Deep Earth DIALOG

Number 11 Fall, 2001

This is the eleventh issue of the newsletter of SEDI, an IUGG Union Committee to Study the Earth's Deep Interior. Requests 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, faxed to (904) 644-8972 or emailed to loper@gfdi.fsu.edu. Items for the next issue or notifications of change of address should be sent to bergman@simons-rock.edu.
 

7th SEDI Symposium, University of Exeter, UK, 30th July - 4th August 2000.

The seventh SEDI symposium was held at the University of Exeter, UK, from July 30th to 4th August 2000. The local organising committee comprised Chris Jones, Andrew Soward and Keke Zhang. A special issue of Physics of the Earth and Planetary Interiors will be based on the contributed papers, and some of the invited talks will form the basis of the book Earth's Core and Lower Mantle to be published by Gordon and Breach. Brief summaries of the sessions are found below.
 

At the conference dinner, the Doornboos prizes for outstanding contributions from young scientists were awarded, commemorating the life and work of Durk Doornboos. The recipients were Mike Bergman, Emmanuel Dormy and Ikuro Sumita. In addition to being scientifically productive, the meeting was an enjoyable social experience. Particularly memorable was the hike on Dartmoor, led by Andrew Soward, though it has to be admitted the weather could have been kinder!
 

Session 1: The structure of the inner core and the CMB

  Invited speakers: Guy Masters and Thorne Lay

Masters discussed the recent seismic evidence indicating that the inner core is anisotropic. This anisotropy has allowed the superrotation of the inner core relative to the mantle to be estimated, although as yet there appears to be no general agreement as to the magnitude of this effect. Inner core superrotation can be estimated both from the splitting of the free oscillation modes and from body wave data, but there are problems in trying to produce a simple model which can account for both data sets. Mapping the nature of the inner core anisotropy is an active research area, and four papers, Collier & Hellfrich, Creager & Ouzounis, Garcia & Souriau, and Isse & Nakanishi all addressed aspects of this problem.
  Collier & Hellfrich reported results from a regional seismic network data yielding an inner core rotation rate relative to the mantle of 0.75 +/- 0.08 deg/year. Creager & Ouzounis found evidence of regions of strong anisotropy and regions of isotropy in the inner core from PKP studies; similar results were found by Garcia & Souriau and Isse & Nakanishi. The latter authors obtained only a small inner core superrotation rate.
  Lay considered the Core-Mantle Boundary layer. Seismic tomography has revealed large scale strucures suggesting that the dynamics of mantle convection affects the D'' boundary layer. Anisotropy has also been detected in the boundary layer regions. The low and ultra-low velocity zones in the lowermost mantle are also active areas of investigation, but the physical origins of the low velocity remain problematic. Buffett, Jeanloz & Garnero proposed that sediments accumulating and solidifying at the top of the core may be responsible. Partial melting in the D'' layer remains another possibility.
  Alfe, Gillan and Price discussed their new results from ab initio quantum calculations on modelling the composition of the inner and outer core. Their work suggests that binary Fe/S mixtures are inadequate to explain the observed densities, and that a ternary Fe/Si/O mixture is more plausible. Calderwood investigated the hypothesis that the D'' layer is an accumulated layer of subducted oceanic crust.
 

