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.