanese SEDI Activities

A project, "The Earth's Central Core," which was described in Dialog 3, actually started in April, 1990, as a three-year program, funded by the Ministry of Education. The program includes four topics: (1) Structure and physical properties, (2) long-term geomagnetic variations and liquid-core motions, (3) Energy and material balance, (4) structure of internal boundaries and related dynamics. Under the first topic, for example, high-pressure experiments using a sintered-diamond anvil are now possible, allowing the use of larger specimens than before and ensure a more uniform temperature control inside the specimens. Under the last topic, plans are being developed to deploy a superconducting gravimeter in the antarctic to make h igh-precision measurement of gravity variations in an attempt to detect the core-undertone modes related to internal gravity waves.

 The program started with a symposium held in Tokyo on April 5th, under the co-sponsorship of the academic societies related to earth sciences. The symposium, which had the same title as the project, was convened by Y. Fukao, Y. Honkura and T. Yagi and a ttended by about 200 people. 33 papers were presented at the symposium.
 

Joint Japanese - U.S. Program for Undersea Cable.

An international program has been started with the aim of making geophysical observations with retired submarine cables, which had been used for telecommunications across the Pacific Ocean. On November 1, 1990, the segment of the Trans-Pacific Cable -1 (TPC-1) between Japan and Guam will be transferred from KDD and AT&T to the Japanese and U.S. geophysical communities, represented by the Earthquake Research Institute (ERI) of the University of Tokyo and the Incorporated Research Institutions for Seismol ogy (IRIS) of the U.S.A. These latter organizations will maintain the cables and make them available for scientific observations.

 ERI has begun preliminary experiments under a shallow sea, proposing to finally install three observation sites between Japan and Guam: two seismic and one geophysical. For the geophysical station, several kinds of observations are planned, including se ismic, geomagnetic-geoelectric, tsunami (pressure), and oceanic currents. Besides these local observations, a long span measurement of variations in the electrical potential difference between Japan and Guam is also being considered. When the acoustic l inkage system is developed in the future, a wider variety of geophysical and geochemical measurements will be possible.

 A workshop on "Scientific uses of undersea cables", sponsored by IRIS and the Joint Oceanographic Institutions, Inc., (JOI), was convened by Rhett Butler (IRIS) and Thomas Pyle (JOI) in Honolulu from January 30 to February 1, 1990 , and attended by parti cipants from the U.S. and Japan. A steering committee for Scientific Uses of Undersea Cables has been formed within IRIS, with Alan Chave (Bell Laboratories) and chairman. In addition to coordinating the joint program with the Japanese group, this commi ttee will examine the possibility of reusing other cables in the Pacific and Atlantic Oceans.

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