Session 2: Seismic observations of the lower mantle

  Invited speakers: Adam Dziewonski and John Woodhouse

Dziewonski gave a critical assessment of seismic tomography of the mantle, aimed at reconciling the differences between large-scale low-resolution models and small-scale high resolution studies. On the basis of research from a number of groups, he concluded (i) that long-wavelength anomalies in the mantle are now well-established: (ii) The features that are interpreted as narrow penetrating slabs in high resolution models are actually more complex than this: and (iii) the spectrum of seismic heterogeneity shows strong radial dependence at a depth of 660km, indicating a major change in flow pattern there.
  Woodhouse discussed seismic imaging of the mantle concentrating on the upper mantle. Recent studies have confirmed the existence of global mantle discontinuities in addition to the 660km discontinuity. Seismically determined velocities are generally in agreement with models based on the obduction of subducted material in the mantle.
  Two papers, Ni & Helmberger and Thomas, Kendall, & Heesom discussed the seismology of the D'' layer, the first considering the horizontal structure of the low and ultra-low velocity zones, the second focussing on anisotropy in the D'' region. Romanowicz, Tkalcic & Houy presented data from the splitting of free oscillation modes, suggesting chemical heterogeneity near the base of the lower mantle. Mantle discontinuities at 200km and 410km deduced from short-period PP precursors were discussed by Rost & Weber.
 

Session 3: Core-Mantle-Inner Core coupling

  Invited speakers: Dominic Jault and Peter Olson

Jault discussed electromagnetic and topographic core-mantle coupling and how they relate to LOD variations. He observed that while the torques between the mantle and core give an important signal about the core dynamics, they do not influence that dynamics very strongly. He suggested that topographic coupling was likely to dominate over electromagnetic coupling at the CMB, so that the average differential rotation between core and mantle was determined by a zero pressure torque condition. He also noted that the inner core can have a significant effect on core-mantle coupling.
  Olson talked on thermal coupling between core and mantle, which can have a significant effect on core dynamics. There are likely to be strong lateral variations in the convective heat flux just below the CMB. This could (i) lead to locking of the nonaxisymmetric core flow to the mantle, (ii) lead to changes in the character of the secular variation, and (iii) could affect the frequency of polarity reversals.
  This theme was taken up by theoretical studies from Gibbons & Gubbins and Yoshida & Shudo, examining the effect of inhomogeneous heat flux on core convection.
  Rogister & Denis presented an observational and theoretical study of the periods of free core nutation and Chandler wobble. Greff-Lefftz, Legros, & Dehant examined the influence of inner core viscosity on the rotational eigenmodes of the Earth. The main effect is found to be on the period of Free-Inner-Core-Nutation mode. Bellanger et. al. looked at the connection between geomagnetic jerks and the Chandler wobble. Dehant et. al. reviewed the work of the Special Bureau for the Core, monitoring angular momentum transfers. Ponsar et. al. considered angular momentum conservation on diurnal timescales. Zatman looked at the phase relations between torques and oscillating flows, and how these affect high frequency core-mantle coupling. Gusev and Petrova considered the heat flux from the CMB of the moon.
 

Session 4: High pressure physics of the Earth

  Invited speakers: Reinhard Boehler and David Price

Boehler pointed out that the key parameters required for mantle convection studies are the temperature profile and its relation to melting temperatures, thermal expansivity, thermal conductivity and viscosity. Progress has been made in measuring the first two on this list. The temperature on the core side of the CMB is now well-constrained to around 4000K, and good estimates exist for the solidus temperature in the mantle. However, there are still controversies concerning the chemical interaction of core and mantle material at the CMB, and about the stability of Mg-Si perovskite. Some first measurements of chemical partitioning in deep mantle conditions were reported.
  Price talked about the recent progress made by ab initio quantum mechanical calculations of material properties at high temperature and pressure. Although these calculations are difficult, they do not face the formidable challenges experimentalists contend with in trying to reproduce deep Earth conditions. Density functional theory has been the basis of these calculations. The main results coming out of these calculations, and their implications for core dynamics, were reviewed. Vocadlo et. al. presented a paper in which the results of ad initio quantum calculations of the properties of iron were compared with experimental results of X-ray scattering at high pressure.
  Aizawa reported on a new experimental technique for compression waves at high pressure and temperature. Brown reported on shock-wave Hugoniot data suggesting the presence of a new first order phase transition around 200GPa, and suggested this might be relevant to inner core anisotropy. Chopelas also investigated possible phase changes using spectroscopic data. Jephcoat and Olijnyk used Raman spectroscopy to investigate phonon modes in high pressure materials to measure the elastic properties relevant to the structure of the inner core. Stacey derived a new thermodynamic inequality on the Gruneisen parameter, constraining the possible equations of state for the core and lower mantle. He compared this constraint with some well-known strain models.
 

Session 5: Experiments and short timescale phenomena

  Invited speakers: Henri-Claude Nataf and Keith Aldridge

Nataf reported on the succesful dynamo experiments in Riga and Karlsruhe, which have generated magnetic fields in stirred liquid sodium. These experiments will open the way to others more directly relevant to conditions inside the core. There are also a number of thermal convection experiments being developed, both with rotation and magnetic fields. These experiments will help to sort out the very complex dynamics of the liquid core.
  A report on the Riga experiment was made by Stefani et. al. Tilgner & Busse reported on numerical simulations of the Karlsruhe experiment and Glatzmaier reported on simulations of the Lathrop convective dynamo experiment. Two papers from the Grenoble group reported on results and techniques for rotating convection experiments, and Sumita & Olson described an experiment designed to model heterogeneous boundary heat flux in rotating spherical convection. Bergman & Jones reported on solidification experiments which could shed light on inner core anisotropy and on the conditions obtaining during the formation of the inner core. Experiments designed to model aspects of mantle convection were also reported. Kumagai reported on thermal plume experiments, Le Bars & Davaille and Yanagisawa & Kurita on two-layer convection, while Namiki & Kurita explored tilted upper boundary convection for modelling the D'' layer.
  Aldridge discussed inertial modes and their role in outer core dynamics. He pointed out that short timescale phenomena can lead to instabilities which grow on long timescales, so that inertial waves could affect core dynamics over timescales much longer than a day. Inertial waves are currently being actively studied, as they give rise to interesting mathematical problems. Further impetus has been given to the subject by the development of VLBI and superconducting gravimeters, which are improving the observational data on high frequency modes. Experiments on precessing spheres were reported by Noir et. al. and Vanyo, and theoretical analyses of precessing flows were given by Lorenzani & Tilgner, Mason & Kerswell and Noir, Jault, & Cardin. Viscous damping of inertial modes was considered by Liao & Zhang, and free core nutation in the moon was discussed by Petrova & Gusev.
 

Session 6: Paleomagnetic data

  Invited speakers: Cathy Constable and Jean-Paul Valet

Constable divided the paleomagnetic data into two categories; those that record times of occurrence and those that record directional and intensity measurements at specific locations. Despite much effort, uncertainty still surrounds the behaviour near transition, so that it is unclear whether reversals are fundamentally distinct from secular variation and excursions or whether a reversal is merely an excursion large enough to be unable to find its way back to normality. Generalised Poisson process models have been developed to give a statistical description of the reversal process, and these can be used to compare the power spectrum of intensity with paleomagnetic records. However, these models lack any mechanism for incorporating the non-dipole components during reversal, and so cannot address questions such as preferred VGP paths.
  Valet addressed the question of the robustness of paleomagnetic observations. Robust observations include: (i) The existence of excursions on the 1-10 kyr timescale: (ii) the intensity lows and non-dipolar character of the field during reversals: (iii) oscillations of dipole intensity on the timescale of 2-6 dipole decay times: (iv) the ratio of g20/g10 at about 5% on the 0.1-5 Myr timescale. For longer timescales the only robust features are the maximum reversal frequency and the existence of superchrons.
  Hatakeyama & Kono considered the effect of paleosecular variation on mean magnetic field direction; the nonlinear relation between Gauss coefficients and field directions imply that data means can be affected by fluctuations of Gauss coefficients. One paper on paleointensity by Pan, Zhu, & Shaw analysed paleointensity measurements in China around the time of the Laschamp and Mono Lake excursions, while another considered paleointensity during the Cretaceous. Sumita et. al. also considered paleointensity measurements dating back 2.1 Gyr in Western Australia, finding an intensity roughly 1/3 the present value.
 

Session 7: Dynamo models, theory and reversals

  Invited speakers: Uli Christensen and Paul Roberts

Christensen reported on systematic parameter studies of dynamos at low supercritical Rayleigh number and high magnetic Prandtl number. Dynamos with larger Rayleigh number and lower magnetic Prandtl number have also been computed, but these models employ hyperdiffusivity which makes them physically less consistent. At low supercritrical Rayleigh number dynamos are regenerated by helical motion (alpha-squared dynamos in mean-field terminology) whereas in the high Rayleigh dynamos, which show reversals, differential rotation is important in generating the toroidal field. To date, the gap between these models has not yet been bridged, so that no models without hyperdiffusivity have been calculated for the large Rayleigh number regime.
  Roberts discussed the role of anisotropic turbulence in the geodynamo. There is no immediate prospect of attaining the resolution necessary to resolve scales of order 1km, so a turbulence model is required. Some proposals for estimating the anisotropic eddy viscosity were reviewed.
  There were twenty one contributed papers on dynamo models. Dellar described a lattice Boltzmann approach to formulating the dynamo problem. Ivers discussed the different numerical methods used to solve the dynamo equations, and Sarson et. al. discussed the relation between 2.5d and fully 3d codes. Grote et. al. investigated the role of hyperviscosity in dynamo calculations.
  Cupal et. al. investigated the Stewartson layers generated by the inner core, Fearn considered the role of inertia in dynamo models, while Katayama et. al. and Kutzner & Christensen investigated the role of the boundary conditions and choice of buoyancy sources. Wicht & Christensen considered the role of a conducting inner core on dynamo simulations. Papers using anisotropic diffusion were presented by Phillips & Ivers and Matsushima & Roberts.
  Four papers were devoted to analysis of the output from geodynamo models: Buffett & Bloxham discussed the energetics, Dumberry & Bloxham looked for evidence of torsional oscillations, Kono & Roberts investigated the geophysical implications of dynamo models, while Matsui & Oya investigated the dynamical balance obtaining in their dynamo model. Loper & Shimizu considered small scale structures in buoyancy driven flows, while Ponty et. al. investigated convective instability driven by an unstable Ekman layer. Rotvig & Jones investigated low Ekman number convection driven dynamos using the 2.5d approximation. Holme has calculated optimised kinematic dynamos seeking flows with minimum shear that can support dynamo action. Starchenko proposed a new approach to the dynamo equations based on almost rigid rotation of the planetary core. Botvinovsky modeled the geomagnetic field in terms of current loops.
 

Session 8: Secular variation, theory and the dynamo

  Invited speakers: Jeremy Bloxham and Fritz Busse

Bloxham discussed the constraints on dynamo models imposed by observations of the geomagnetic field. A number of rather different models, with fundamental differences in the underlying physics, are capable of generating fields which look quite similar. There is, however, a wide range of phenomena that have been inferred from geomagnetic observations, ranging from timescales of 1 yr to 10^9 yrs. The extent to which dynamo models are consistent with these observations was reviewed.
  Busse considered convection in rotating spheres and its application to planetary dynamos. Even though current dynamo models are not yet in the right parameter regime, some trends can be perceived; the persistence of structures governed by Coriolis force, even in the presence of magnetic field, is one such trend. Another is the braking of strong differential rotation by Lorentz forces. He also discussed the recently discovered hemispherical dynamos, in which dynamo action is found to occur in one hemisphere but not in the other, leading to a strong quadrupolar component.
  There were five papers on secular variation in this session. Chulliat & Hulot considered tangentially geostrophic flow under the frozen flux assumption, while Hulot et. al. derived core surface flows from the Oersted satellite data. Lannglais et. al. compared Oersted data with Magsat data. Thomson & Macmillan derived geomagnetic models from a combination of ground based and satellite data. Kuang considered whether dynamo models could be used to predict secular variation. Greiner-Mai et. al. presented calculations of how the Earth's magnetic field could be continued into a core layer with differential rotation, while Stefani & Gerbeth investigated inverse dynamo models, whereby the alpha effect is calculated from the observed field.
  There were seven papers in this session on fluid mechanics problems related to core dynamics. Recent advances in the theory of the structure of convection in rotating spheres and spherical shells was considered by Jones et. al., Dormy et. al., and Takehiro & Sakuraba. Revallo & Sevcovic investigated convection in the rotating MHD duct. Dormy et. al. considered super-rotating shear layers driven by rotation of the inner core, while London has developed asymptotic approximations for resistive instabilities in the core.
 

Session 9: Mantle Convection

  Invited speakers: Gerry Schubert and Jerry Mitrovica

Schubert considered the recent proposal that an isolated, compositionally dense, heat source rich layer exists in the lower half of the mantle. This model is motivated by geochemical arguments for an isolated reservoir in the lower mantle, and by the apparent disappearance of slab heterogeneity in the lower mantle. However, Schubert argued that partitioning the lower mantle in this way would lead to a thermal boundary layer at the interface, which is not observed in the seismic data. He concluded that this two layer model of mantle convection is not tenable.
  Mitrovica considered the relation between mantle convection studies and the geological record. He proposed that mantle flow driven by down-welling slabs is largely responsible for transient large-scale marginal and intracratonic sedimentary basins. Dynamic uplift of continents could give constraints on the buoyancy of lower mantle structures. Also, simulations of mantle convection can model the true-polar wander realistically, providing a further link between mantle convection studies and the geologic record.
  Calderwood presented three papers on the distribution of radioactive elements between the mantle and the core, and how this affects mantle convection. He proposed that the heat flux coming out of the core is around 21TW, much larger than previous estimates. This extra core heat is generated by potassium in the core, and means that mantle convection is primarily driven from below. Coltice et. al investigated the reliability of geochemical box models. Steinberger & Castle considered the D'' layer as a thermal boundary layer, and Kurita & Namiki reported on some experiments modelling the effects of lateral variations of the D'' layer. Sotin et. al. applied scaling laws to the thermal boundary layers in the mantles of other planets. Four papers were devoted to thermal plumes; Laudenbach & Christensen made a laboratory investigation in viscous fluids, while Labrosse & Sotin have done numerical simulations of mantle plumes. Samuel & Farnetani investigated thermochemical convection to examine He concentrations in mantle plumes, while Gusev et. al. considered plumes under the Volga-Kama's region. Grigne and Labrosse investigated thermal blanketing of the mantle by the continents, while Ribe developed an asymptotic theory of thin viscous sheets for modellling subduction regions.

[Report by Chris Jones]
 

IAGA/IASPEI Joint Association Meeting, Hanoi 2001.

This joint association meeting was held at the International Conference Center in Hanoi, Vietnam, from 19th-31st August 2001.

SEDI co-sponsored 4 joint sessions linked to separate Association sessions:

JS0 Reversals: Paleomagnetism, the Geodynamo, and the Earth's Deep Interior
JS1 Inner Core - Outer Core Lower Mantle Structure and Interactions
JS2 Geodynamical Constraints at Different Scale
JS3 Plate Reconstructions: Paleomagnetism, Hot Spots and Marine Magnetic Anomalies
JG3 Advances in Geophysical Data Collection and Dissemination

and 3 Association sessions directly:

S4 Earth Structure and Geodynamics
S4W Reference Earth Models

There were some minor changes to the programme forced by variations in attendance at the meeting.

The Joint Association/SEDI Lecture was delivered by David Gubbins on ``Core-Mantle Interactions: Theory and Observation''

More details of this successful meeting can be obtained from the web site: www.iagaandiaspei.org.vn

[Report by David Gubbins]
 

VIITH GEOMAGNETISM AND PALEOMAGNETISM RETREAT, SKIPTON, UK, 24-28 SEPTEMBER, 2001.

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